BR112018014112B1 - SYSTEM FOR FORMING A FISTULA BETWEEN TWO VESSELS - Google Patents

Abstract

The present invention relates to devices, systems and methods for forming a fistula between two blood vessels. The systems may comprise a first catheter including a housing and an electrode having a proximal end and a distal end. The proximal end is fixed relative to the housing and the distal end is slidable longitudinally within the housing.

Description

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[001] This patent application claims the benefit of U.S. Provisional Patent Application No. 62/399,471, filed on September 25, 2016, and entitled "DEVICES AND METHODS FOR FORMING A FISTULA", and U.S. Provisional Patent Application No. 62/279,603, filed on January 15, 2016, and entitled "DEVICES AND METHODS FOR FORMING A FISTULA", both of which are incorporated herein by reference in their entireties.

FIELD

[002] The present invention relates to devices and methods for forming a fistula. The devices and methods can be used to form a fistula between two blood vessels.

BACKGROUND

[003] A fistula is generally a passage between two internal organs. The formation of a fistula between two blood vessels may have one or more beneficial functions. For example, the formation of a fistula between an artery and a vein may provide access to the vasculature of hemodialysis patients. Specifically, the formation of a fistula between an artery and a vein allows blood to flow rapidly between the vessels while bypassing the capillaries. In other cases, a fistula may be formed between two veins to form a venovenous fistula. Generally, fistula formation requires surgical dissection of a target vein, and transection and movement of the vein for surgical anastomosis to the artery. It may therefore be useful to find improved ways to form a fistula between two blood vessels.

SUMMARY

[004] In the present descriptive report, devices, systems and methods for forming a fistula between two or more blood vessels are described. Generally, the system for forming a fistula between two vessels, described in the present descriptive report, comprises a first catheter, including a housing and an element having a proximal end and a distal end. The proximal end can be fixed relative to the housing and the distal end can be slidable longitudinally within the housing.

[005] In some variations, the housing may comprise an opening, and the electrode may comprise a portion intermediate between the proximal end and the distal end. In some variations, the intermediate portion may extend into and out of the opening. In some variations, the electrode may comprise a leaf spring. In some variations, the first catheter may comprise a fluid seal to prevent fluid from entering the first catheter at the proximal end of the electrode. In some variations, the housing may comprise a thermally insulating portion adjacent to at least the proximal end of the electrode. In some variations, the electrode may be configured to self-expand from the housing. For example, a user may need to actuate the electrode between a low-profile configuration and an extended configuration.

[006] In some such variations, the intermediate portion of the electrode may comprise multiple bends. In other such variations, the intermediate portion may comprise a bend of less than about 40 degrees. In some variations, the electrode may comprise a low-profile configuration in which the electrode is retracted into the housing. In some variations, the housing may comprise a reservoir between the proximal end of the electrode and a distal end of the first catheter. The reservoir may be configured to hold fluid. The electrode may be configured to generate plasma from the fluid in the reservoir. In some variations, a feedback circuit may be configured to apply a square-wave voltage to the electrode.

[007] In some variations, a second catheter may comprise a second housing and a protruding strut. In some such variations, the protruding strut comprises a compression region configured to oppose the electrode and compress tissue therebetween. In some such variations, the electrode is configured to remove tissue along a length based on a length of the compression region. In some such variations, the second catheter may comprise a retracted portion opposing the protruding strut. The retracted portion may have a shape complementary to that of the protruding strut. In some such variations, the system may comprise an introducer sheath. The protruding strut and the retracted portion may reside within the intubating sheath.

[008] In some variations, the system may further comprise a second catheter comprising a retracted strut. The retracted strut may have a shape that is complementary to a portion of the electrode. In some such variations, the electrode may comprise a portion intermediate between the proximal end and the distal end. The retracted strut may have a shape that is complementary to the intermediate portion of the electrode. In some such variations, the electrode may comprise an extended configuration in which the electrode is extended from the housing. The complementary shape may correspond to a shape of the electrode in the extended configuration. In some such variations, the shape may comprise a concave portion comprising an opening configured to receive the electrode. In some such variations, the electrode may be configured to be propelled into the extended configuration.

[009] In some variations, the first catheter may comprise a first coaptation region comprising a flat junction surface. In some such variations, the first junction region may have a square or rectangular cross-section. In other such variations, the system may further comprise a second catheter comprising a second junction region comprising a flat junction surface. In some such variations, the second junction region may have a square or rectangular cross-section. In some such variations, the first junction region may comprise a first magnet and the second junction region may comprise a second magnet.

[0010] The system may include one or more additional items. In some variations, the first catheter may comprise a first cable, and the system may further comprise a second catheter comprising a second cable. Both the first cable and the second cable comprise a flat surface. In some such variations, the first catheter may comprise a first rod and the second catheter may comprise a second rod. The first rod and the second rod may both comprise braiding to improve torsional rigidity. In some such variations, the first cable may comprise a first magnet and the second cable may comprise a second magnet.

[0011] In some variations, the first catheter comprises a rotation indicator, which comprises a radiopaque material. In some such variations, the rotation indicator may comprise a radiopaque film. In some such variations, the radiopaque film may have a thickness of about 0.025 mm. In other such variations, the rotation indicator may have a square or rectangular cross-section. In other such variations, the rotation indicator may have a cross-section having a written character shape. In other such variations, the rotation indicator may comprise a cube, which comprises a cylindrical cross-section. In other such variations, the rotation indicator may comprise an arrow-shaped cross-section.

[0012] Other systems for forming a fistula between two blood vessels are also described herein. In general, these devices described herein may comprise a first catheter having a fixed height electrode comprising a wire comprising an inner portion and an outer portion, and a protrusion. In some variations, the fixed height electrode may be configured to be supported by the protrusion. In some variations, the outer portion of the fixed height electrode may extend from the housing by up to about 3 mm.

[0013] Also described herein are methods of forming a fistula between two blood vessels. In one variation, a method of forming a fistula between two blood vessels comprises advancing a first catheter into a first blood vessel. The first catheter may comprise an electrode. The method may further comprise removing tissue with the electrode. Removing tissue may comprise applying a constant square wave voltage to the electrode.

[0014] In some such variations, the catheter may comprise a housing and the electrode may comprise a proximal end and a distal end. The proximal end may be fixed relative to the housing, and the distal end may be slidable longitudinally within the housing. In other such variations, a second catheter, comprising a strut, may be advanced to a second blood vessel.

[0015] In some such variations, the first catheter may comprise a first junction region comprising a first magnet, and the second catheter may comprise a second junction region comprising a second magnet. The magnetic attraction forces between the first magnet and the second magnet may compress the tissue between the abutment and the electrode.

[0016] Other methods of forming a fistula between the two vessels are also described herein and may comprise advancing a first catheter into a first blood vessel and a second catheter into a second blood vessel. The first catheter may comprise a first electrode, and the second catheter may comprise a conductive portion. Tissue in the first blood vessel and the second blood vessel may be removed with the first electrode. The first electrode may contact the conductive portion after the tissue is removed. Tissue in the second blood vessel may be removed with the conductive portion, while the conductive portion remains in contact with the first electrode. In some variations, the first blood vessel may comprise a venous blood vessel, and the second blood vessel may comprise an arterial blood vessel.

[0017] In some variations, a method of forming a fistula between two vessels may comprise advancing a first catheter to a first blood vessel and a second catheter to a second blood vessel. The first catheter may comprise a first electrode, and the second catheter may comprise a second electrode. A removal cycle may be performed and comprises: measuring a first impedance between the first electrode and the second electrode; selecting a removal parameter based on the first impedance; removing tissue based on the selected removal parameter; measuring a second impedance between the first electrode and the second electrode; and determining that a fistula has been created based on the second impedance.

[0018] In some variations, determining that the fistula has been created may comprise measuring a second impedance of about 150 ohms or less between the first electrode and the second electrode. In some such variations, measuring may comprise measuring the first cleaning air stream and the second cleaning air stream for a measurement period of about 20 ms, and removing tissue comprises removing tissue for a removal period of about 40 ms. In some such variations, measuring may comprise measuring the first impedance and the second impedance for a measurement period of about 1 second, and removing tissue comprises removing tissue for a removal period of about 500 ms. In some such variations, a first current and a second current may be applied to the electrode to induce neuromuscular stimulation, and a distance from a nerve to a fistula site may be calculated based on the induced neuromuscular stimulation. In some of these variations, the first current may comprise about 1 microampere, and the second current may comprise about 3 microampere. In other such variations, the first catheter and second catheter may be repositioned based on the distance from the nerve to the fistula site. In some variations, a denaturation parameter may be selected based on the first impedance, and tissue may be denatured based on the denaturation parameters selected to reduce tissue density.

[0019] Also described herein are methods of positioning a first catheter and a second catheter within two vessels, which may comprise advancing a first catheter into a first blood vessel and a second catheter into a second blood vessel. The first catheter may comprise a first radiopaque portion, and the second catheter may comprise a second radiopaque portion. An orientation of the first radiopaque portion and the second radiopaque portion may be fluoroscopically imaged using an X-ray beam. The orientation of the first radiopaque portion may be matched to that of the second radiopaque portion. The X-ray beam may be non-perpendicular to the first and second radiopaque portions.

[0020] Also described herein are methods of positioning a first catheter within a first blood vessel and a second catheter within a second blood vessel, comprising advancing the first catheter into the first blood vessel and the second catheter into the second blood vessel. The first catheter may comprise a first electrode, and the second catheter may comprise a second electrode. An impedance may be measured between the first electrode and the second electrode. The first catheter may be aligned with the second catheter based on the measured impedance.

[0021] In some variations, aligning the first catheter with the second catheter may comprise rotationally and axially aligning the first catheter with the second catheter. In some variations, an audio and/or visual alignment signal may be generated based on the measured impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1A is a perspective view of a portion of a catheter comprising an electrode. Figure 1B is a cross-sectional perspective view of a portion of the catheter of Figure 1A.

[0023] Figures 2A and 2B are cross-sectional side views of a portion of the catheter of Figure 1A, showing the electrode in extended (Figure 2A) and low profile (Figure 2B) configurations.

[0024] Figures 3A through 3C are cross-sectional side views (Figure 3A) and cross-sectional perspective views (Figures 3B and 3C) of portions of the housing and electrode of the catheter of Figure 1A.

[0025] Figure 4 is a cross-sectional side view of a portion of the housing and electrode of the catheter of Figure 1A.

[0026] Figure 5 is a perspective view of a portion of another variation of a catheter housing and electrode.

[0027] Figures 6A and 6B are cross-sectional side views of portions of other variations of catheters comprising curved electrodes. Figure 6C is a perspective view of a portion of another variation of a catheter comprising a curved electrode.

[0028] Figures 7A and 7B are representations of a portion of another variation of the catheter comprising a fixed electrode. Figure 7A is a perspective view, and Figure 7B is a cross-sectional side view.

[0029] Figure 8 is a plan view of another variation of an electrode.

[0030] Figures 9A through 9P are illustrative representations of variations in electrode shapes.

[0031] Figures 10A and 10B are plan views of an electrode having variations of an electrode shape.

[0032] Figures 11A and 11B are illustrative representations of a fistula formed by an electrode, having a shape as in Figure 9L. Figure 11A is a side view and Figure 11B is a cross-sectional view of two vessels having a fistula formed between them.

[0033] Figures 12A and 12B illustrate side views of fistulas with strain relief ends.

[0034] Figures 13A and 13B illustrate portions of another variation of a system comprising a first catheter having an electrode and a second catheter having a support. Figure 13A is a perspective view of a portion of the second catheter. Figure 13B is a cross-sectional side view of portions of the first catheter and the second catheter.

[0035] Figures 14A through 14C illustrate portions of another variation of a system comprising a first catheter having an electrode and a second catheter having a support. Figure 14A is a perspective view of a portion of the second catheter. Figure 14B is a side view of a portion of the second catheter. Figure 14C is a cross-sectional side view of portions of the first catheter and the second catheter.

[0036] Figures 15A and 15B illustrate perspective side and cross-sectional views, respectively, of a portion of another variation of a system comprising a first catheter having an electrode and a second catheter having a support.

[0037] Figure 16 is a cross-sectional side view of a portion of another variation of a system comprising a first catheter having an electrode and a second catheter having a support.

[0038] Figures 17A through 17C illustrate portions of another variation of a system comprising a first catheter having an electrode and a second catheter having a support. Figure 17A is a perspective view of a portion of the second catheter. Figure 17B is a cross-sectional perspective view of a portion of the second catheter. Figure 17C is a cross-sectional side view of a portion of the first catheter and the second catheter.

[0039] Figures 18A and 18B are cross-sectional side views of portions of variations of systems comprising a first catheter having an electrode and a second catheter having a support.

[0040] Figures 19A to 19D illustrate another variation of a system comprising first and second catheters comprising respective first and second electrodes. Figure 19A is a perspective view of a portion of the first catheter and the second catheter. Figure 19B is a side view of a portion of the first catheter and the second catheter. Figures 19C and 19D are cross-sectional side views of portions of the first catheter and the second catheter.

[0041] Figures 20A through 20D illustrate various variations of a system comprising a first catheter comprising an electrode and a second catheter comprising a conductive portion. Figure 20A is a perspective view of a portion of the second catheter comprising the conductive portion. Figures 20B through 20D are cross-sectional side views of portions of the first and second catheters in blood vessels.

[0042] Figures 21A through 21E are side views of various variations of a catheter comprising a conductive portion.

[0043] Figures 22A to 22D illustrate another variation of a system comprising a first catheter and a second catheter having respective first and second electrodes. Figures 22A and 22B are plan views, and Figures 22C and 22D are cross-sectional views.

[0044] Figures 23A-23D illustrate another variation of a system including a first catheter comprising a proximal end and a second catheter comprising a support. Figures 23A and 23B are perspective views of portions of the first catheter and the second catheter. Figures 23C and 23D are cross-sectional side views of portions of the first catheter and the second catheter.

[0045] Figures 24A and 24B are illustrative representations of a variation of a rotation indicator. Figure 24C illustrates the width of the rotation indicator displayed as a function of the rotation error of the rotation indicator of Figures 24A and 24B.

[0046] Figures 25A and 25B are illustrative representations of another variation of a rotation indicator. Figure 25A is a perspective view, and Figure 25B is a side view of the rotation indicator. Figure 25C illustrates a fluoroscopic view of rotation indicators similar to those illustrated in Figures 25A and 25B having varying thicknesses and orientation.

[0047] Figures 26A through 26C are illustrative representations of another variation of a rotation indicator. Figures 26A and 26C are perspective views, and Figure 25B is a side view of the rotation indicator. Figure 26D illustrates a fluoroscopic view of rotation indicators similar to those illustrated in Figures 26A through 26C having varying bore diameters and orientation.

[0048] Figure 27 is a cross-sectional side view of a distal portion of another variation of a system comprising a first catheter and a second catheter, each comprising a junction region.

[0049] Figures 28A through 28C illustrate fluoroscopic views of another variation of a rotation indicator having different orientations.

[0050] Figures 29A and 29B are side and perspective views, respectively, of another variation of a rotation indicator.

[0051] Figure 30 is a cross-sectional side view of another variation of a rotation indicator.

[0052] Figure 31 is a perspective view of another variation of a rotation indicator.

[0053] Figures 32A and 32B are fluoroscopic views of another variation of a system comprising a first catheter and a second catheter, each comprising rotation indicators.

[0054] Figure 33 is a perspective view of a variation of cables of the first catheter and the second catheter.

[0055] Figures 34A through 34C illustrate another variation of a catheter handle portion. Figure 34A is a perspective view of the handle. Figure 34B is a perspective view of the handle with a butt portion removed. Figure 34C is a cross-sectional perspective view of the handle.

[0056] Figure 35 is a flowchart illustrating a variation of a method for forming a fistula.

[0057] Figures 36A through 36G are various side and perspective views of portions of another variation of a catheter system. Figure 36A shows a side view of a distal portion of the catheters, Figure 36B shows a cross-sectional side view of the catheter illustrated in Figure 36A, Figure 36C shows a catheter electrical plug, Figure 36D shows a side view of the electrode and support illustrated in Figure 36A, Figure 36E shows a perspective view of the catheter system, Figure 36F shows an intubating sheath over the catheter, and Figure 36G shows a catheter handle. DETAILED DESCRIPTION

[0058] In general, devices, systems, and methods for forming a fistula are described herein. In some variations, the devices and methods may be used to form a fistula between two blood vessels (e.g., an arteriovenous fistula between an artery and a vein, or a venovenous fistula between two veins). Generally, to form such a fistula between two blood vessels, one or more catheters may be advanced in a minimally invasive manner through the vasculature to a target site. In some cases, a single catheter may be placed in a blood vessel to form a fistula with an adjacent blood vessel. In other cases, a system comprising multiple catheters may be used to form a fistula. For example, in some cases, a catheter may be placed in each of two blood vessels. In these cases, it should be considered that each catheter may or may not have the same configuration of elements, and that some catheters may be different from and/or complementary to other catheters, as will be described in more detail below. In some variations, the catheter can be removed from the vasculature after fistula formation, without leaving foreign objects in the body, such as a stent, coil, plug, and so on. In some cases, a catheter configuration can be selected based on the fistula to be formed and the vessels in which the fistula will be located. The variations described below can improve treatment outcomes and reduce complications associated with fistula formation.

[0059] Generally, the systems described herein comprise one or more catheters. The at least one catheter may generally comprise a housing and a fistula-forming element, such as an electrode. The electrode may be secured to the housing and used to remove tissue to form a fistula. During removal, sufficient energy must be delivered to the tissue so that the tissue is removed to form a fistula. A size and shape of an electrode and the energy applied to the electrode may be selected to form a fistula with a minimum of energy to reduce collateral damage to the tissue. The electrode described herein may provide a desired fistula formation that can accommodate a wide range of patients and fistula requirements. In some cases, an electrode may be configured to be advanced through blood vessels of varying diameters without damaging tissue. Once positioned at a fistula-forming site, the electrode may, in some variations, naturally extend to the desired position without user manipulation. During fistula formation, the electrode may continue to extend as tissue is removed. As such, a separate electrode actuator mechanism may be irrelevant. The electrode may be formed into a compact shape to reduce catheter size and/or manufacturing complexity.

[0060] In some variations of the system, a first catheter and a second catheter may complement each other, wherein, for example, the first catheter may comprise an electrode, and the second catheter may comprise a strut, which may determine and control the removal of tissue performed by the electrode of the first catheter. In other variations, a first catheter and a second catheter may both comprise at least one electrode. In some such variations, the first catheter and the second catheter may both comprise at least one electrode, and the electrode(s) of one of the catheters may be an active electrode, while the electrode(s) of the other catheter may be a return electrode. In other such variations, the first catheter and the second catheter may form a dual removal system, which may be energized to remove tissue from opposite sides. This may, in some cases, reduce a removal time. The electrodes may be activated in a simultaneous removal mode, an alternating mode, or a combination thereof. Dual removal systems, described in more detail in this specification, may improve safety, decrease fistula formation time, and/or provide a compact catheter system. In other variations, synergistic removal may be provided when a conductive portion of one catheter is activated by activation of the electrode of another catheter. For example, an activated first electrode may contact a second catheter to energize a conductive portion of the second electrode.

[0061] The catheter may further comprise one or more alignment items, which assist in aligning a catheter relative to another catheter in adjacent blood vessels, and/or in placing the catheters (and blood vessels) in close proximity relative to each other. Aligning the catheters relative to each other may position the fistula-forming element(s) of the catheters at a desired axial location and/or at a rotational angle relative to each other, when both catheters comprise at least one fistula-forming element. When only one of the two catheters comprises a fistula-forming element, aligning the catheters relative to each other may position the fistula-forming element(s) of a first catheter at a desired axial angle and/or a rotational angle relative to a corresponding component of a second catheter (e.g., a strut). In some cases, alignment items on two catheters can hold the respective blood vessels in a desired position during fistula formation, and may help achieve efficient fistula formation with reduced collateral damage. In these or other cases, alignment items may stretch and/or compress tissue at a fistula site in a manner that allows tissue to be removed more quickly and with less energy.

[0062] Furthermore, the alignment items discussed in detail herein may increase the user's confidence in achieving alignment of the catheters, and may be done with a reduced degree of effort. In some variations, the alignment items may comprise one or more magnets, junction surfaces (e.g., flat surfaces), visual alignment aids, and/or cables. For example, opposing magnetic junction surfaces may place the catheters in rotational alignment with each other and together closer to blood vessels. In some variations, at least a portion of the catheters described herein may have a square cross-sectional shape to promote rotational alignment. Magnets located within these catheters may also have a square cross-sectional shape. In some variations, the catheters may comprise magnet arrays comprising multiple square magnets. Additionally or alternatively, a cable may be used to align at least a portion of a catheter relative to at least a portion of another catheter. In some variations, a rotation indicator can be visualized under fluoroscopy, so that a user can visualize the catheters in the blood vessels and manipulate the catheter(s) to a desired position.

[0063] The electrodes described herein may, in some variations, not only remove tissue, but may also measure tissue and/or fistula characteristics (e.g., impedance), such as for catheter alignment and confirmation of fistula formation. The electrodes may be connected, for example, to a generator under the control of an electrosurgical controller. The power supply may be adjusted by the controller to optimize fistula formation and limit collateral tissue damage, based, for example, on measured impedance data.

[0064] One or a combination of catheters described herein may be used to form a fistula, as will be described in more detail herein. Generally, the methods described herein for forming a fistula between two blood vessels may comprise advancing a first catheter into a blood vessel in a minimally invasive manner through the vasculature. After aligning the catheters in a desired location, the vessels may optionally be measured to determine tissue characteristics for tissue modification based on the measurement. Energy may then be delivered to one or more electrodes to remove tissue. For example, an electrosurgical controller may control energy delivery based on real-time measurements to improve energy efficiency and reduce removal time. In other variations, the amount of energy delivered may be fixed or predetermined. The catheters may be removed after confirmation of fistula formation, which may optionally be confirmed by measurement of one or more properties. As described in more detail in this descriptive report, the methods described in this descriptive report can improve fistula patency and longevity using less energy, less time, and with less tissue damage.

1. SYSTEMS A. CATHETERS

[0065] Generally, the systems and devices described herein may be useful in measuring, modifying, and removing tissue to form a fistula. The systems described herein typically comprise one or more catheters. The one or more catheters may comprise one or more fistula-forming elements. The catheters may be configured to be advanced through the vasculature in a minimally invasive manner. In some variations, a fistula may be formed by one or more active electrodes of a catheter. In other variations, two catheters, each comprising one or more electrodes, may simultaneously remove tissue from opposite sides to form a fistula. In some other variations, a first catheter comprising one or more electrodes may form a fistula with a second catheter comprising one or more supports opposing the one or more electrodes. In still further variations, a first catheter comprising one or more electrodes may form a fistula with a second catheter comprising one or more conductive portions, wherein the one or more conductive portions form a return electrode or are energized by contact with one or more electrodes of the first catheter.

[0066] The catheters may be of any diameter suitable for intravascular use, such as, for example, about 4 on the French scale, about 5.7 on the French scale, about 6.1 on the French scale, about 7 on the French scale, about 8.3 on the French scale, between about 4 and about 9 on the French scale, between about 4 and about 7 on the French scale, between about 4 and about 6 on the French scale, or the like. The catheters described may further comprise elements to aid in visualization and/or alignment of one or more catheters, as described in more detail herein. Any or all catheters may be used with the systems described herein to form fistulas using the methods described herein.

[0067] Figure 1A is a perspective view of a distal portion of an illustrative first catheter 100 that may be used to form a fistula between two vessels. The first catheter 100 may comprise a housing 102. An electrode 106 may protrude from an opening 104 of the housing 102 and may be activated to form a fistula. In some variations, the housing 102 may comprise one or more insulating materials that may shield or otherwise protect the catheter 100 and its components from the heat generated by the electrode 106 during use. For example, one or more portions of the housing 102 adjacent to the electrode 106 may comprise a thermally insulating portion that may be ceramic.

[0068] Catheters may further comprise one or more lumens or passageways extending at least partially along or through the catheter. The distal end of the catheter may be configured to aid in advancement of the catheter and/or to be atraumatic. In some variations, the distal end may comprise one or more quick-change portions or other lumens for advancement of the catheter over a guidewire. In still other variations, the distal end may have a guidewire secured to, or otherwise formed integrally with, the catheter. B. FISTULA-FORMING ELEMENTS

[0069] As mentioned above, the catheters described in the present specification may comprise one or more elements for forming a fistula. A fistula-forming element may comprise an element capable of forming a fistula between two vessels. For example, the fistula-forming element may comprise one or more electrical mechanisms (e.g., electrodes or electrocautery mechanisms). Generally, at least a portion of each electrode may be exposed to the surrounding environment (e.g., through one or more openings in a catheter housing, when the catheter is in a fistula-forming configuration). This exposed electrode surface may be configured to contact surrounding tissue (e.g., a blood vessel wall) and/or fluids, and may act as a removal surface such that current may be delivered to and/or conducted from tissue and fluid by the removal surface to facilitate removal (e.g., dissolution of solids) or vaporization (e.g., phase change from fluid to gas) of tissue. In some variations, the exposed electrode surfaces may additionally or alternatively be used to perform functions other than fistula formation. For example, the exposed electrode surface may be used to deliver an amount of energy that causes it to act as a heating surface to heat and modify tissue, rather than remove tissue to form a fistula. Additionally or alternatively, in some variations, the electrode may be used to deliver neurovascular stimulation. Additionally or alternatively, in some variations, the electrode can be used to measure tissue and/or fistula characteristics.

1. LOW PROFILE AND EXTENDED SETTINGS

[0070] In some variations, the electrodes described herein may be configured to have a low-profile configuration and an extended configuration. In the low-profile configuration, the electrode may be configured to be atraumatic when the catheter comprising the electrode is delivered to a fistula site. For example, in the low-profile configuration, the electrode may be retracted into the catheter body such that the outer surface of the electrode does not extend radially beyond the outer surface of the catheter body. As such, the catheter comprising the electrode may be delivered by a tubular body (e.g., a blood vessel, a sheath) in an atraumatic manner. In some variations, the outer surface of the electrode may be flush with the outer surface of the catheter body in the low-profile configuration, while in other variations, the outer surface of the electrode may be below the outer surface of the catheter body in the low-profile configuration. In the extended configuration, at least a portion of the electrode may extend radially outward from the outer surface of the catheter body (i.e., radially beyond the outer surface of the catheter body), and a portion of the electrode may be spaced from the outer surface of the catheter body. As such, the electrode may contact, and in some cases be compressed into, tissue to form a fistula (as described in more detail herein).

[0071] In some variations, the electrodes described herein may be propelled in the direction of the extended configuration. That is, the electrode may be configured to self-expand from the low-profile configuration to the extended configuration. In some variations, the electrode may be held in the low-profile configuration by the inner surface of a vessel wall during delivery. The electrode may then self-expand in the direction of the extended configuration as delivery of energy by the electrode results in tissue removal (as described in more detail herein). In other variations, the electrode may be held in the low-profile configuration by another component of the catheter system, such as, but not limited to, a sheath.

[0072] The electrode may have any suitable shape in the low profile configuration and in the extended configuration. For example, in some variations, the electrode may be curved such that in an extended configuration it forms a convex curve extending from the outer surface of the catheter body. As the electrode moves from a low profile configuration to an extended configuration, the radius of curvature of the electrode may decrease causing the electrode to protrude from the catheter body. Conversely, as the electrode moves from an extended configuration to a low profile configuration, the radius of curvature of the electrode may increase causing the electrode to be retracted into the catheter body. As another example, in some variations, the electrode may comprise one or more bends, which allow it to move between the low profile configuration and the extended configuration.

[0073] In some variations, the electrodes described herein may be configured such that one or both ends of the electrode slide within the catheter body as the electrode moves between the low-profile configuration and the extended configuration. For example, an electrode may comprise a first end and a second end, wherein both the first and second ends are located within the catheter body. A first end of the electrode may be fixed, while a second end of the electrode may be slidable within a lumen within the catheter body. As the second slidable end of the electrode moves toward the first fixed end of the electrode, the electrode may move toward the extended configuration. As the second slidable end of the electrode moves from the first fixed end of the electrode, the electrode may move toward a low-profile configuration. If the electrode is curved, for example, as the second slidable end moves toward the first fixed end of the electrode, the radius of curvature of the electrode may decrease, causing the electrode to protrude from the catheter body.

[0074] An example of such a curved electrode is shown in Figures 1A-2B. Shown in Figure 1A is a distal portion of an exemplary catheter 100 comprising an electrode 106, with the electrode 106 shown in an extended configuration. Figure 1B is a cross-sectional perspective view of the catheter 100. The housing 102 of the catheter comprises an opening 104 through which the electrode 106 may extend when in the extended configuration. As shown in Figure 1B, the electrode 106 may comprise a proximal end 108, a distal end 112, and an intermediate portion 110 between the proximal end 108 and the distal end 112. The electrode 106 may comprise a proximal bend 114 between the proximal end 108 and the intermediate portion 110, an intermediate bend 116 within the intermediate portion 110, and a distal bend 118 within the distal end 112. The proximal end 108 of the electrode 106 may be fixed relative to the housing 102, to fix an axial and/or rotational orientation of the distal end 112 of the electrode 106 relative to the housing 102. The distal end 112 of the electrode 106 may be located within a lumen 120 within the catheter housing 102. The distal end 112 of the electrode 106 may be capable of sliding distally and proximally within lumen 120 (e.g., sliding longitudinally within housing 102), such that intermediate portion 110 of electrode 106 extends into and out of opening 104 in housing 102.

[0075] Figures 2A and 2B illustrate cross-sectional side views of electrode 106 in various states of extension and recession with respect to a catheter housing 102. Figure 2A illustrates electrode 106 in an extended configuration. In the extended configuration, intermediate portion 110 of electrode 106 extends from opening 104 in housing 102 and is thereby extended radially from housing 102. Figure 2B illustrates electrode 106 in a low-profile or retracted configuration. In the low profile configuration, the intermediate portion 110 of the electrode 106 is retracted into the opening 104. As shown, a small portion of the intermediate portion 110 of the electrode 106 may extend slightly radially beyond the outer radius of the catheter housing 102, but in other variations, the intermediate portion 110 of the electrode 106 may be flush with or below the outer radius of the catheter housing 102. In the retracted low profile configuration, the electrode 106 may be capable of being advanced atraumatically through the vasculature for fistula-forming positioning. It should be appreciated that in the variation shown in Figures 2A and 2B, the proximal end 108 of the electrode 106 remains fixed relative to the catheter housing 102 regardless of the position of the intermediate portion 110 and the distal end 112 of the electrode 106.

[0076] In some variations, the electrode 106 may be urged toward the extended configuration from the low-profile configuration. That is, the electrode 106 may be configured to self-expand from the low-profile configuration to the extended configuration. Stated another way, the electrode 106 may be in its natural resting state in the extended configuration, with the intermediate portion 110 of the electrode 106 extending through the opening 104 in the catheter housing 102 at a predetermined distance from the outer surface of the catheter housing 102. In such variations, a force may be required to maintain the electrode 106 in the low-profile configuration. Such a force may be, for example, a radially inward or outward force applied to the intermediate portion 110 of the electrode 106, or a longitudinal force applied to an end of the electrode (e.g., a distal end 112 of the electrode 106 configured to slide proximally and distally within a lumen of the catheter). When an external force is no longer applied to the intermediate portion 110 of the electrode 106, the electrode 106 may return to the extended configuration as shown in Figure 2A, and thereby allow the catheter to be compact. This design may eliminate the need for a complex and/or bulky electrode actuator mechanism.

[0077] For example, the electrode 106 may be held in the low profile configuration due to radially inward or outward force applied by one or more of a vessel wall (not shown for clarity), a second catheter, a sheath, or other object that may compress the intermediate portion 110 of the electrode 106 into the opening 104. This force may also cause the distal end 112 of the electrode 106 to slide longitudinally within the housing 102. For example, in some variations, when the catheter 100 is delivered through the vasculature to a target site, the intermediate portion 110 of the electrode 106 may contact the inner surface of a vessel wall. The inner surface of the vessel wall may exert a radially inward force on the intermediate portion 110 of the electrode 106, which forces the electrode 106 into a retracted, low-profile configuration in which the electrode 106 is retracted into the opening 104 while the electrode 106 contacts and advances through the vessel. The distal end 112 of the electrode 106 may slide distally within the lumen 120 as the intermediate portion 110 of the electrode 106 retracts into the opening 104. As shown, the electrode 106 may comprise a leaf spring—that is, an electrode formed of a curved slat or ribbon having a curvature configured to flex when an external force is applied against it.

[0078] That is, as the electrode 106 of a catheter 100 is advanced through a vessel, the electrode 106 may extend in and out of the opening 104 based on the force encountered. Accordingly, tissue trauma may be reduced as the electrode 106 is advanced through a blood vessel. For example, the electrode 106 may be in the retracted configuration while the electrode 106 contacts a vessel and advances through it. In the retracted configuration, the electrode 106 may be compressed such that substantially all of the electrode 106 is within the opening 104. The compressed electrode 106 may store energy, which allows the electrode 106 to automatically return to its natural extended configuration once the external force is removed. In some variations, a smaller diameter vessel may compress electrode 106 into opening 104, while a large diameter vessel may allow electrode 106 to extend from housing 102 before contacting a vessel wall.

[0079] Propulsion of the electrode 106 toward the extended configuration may increase the energetic efficiency of fistula formation by applying the energy to the tissue to be removed rather than to fluid in the vessel. In some variations, when a leaf spring electrode as described herein is energized and tissue is removed, the electrode may extend beyond the opening by virtue of its elastic force to maintain continuous contact with the tissue. In this manner, the electrode may naturally conform to the size of the vessel encountered, and may eliminate the need for a user-actuated mechanism to deploy and/or extend the electrode from the housing, while still allowing the catheter comprising the electrode to be delivered atraumatically through the vasculature. Stated another way, instead of the catheter comprising a user-actuated control that, in a first state, maintains the electrode in an atraumatic position for delivery through the vasculature, and, in a second state, allows the electrode to enter a configuration for tissue removal, in some variations described herein, the catheter may not comprise such a user-actuated control. Although the electrode may be movable between the low-profile configuration and the extended configuration, this movement may occur as a natural consequence of the electrode's propulsion in combination with external forces (such as from a vessel wall and/or tubular body), and the electrode remains in a single state during use. That is, during both delivery and tissue removal, the catheter is in a state in which the electrode will be able to remain in the extended configuration in the absence of an external force on the catheter (e.g., from a vessel wall) being compressed on the electrode. Stated another way, in these catheters described in this specification, the electrode does not need to be released or disposed of from a delivery configuration by the user prior to fistula formation. As such, the electrode variations described in this specification may improve usability by reducing the number of steps and complexity of catheter operation, increase catheter reliability by reducing the number of internal moving parts, and simplify catheter manufacturing by reducing the number of components.

[0080] In some variations, an electrode removal surface may optionally be temporarily covered (e.g., by a sheath or tubing) such that the electrode does not contact tissue and/or other components as it is advanced through one or more vessels. In such cases, the temporary covering may be moved, removed, or reciprocated to expose the removal surface to the surrounding environment. The covering may be slidable longitudinally along an external surface of the catheter and maintain the electrode in a low-profile configuration. In some variations, a temporary covering disposed over a catheter may aid in advancing the catheter past an access site (e.g., a hemostasis valve) without damaging the catheter or the access site. For example, a sheath slidably located on a catheter may be configured such that the catheter is advanced past a hemostasis valve, and in the vasculature, the distal end of the sheath protects the electrode from contacting the valve. In this manner, the lead in a low profile configuration may be covered by the sheath to protect the lead from passing into the extended configuration and touching the valve as it is advanced into a patient. In other variations, the lead configuration as described in this specification may aid in temporary coverage and/or packaging of a catheter set, as the lead and housing may be easily advanced through a tubular body (e.g., packaging) as if into a blood vessel.

[0081] Figures 3A-3C are detailed representations of the proximal end 108 of the electrode 106 at its point of attachment to the catheter housing 102. An electrical conductor 130 may be coupled to the proximal end 108 of the electrode 106. Wire insulation 132 may cover the electrical conductor 130. A proximal end of the electrical conductor 130 may be coupled to a power source, such as a radiofrequency current generator, as discussed in detail herein, for protection against heat generated when the electrode 106 is activated. The proximal end 108 of the electrode 106 may be attached to the catheter housing 102 in any suitable manner. For example, Figure 3B illustrates an adhesive 140 applied between the proximal end 108 of the electrode 106, the wire 130, the insulation 132, and the housing 102. The adhesive 140 may protect the insulation 132 from heat and/or plasma generated from fluid in the opening 104, securely attach the electrode 106 relative to the housing 102, and/or prevent fluid from ingress from the opening 104 into other portions of the catheter. In other variations, the proximal end 108 of the electrode 106 may be mechanically secured to the housing 102. Figure 3C provides a cross-sectional perspective view of the proximal end 108 of the electrode 106 being reduced in width (e.g., tapered) by the housing 102 to further secure and secure the proximal end 108 of the electrode 106 in the housing 102. In some variations, the opening 104 may form a reservoir for filling with fluid. When electrode 106 is in an extended configuration, fluid may enter and be retained in a reservoir beneath an outer surface of electrode 106 and be relatively immune to fluid flow around the catheter, such as blood flow through a vessel. As discussed in more detail herein, electrode 106 may be configured to generate plasma from fluid in the reservoir. Proximal end 108 of electrode 106 may be sealed from any ingress of fluid from the reservoir, such as by adhesive 140.

[0082] In variations in which the distal end of the electrode is configured to slide within the catheter to alternate the electrode between the low-profile configuration and the extended configuration, the distal end of the electrode may comprise one or more features that allow it to move smoothly within the catheter. The distal end of the electrode may move longitudinally between a first proximal position when the electrode is in an extended configuration and a second distal position when the electrode is in a low-profile configuration. For example, as shown most clearly in Figure 4, the distal end 112 of the electrode 106 may comprise a feature such as a kink and/or a turned-down shape. The distal end 112 of the electrode 106 may be shaped to promote smooth sliding of the distal end 112 of the electrode 106 within the lumen 120 of the housing 102 without contact or snagging. In the variation shown, the distal end 112 of the electrode 106 is curved such that the curve moves along the wall of the lumen 120, rather than the tip of the distal end of the electrode 106 contacting the wall of the lumen 120. In other variations, the distal end 112 of the electrode 106 may have other configurations to allow for smooth translation within the lumen. For example, the distal end 112 of the electrode 106 and/or the wall of the lumen 120 may comprise a lubricating coating. The lumen 120 of the electrode 106 may extend distally from the aperture 104 and have a length configured to accommodate a full distal extension of the distal end 112 of the electrode 106 when in a low-profile configuration (i.e., the intermediate portion 110 of the electrode 106 is retracted within the aperture 104).

[0083] Although a catheter may comprise an electrode having a fixed proximal end and a movable distal end, it should be appreciated that in some variations, one or more ends of the electrode may be partially fixed, such that the end may translate within a fixed range of motion. Figure 5 is a detailed perspective view of a variation of a distal end of an electrode 504, having a distal end with a fixed range of motion. Shown therein is a portion of a catheter housing 500 having an opening 502 through which the electrode 504 may extend in the extended configuration. The distal end of the electrode 504 is coupled to an anchor 506 via a fenestration 508. As shown in Figure 5, the anchor 506 may comprise a pin securely attached to a distal end of the housing 500. The distal end of the electrode 504 may be longitudinally slidable to move the elongated fenestration 508 of the electrode 504 relative to the anchor 506 such that the fenestration 508 moves between a first position with the anchor at its proximal end and a second position with the anchor at its distal end. The anchor 506 may serve as an axial grip structure to prevent the distal end of the electrode 504 from coming loose from the housing 500 and passing through the opening of the housing 502.

[0084] Although in some variations, such as the electrode 106 described herein, an intermediate portion 110 of the electrode 106 may have a curved shape therethrough, in other variations, the intermediate portion of the electrode may comprise one or more distinct bends that allow the electrode to extend and retract into a housing. For example, Figure 6A shows a cross-sectional side view of part of a catheter comprising a housing 600 and an electrode 604, wherein the electrode 604 comprises a proximal end 606, an intermediate portion 608 comprising a first bend 612 and a second bend 614, and a distal end 610. The first and second bends 612, 614 may have the same or different angles. As shown, the first fold 612 may be at a smaller angle than the second fold 614. In some variations, an advancing angle of the intermediate portion 608 of the electrode 604 (e.g., the angle of the second fold 614) may be maintained at approximately less than a 40 degree angle with respect to a longitudinal axis of the electrode 604 with the housing 602 as the electrode 604 transitions between a low profile configuration and an extended configuration. In some such variations, the first fold 612 may flatten as the electrode 604 transitions from the extended configuration (shown in Figure 6A) to the low profile configuration. The housing 600 may comprise an opening 602 into which the electrode 604 may be retracted in the low profile configuration. In the low profile configuration, a distal end 610 of the electrode 604 may be smoothly movable through a lumen of the housing 600. In some such variations, the second bend 614 of the electrode 604 may be flattened to a lesser degree than the first bend 612 of the electrode 604.

[0085] Figure 6B shows a cross-sectional side view of another exemplary electrode 644 comprising a proximal end 646, an intermediate portion 648, and a distal end 650, wherein the intermediate portion 650 of the electrode 644 comprises a single fold 656. The catheter housing 640 may comprise an opening 642 into which the electrode 644 may be retracted into the low-profile configuration. In some variations, the fold 656 may be flattened as the electrode 644 transitions from the extended configuration (shown in Figure 6B) to the low-profile configuration. As shown in Figure 6B, the bend 656 may be located substantially centrally between the proximal end 646 and the distal end 650 of the electrode 644. In the low-profile configuration, a distal end 650 of the electrode 644 may move smoothly through a lumen of the housing 640. It should be appreciated that the bend 656 may be located at a suitable location along the intermediate portion 650 of the electrode 644, such as closest to a proximal end 646 or a distal end 650 of the electrode 644.

[0086] Figure 6C is a perspective view of another variation of a curved electrode. The electrode 668 includes a first fold 678 and a second fold 680, wherein the angles of the first fold 678 and the second fold 680 are approximately equal, and the portion of the electrode 668 between the first and second folds 678, 680 is substantially parallel to a longitudinal axis of the catheter. When the electrode 668 is in a low-profile configuration, it may be retracted into the opening 662 of the catheter housing 660. In some variations, the first and second folds 678, 680 may be flattened as the electrode 668 transitions from the extended configuration (shown in Figure 6C) to the low-profile configuration. The first fold 678 may be located closer to a proximal end of the electrode 668, and the second fold 680 may be located closer to a distal end of the electrode 668. The distance between the first and second folds 678, 680 may correspond to an electrode removal surface and a length of a fistula formed by the electrode 668. The catheter housing 660 may further comprise a first junction region portion 682 and a second junction region portion 684, with the opening 662 therebetween. The junction region may aid in visualization and positioning of one or more catheters relative to each other and their corresponding blood vessels, as will be discussed in more detail herein.

[0087] It should be appreciated that a proximal end is not limited to one or two bends, and an intermediate portion of the electrode may comprise multiple bends. In general, the shape of the electrode may be configured to allow the electrode to extend to a maximum extended configuration (i.e., radially from the outer surface of the catheter), while also being capable of retracting entirely to a low-profile configuration (i.e., into an opening of the catheter housing, such that the outer surface of the electrode is flush with, or below, the outer surface of the catheter), without permanently deforming (e.g., maintaining its shape memory), and configured to support proximal-to-distal and distal-to-proximal insertion into a tubular body (e.g., movement of the catheter proximally or distally within a vessel), without decoupling the electrode from the catheter.

[0088] The electrodes discussed in this specification may be made from any suitable material or combination of suitable materials. In some variations, the electrode may comprise one or more refractory metals. For example, an electrode may comprise tungsten, molybdenum, niobium, tantalum, rhenium, combinations thereof, or alloys.

[0089] Although the catheters shown in Figures 1A through 6C have a single electrode, a catheter may have any suitable number (e.g., zero, one, two, three, or four, or more) and combinations of electrodes as described herein. The electrodes may be located on one or any suitable portion of the catheter (e.g., a distal end, a midportion, or combinations thereof). In variations in which a catheter comprises two or more electrodes, multiple electrodes may be used to create multiple fistulas, simultaneously or sequentially. In other variations, multiple electrodes may interact to form a single fistula.

2. FIXED HEIGHT ELECTRODE

[0090] It should be appreciated that in other variations the fistula-forming elements described herein need not extend into or from the opening in a catheter housing. In some variations, the devices described herein may include a fixed or static electrode. Figures 7A and 7B are illustrative representations of a portion of a catheter comprising a fixed electrode. As shown, the catheter may comprise a fixed electrode 712. At least a portion of the electrode 712 may extend radially from the outer surface of the catheter housing 700 so that it may contact or compress tissue to be removed. In the variation shown, the electrode 712 may comprise a wire having a proximal end 706 secured to the catheter, a distal end 710 secured to the catheter, and an intermediate portion 708 between the distal end and the distal end that is a fixed distance from the outer surface of the catheter.

[0091] In some variations of catheters comprising a fixed electrode, a portion of the electrode may be supported by a protrusion of a catheter. For example, the housing 700 may comprise a protrusion 702, which projects from the housing 700 by a predetermined degree, to support an intermediate portion 708 of the electrode 712. In some variations, the protrusion 702 may extend up to about 3 mm from the housing 700. The intermediate portion 708 of the electrode 712 may rest on the protrusion 702. The protrusion 702 may prevent the electrode 708 from deforming under compression. In some variations, on the side of the catheter opposite the protrusion 702 and the intermediate portion 708, the housing 700 may comprise a recess 704 to maintain an approximately consistent cross-sectional diameter of the catheter along a length of the housing 700. For example, the depth of the recess 704 may correspond to a height of the protrusion 702. This may help, for example, for the entire catheter to pass through an intubating sheath. In some variations, the recess 704 may be electrically insulated by epoxy.

3. FORM

[0092] The size, shape, and orientation of an electrode used to form a fistula can determine the size or other characteristics of the fistula, including fistula resistance and flow rate. The electrode removal surface can be any shape or size suitable for removing tissue and forming a fistula of a desired size and shape. For example, a wider electrode removal surface can produce a wider fistula opening, resulting in lower fistula resistance and improved flow rate. The electrode shape can, in some cases, be selected to promote wound healing as well as prevent fistula dilation and other undesirable complications. In some variations, a desired electrode shape having a desired width can be stripped and then formed into a leaf spring. In some variations, a secondary electrode shape can be welded to an electrode to provide a desired shape and width in a formed fistula.

[0093] In some variations, one portion of an electrode may comprise a coiled portion of material (e.g., a flattened ribbon) configured to contact tissue for fistula formation, while a different portion of the electrode may comprise an uncoiled portion (e.g., a round wire). The coiled portion may be flattened relative to the uncoiled portion. For example, returning to Figure 1B, the intermediate portion 110 and distal end 112 of electrode 106 may comprise a coiled portion of material (e.g., flattened ribbon), while the proximal end 108 of electrode 106 may comprise an uncoiled portion. In some variations, the proximal end 108 of electrode 106 may comprise a transition from the uncoiled portion to the coiled portion. The distal end 112 of electrode 106 may optionally comprise a transition from coiled material to uncoiled material (not shown). Similarly, although electrode 912 in Figures 7A - 7B is shown as an unwound wire, it should be appreciated that in some variations, all or a portion of electrode 712 may be coiled.

[0094] Figure 8 is a plan view of a coiled portion of an electrode 800. As shown therein, the coiled portion of the electrode 800 comprises a proximal end 802, an intermediate portion 804, and a distal end 806. In some variations, a proximal end 802 may comprise an attachment portion 808 for attaching the proximal end 802 relative to a catheter housing (not shown). The coiled portion of the electrode 800 may comprise a flat ribbon shape. The electrode 800 may vary in width along its length, wherein, for example, an intermediate portion 804 is wider than a proximal end 802 and/or a distal end 806. The wider intermediate portion 804 may correspond to a removal surface that contacts and removes tissue. The electrode may comprise, for example, a ribbon of tungsten or a tungsten-rhenium alloy formed in a leaf spring configuration. In some variations, the intermediate portion of an electrode may be formed with one or more shapes to aid in the formation of fistulas having a predetermined configuration. For example, a secondary electrode, such as those illustrated in Figures 9A through 9P, may be welded to an intermediate portion 804 of electrode 800.

[0095] Figures 9A-9P illustrate exemplary variations of electrode shapes that may be desirable for fistula formation. Exemplary electrode shapes may include a longitudinally extending bar 902, 904, 906 shown in Figures 9A-9P, a laterally extending bar 908, 910, 912 shown in Figures 9D-9F, one or more circles 914, 916 shown in Figures 9G and 9H, a dumbbell 918, 928 shown in Figures 9I and 9N, an asterisk 920 shown in Figure 9J, a horseshoe 922 shown in Figure 9K, a plus sign 924 shown in Figure 9L, and a Y-shape 926 shown in Figure 9M. An exemplary electrode 932 having a plurality of triangular openings 930 is shown in Figure 9O. Another exemplary electrode 934 having a circular distal end and a semicircular opening 936 is shown in Figure 9P. These shapes may be provided on a surface of the fistula-forming member on a side opposite a catheter opening.

[0096] In some such variations, these shapes may be welded or formed onto the electrode. For example, the electrode shapes may be welded onto a removal surface of an electrode, such as a mid-portion 804 of the flat ribbon electrode 800 of Figure 8. Figure 10 is an exemplary representation of the electrode shape of Figure 9L welded onto the electrode illustrated in Figure 8. In particular, Figure 10A illustrates a flat ribbon electrode 1000 having a plus-shaped electrode 1002 welded thereto. In variations of the systems described herein comprising a first and a second catheter, both comprising an electrode, both electrodes may have the same shape. In other such variations, an electrode from one catheter may be configured to nest with an electrode from the other catheter. In still other such variations, the electrodes from both catheters may have similar shapes, but one electrode may be larger than the other. This may be desirable, for example, to make a larger opening in a first vessel (e.g., a vein) than in a second vessel (e.g., an artery). It should be noted that in some cases, fistula dilation may be reduced by forming a fistula in a circumferential direction of a blood vessel, as opposed to an axial direction. For example, as shown in Figure 10B, a shape may be welded or formed into an electrode having a longitudinal direction perpendicular to the longitudinal axis of the catheter. Figure 10B is an exemplary representation of the electrode shape of Figure 9N welded into the electrode illustrated in Figure 8. In particular, Figure 10B illustrates a flat ribbon electrode 101 having a dumbbell-shaped electrode 1012 welded thereto.

[0097] In some variations, the electrode shape can be configured to have beneficial effects. For example, an electrode can be configured to form tissue flaps. Such tissue flaps can facilitate the generation of neointima from the extravascular space and can promote wound healing and reduce the likelihood of thrombosis. Exemplary electrode shapes that can form tissue flaps include an asterisk shape 920, a plus shape 924, or a Y shape 926, as shown in Figures 9J, 9L, and 9M, respectively. For example, Figure 11A shows a side view of a fistula 1108 formed using an electrode having a plus shape, such as an electrode comprising the plus shape 924 shown in Figure 9L. Figure 11B shows a cross-sectional view of two vessels 1104, 1106 between which the fistula 1108 is formed. As shown in Figures 11A-11B, the fistula 1108 may form tissue flaps 1102. As shown in the cross-sectional view of Figure 11B, the tissue flaps 1102 may embrace the first vessel 1104 in the second vessel 1106 when a fistula 1108 is formed. As another example, the electrode shape may be configured to form a fistula, with a shape configured to distribute the fluid flow stress over a wider surface area, to prevent enlargement, dilation or tearing of the fistula due to arterial blood pressure. For example, Figure 12A illustrates a side view of a fistula 1200 with strain relief end 1202 formed by an electrode comprising a dumbbell shape with rounded ends, as shown in Figure 9N. Figure 12B illustrates a side view of a fistula 1204 with strain relief end 1206 formed by an electrode comprising a dumbbell shape with flat ends as shown in Figure 9I.

[0098] A size (e.g., radius) and shape of tissue removal created by use of a given electrode may depend not only on the size and shape of the electrode, but also on the energy delivered to the electrode and the surrounding tissue and/or fluid. For example, a lower voltage may remove tissue to form a cut that matches the shape of the electrode, while a higher voltage may increase a radius of removal of the electrode so that the area removed is larger than the electrode. In some variations, energy may be applied so that the fistula does not exceed the dimensions of the electrode by more than about 0.1 mm.

[0099] In some variations, an electrode may have one or more openings or recesses so that energy applied to the electrode to remove tissue may remove an outer circumference of tissue while generally leaving intact one or more internal portions of tissue within the outer circumference. In variations having recesses, the recesses may vary in height. After initial fistula formation with the electrode, blood flow through the fistula may naturally push against the internal portions of the fistula tissue. An exemplary electrode 932 having a plurality of triangular openings 930 is shown in Figure 9O. In some such variations, it may be desirable for the opening to be of a diameter equal to or less than about 0.07 mm to prevent detached tissue released into the blood flow from forming an embolism. In electrode 934 shown in Figure 9P, aperture 936 may comprise asperities, which aid in bubble formation and create electric field concentrations at the electrode, thereby increasing localized current densities and increasing localized resistive heating. This may accelerate removal by reducing a vapor generation time preceding plasma generation.

C. SINGLE-SIDED ELECTRODE SYSTEMS

[00100] In the systems described in this specification comprising two catheters, each catheter may comprise a fistula-forming element, although it need not. In some variations, only one catheter may comprise a fistula-forming element. In some such cases, a second catheter, which lacks a fistula-forming element, may not directly remove tissue, but may help align the catheters, bring the catheters into apposition, and/or otherwise enhance the formation of a fistula. Generally, in such variations, the second catheter may comprise a strut, configured to be positioned within a vessel such that the strut opposes a fistula-forming element of the first catheter. The strut of a second catheter may project from one side of the body of the second catheter. In some variations, the strut may have a shape corresponding to a fistula-forming element of a first catheter to extend into the second catheter, which may in some cases increase a removal surface and therefore a length of a fistula. For example, the strut may comprise one or more concave or convex portions configured to be corresponding and complementary to the fistula-forming element of the first catheter. In some variations, the second catheter may comprise a recessed portion on the opposite side of the catheter body from the strut. In some variations, a catheter comprising a protruding strut may encounter difficulty passing through a tubular body, such as an intubator sheath. A recessed portion may define a relief cut to accommodate passage of the catheter through a tubular body, such as an intubator sheath. This may allow a device of approximate cross-section to be held by the region comprising the strut. Thus, a catheter may be able to fit through a lumen of an intubating sheath and/or a blood vessel, since the diameter of a catheter may not exceed, even at the site of support, a diameter of the lumen of an intubating sheath and/or a blood vessel.

[00101] The support may be configured to comprise tissue in a region localized for removal by a fistula-forming element of a first catheter. Compression of tissue between a support and a fistula-forming element may reduce the required removal height or reach of the fistula-forming element. The support may comprise any suitable non-conductive material. For example, the support may comprise a ceramic and/or polymeric material.

[00102] Figure 13A illustrates a portion of a second catheter, comprising a raised bolster 1302 having a flat surface projecting from a second square catheter 1300. Figure 13B shows the bolster 1302 aligned with an electrode 1306 of a first catheter 1304. In use, after advancing the first catheter and second catheter into respective blood vessels (not shown), the catheter housings may be aligned as shown in Figure 13B. The convex portion 1302 of the second catheter 1300 may oppose the electrode 1306 and compress the tissue therebetween. As shown and discussed in more detail herein, the size and shape of a bolster may be varied based on factors including tissue thickness and density, as well as the desired size, shape, and location of the fistula.

[00103] Figures 14A and 14B show perspective and side views, respectively, of a portion of a second catheter 1400 comprising a protruding support 1402. Opposite support 1402, the second catheter comprises a retracted portion 1404 having a shape complementary to that of protruding support 1402. Figure 14C shows a portion of the second catheter 1400 aligned with a portion of a first catheter 1406 comprising an electrode 1408. In some variations, protruding supports 1302, 1402 may aid in the removal of thicker tissue by further compressing the tissue onto the electrode 1306, 1408.

[00104] Figures 15A and 15B illustrate a protruding support having a shape complementary to that of a corresponding electrode. A system is shown therein comprising a first catheter 1504 comprising an electrode 1506 and a second catheter 1500 comprising a protruding support 1502. Support 1502 comprises a concave portion having a shape that is corresponding and complementary (e.g., inverse, reciprocal) to that of electrode 1506 to mate or conform thereto when the first catheter and second catheters 1504, 1506 are aligned therewith. As best shown in Figure 15B, the shape of the concave portion of support 1502 may correspond to a shape of the intermediate portion of electrode 1506 in an extended configuration. As such, when the first catheter and second catheter 1504, 1500 are located in adjacent vessels and properly aligned with each other, the strut 1502 may promote compression of the tissue between the strut 1502 and the electrode 1506. Although shown in Figures 15A and 15B as comprising a concave curved portion, in other variations, a strut may have any suitable shape based on the shape of an electrode, such as, for example, a shape complementary to that of the electrodes illustrated in Figures 6A through 6C. In some variations, such as those shown in Figures 15A and 15B, the lowest point of the concave portion may be approximately the same height as that of the catheter body adjacent to the strut. However, it should be appreciated that in other variations, the lowest point of the concave portion may be below the height of the catheter body adjacent to the strut - that is, in some variations, the strut may be partially protruding and partially retracted.

[00105] Although Figures 13A-15B show struts projecting from a second catheter, in other variations, the strut may be retracted into the catheter, or the strut may have some portions extending, and some portions retracted, into the catheter. For example, Figure 16 shows a catheter system comprising a catheter with a retracted assembly and evacuation passageway unit having a complementary shape to a portion of an electrode of another catheter. Shown therein is a cross-sectional side view of a system comprising a first catheter 1604 comprising a curved electrode 1606 and a second catheter 1600 comprising a retracted, concave, curved strut 1602. The concave strut 1602 may have a complementary, e.g., inverse, reciprocal shape to mate or conform to a portion of the electrode 1606 of the first catheter 1604.

[00106] In variations of a system comprising a bolster having a shape complementary to that of a corresponding electrode (e.g., Figures 15A-16), a length of a fistula formed may be based on a length of corresponding regions of the bolster and electrode, i.e., a compression region of the bolster. For example, in the variation of Figures 15A and 15B, the fistula formed by the system may have a length corresponding to the peak-to-peak distance of a saddle shape formed by the bolster, shown in Figure 15B as distance 1508. As another example, in the variation of Figure 16, the fistula formed by the system may have a length corresponding to the end-to-end distance of the recess, shown in Figure 16 as distance 1608.

[00107] In other variations, the second catheters described herein may comprise retracted struts that are not complementary in shape to the electrode of a first catheter opposite thereto. For example, in some variations, a second catheter may comprise a rectangular recess configured to receive a portion of an electrode of a first catheter opposite thereto during or after fistula formation. An exemplary retracted strut 1702 is shown in perspective and cross-sectional views in Figures 17A and 17B, respectively. As shown therein, the recess 1702 may comprise a rectangular recess (e.g., a slot) for receiving a portion of an electrode. An exemplary system comprising retracted support 1702 is shown in Figure 17C, comprising a first catheter 1704 comprising a curved electrode 1706, and a second catheter 1700 comprising rectangular retracted support 1702. The portion of electrode 1706 radially furthest from the first catheter 1704 may fit within the rectangular recess. As such, the width of the slit may be configured to be at least the width of electrode 1706.

[00108] A rectangular retracted strut may be of any suitable length and depth. Figures 18A and 18B are cross-sectional views of two systems comprising a first catheter with an electrode and a second catheter with a rectangular retracted strut. Figure 18A shows a variation comprising a first catheter 1804 comprising a curved electrode 1806 and a second catheter 1800 comprising a bolster comprising a rectangular recess 1802. Figure 18B shows the first catheter 1804 of Figure 18A with a different second catheter 1810. As shown therein, the second catheter 1810 comprises a bolster comprising a rectangular recess 1812, wherein the rectangular recess 1812 is shorter and shallower than the rectangular recess 1802. As such, a smaller portion of the electrode 1806 is capable of being received by the recess 1812 as compared to the recess 1802. This may result in the formation of a smaller fistula when formed by delivery of current from the electrode 1806. More generally, the second catheters comprising bolsters with recesses having different depths and/or lengths, may lead to the generation of fistulas of different sizes when using the same electrode. A deeper recess may allow more of the electrode to extend into the recess, thereby increasing the electrode removal surface. In some non-limiting variations, the bolster may be between about 2 mm and about 20 mm in length, and the recess may be between about 1 mm and about 10 mm in length. In one variation, the bolster may be about 10 mm long.

[00109] In single-sided electrode systems, a first catheter comprising an electrode may be connected to a monopolar output of a current generator. In some variations, a single-sided electrode system may comprise one or more return electrodes (e.g., ground pads), which may be disposed on a patient's skin and configured to allow current to pass from an active electrode of the first catheter through the patient and then to the return electrode. A conductive gel may be applied between the return electrode and the skin to improve contact between them.

D. DOUBLE SIDE ELECTRODE SYSTEMS

[00110] In other variations, the catheter systems described herein may be dual-sided catheter systems. A dual-sided catheter system may comprise a first catheter and a second catheter, each including one or more electrodes. In some variations, the dual-sided catheter systems may be dual-removal catheter systems, wherein one or more electrodes on each of the two catheters provide energy to remove tissue at a fistula site from opposite sides, such that the electrodes on each catheter are active electrodes and deliver current. In order for both to deliver energy, both catheters may be connected to an active output of a current generator. A dual-removal system may comprise one or more return electrodes, e.g., ground pads, which may be disposed on a patient's skin and configured to allow current to pass from the active electrodes through the patient and then to the return electrode. A conductive gel may be applied between the return electrode and the skin to enhance contact. A dual-sided removal system described herein may have the ability to remove twice as much tissue in a predetermined amount of time as compared to a single-sided removal system. The energy applied to the tissue may also be evenly distributed when delivered with a dual-sided removal system. In other variations, dual-sided catheter systems may be configured so that one or more electrodes deliver energy, with one or more electrodes on the second of the two catheters providing a return path.

[00111] Figures 19A-19D are illustrative representations of a distal portion of a variation of a dual-sided catheter system comprising a first catheter 1900 and a second catheter 1910, each comprising a first electrode 1902 and a second electrode 1912. The electrodes 1902, 1912 may have a low-profile configuration and an extended configuration, and may be propelled toward the extended configuration, as described in more detail herein. As shown in Figures 19A-19D, each electrode may comprise a curved wire configured to at least partially flatten into the low-profile configuration, as described in more detail herein. In some variations, each electrode may include, for example, a protruding leaf spring element, which may deflect to retract into its respective housing, and may extend when energized to remove tissue and make a cut therethrough, similar to the other leaf spring electrodes described herein. The leaf spring may be sufficiently thin so that if the electrodes are brought into contact with each other, the leaf spring will conform to a flat junction surface.

[00112] Fistula formation using two such catheters may be achieved in a variety of ways, including, for example, in either a monopolar or bipolar configuration. In some variations, when both catheters are designed to deliver monopolar energy, current may be delivered by the first electrode 1902 at the same time as the second electrode 1912. In other variations, the first electrode 1902 may be an active electrode and the second electrode 1912 may be a return electrode, or vice versa. In still other variations, the first electrode 1902 may be activated and followed by the second electrode 1912, or vice versa. In some variations, a combination of these modes may be combined or alternated. In some variations, the electrodes may be activated alternately, for example, for a fixed number of cycles. The electrodes may have the same or different shapes, widths, and geometries. It should be considered that any combination of electrode and catheter designs described in this descriptive report can be used in the dual-side electrode systems considered in this descriptive report.

E. SYNERGISTIC REMOVAL SYSTEMS

[00113] In other variations, the catheter systems described herein may be configured for synergistic removal. Synergistic removal, as referred to herein, means removal of tissue by one blood vessel, which is predicated on removal of tissue by another blood vessel. For example, synergistic removal may begin by removal of tissue with a first catheter disposed in a first blood vessel. Once a predetermined amount of tissue is removed, a second catheter disposed in a second blood vessel may provide concurrent tissue removal until fistula formation is complete. In this manner, tissue removal may begin with one catheter, but be subsequently combined with removal of tissue from a second catheter. As described in detail herein, synergistic removal may improve safety and increase the speed of fistula formation, and may be provided with a compact system.

[00114] More particularly, in synergistic removal, tissue removal may be performed in a first blood vessel by a first catheter for a predetermined period of time and be followed by tissue removal performed in a second blood vessel by a second catheter in conjunction with tissue removal by the first catheter. For example, a first catheter may comprise an electrode, which acts as an active electrode, while a second catheter may comprise a conductive material, which activates only upon contact with the electrode of the first catheter. The electrode of the first catheter may be energized by a power source to perform tissue removal. Once sufficient tissue is removed from the blood vessels, the electrode of the first catheter may make physical contact with the conductive material of the second catheter, such that the conductive material is energized by the power source upon contact with the electrode of the first catheter. It should be noted that even when the conductive portion is not energized, the conductive portion may be stretched and/or compressed to enhance removal by an electrode of the first catheter.

[00115] Figures 20A-20D are illustrative representations of portions of a system including a first catheter 2006 comprising an electrode 2008 and a second catheter 2000 comprising a conductive portion 2002. Figure 20A is a perspective view of a portion of the second catheter 2000. Figures 20B-20D show cross-sections of a portion of the first 2006 and second 2000 catheters disposed in blood vessels 2010, 2012. The first catheter 2006 may comprise an active electrode 2008 coupled to a power source (not shown). The second catheter 2000 may comprise an electrically conductive portion 2002 configured to be contacted by the electrode 2008. Figures 20C and 20D are cross-sectional side views of the first catheter 2006 and the second catheter 2000 disposed in blood vessels 2010, 2012. Upon activation of the electrode 2008, the electrode may generate plasma, and tissue adjacent to the active electrode in a first blood vessel 2012 may be removed (Figure 20C). The active electrode 2008 may advance along the walls of the first and second vessels to contact the electrically conductive portion 2002 of the second catheter (Figure 20C). Contact between the two electrical surfaces may serve to electrically connect the active electrode 2008 to the electrically conductive portion 2002 such that they generally share the same electrical potential, and the electrically conductive portion 2002 serves as an extension of the electrode 2008. Subsequent to contact, energy supplied from the power source may be conducted directly to the electrically conductive portion 2002, thereby promoting the formation of a plasma in the electrically conductive portion 2002. This may serve to further remove or ablate tissue with which the conductive portion 2012 is in contact from within the second blood vessel 2010 until the electrode 2008 has traveled entirely through the walls of both vessels and is in contact with the conductive portion 2002 (Figure 20D). In this manner, a single electrical source may conduct energy to two separate electrodes in a system, the second electrode of which is only activated once the first formed element has made contact with it.

[00116] It should be appreciated that the conductive portion of the second catheter may have any suitable shape. That is, the conductive portion is not particularly limited in size and shape, and may comprise a spring, a raised wire, and/or a rigidly attached metal surface. Figures 21A-21E illustrate side views of various variations of the conductive portion, disposed in various catheters 2100, 2102, 2104, 2106, 2108, including a flat conductive portion 2112, as shown in Figure 21A, a protruding conductive portion having first and second bends 2114, as shown in Figure 21B, a rounded conductive portion 2116, as shown in Figure 21C, a protruding conductive portion having a bend 2118, as shown in Figure 21D, and a protruding conductive portion having a projection 2120, as shown in Figure 21E. It should be considered that any combination of electrode designs described in this descriptive report can be used in the synergistic removal systems considered in this descriptive report.

[00117] In some variations, the first catheter 2006 and the second catheter 2000, shown in Figures 20A-20D, may be used to form an arteriovenous fistula by first removing tissue from the venous side of a joined artery and vein pair. Once a hole has been produced by the advancing electrode 2008 from the venous side, the arterial portion 2002 may be activated by electrical contact with the active electrode 2008 to remove further tissue on the arterial side. Synergistic removal in this manner may prevent incomplete fistula formation due to a rupture in the arterial wall when fistula formation begins on one arterial side. For example, arterial removal may be activated only when the conductive portion 2002 directly contacts the electrode 2008 from the venous side, which occurs only if a hole is formed from the venous lumen into the arterial lumen. In this way, the risk of high arterial blood flow, which undesirably increases the size of a fistula, is reduced.

[00118] In some variations, synergistic removal may allow for increased fistula opening by use of a leaf spring electrode of a first catheter advancing from a venous side of a joined artery and vein pair. When the conductive portion is energized by the electrode, the conductive portion need not include wires or other elements through the second catheter to energize the conductive portion. In some variations, a conductive portion of a second catheter, opposite an electrode (e.g., leaf spring electrode) may be free of electrical components within the proximal section of the catheter (e.g., power source and wire to energize the conductive portion). Accordingly, a proximal junction region of the second catheter, comprising a set of magnets, may be provided with more mass and attraction force, thereby increasing tissue compression and catheter alignment. Additionally or alternatively, the second catheter may be formed with a smaller diameter relative to the first catheter or other alignment elements relative to the first catheter.

F. ENERGY SOURCE

[00119] As described herein, the electrodes described herein may be connected to a radiofrequency current generator (e.g., via a monopolar or bipolar output of the current generator) to energize the electrodes for the various applications described herein (e.g., blood vessel modification, tissue removal). The electrodes may be advanced to a first blood vessel adjacent to a second blood vessel, and in some such variations, a ground electrode may be placed external to the patient, and current may be applied to the tissue, via the ground electrode, to form a fistula between the two vessels. Tissue from a first blood vessel, being located closer to the electrode, may be removed or vaporized more quickly than tissue from a second blood vessel. Additionally, in variations in which the electrode is configured to extend through tissue, during removal, the electrode may first contact and remove tissue from the first blood vessel, before contacting and removing tissue from the second blood vessel.

[00120] In some variations, one or more electrodes may be connected to an electrosurgical generator, power source, or other waveform generator that is configured to generate alternating current under the control of an electrosurgical controller. In some such variations, one or more electrodes may be connected to a monopolar output of a generator. In other variations, two or more electrodes may be connected to bipolar outputs of a generator. In some such variations, a first electrode is attached to the active output of the generator, and a return electrode (e.g., a large metal plate or flexible metalized block) may be attached or temporarily attached to the patient and connected to the return output of the generator. In other such variations, two or more electrodes may be attached to an active output of the generator, and a return electrode may be attached or temporarily attached to the patient and connected to the return output of the generator. In still other variations, a first electrode may be attached to the active output of the generator, and a second electrode may be attached to the return output of the generator. In yet other variations, a first electrode may be connected to an output of a generator, and a second electrode may be floating, that is, not directly connected to any output of the generator in an oriented monopolar configuration.

[00121] In some variations, the radio frequency current generator may be a high voltage generator, having a voltage range of about 180 V peak to about 500 V peak, and a frequency range between about 150 kHz and about 8 MHz. The generator may be configured to generate, for example, sinusoidal and square waveforms.

G. ALIGNMENT ITEMS

[00122] In variations in which a system having multiple catheters is used to create a fistula between two blood vessels, each catheter may be configured to promote rotational and/or axial alignment. Proper axial and rotational alignment between two catheters may facilitate alignment of one or more fistula-forming elements on a first catheter with one or more corresponding elements (e.g., one or more fistula-forming elements, one or more corresponding surfaces - e.g., a bolster) on a second catheter.

[00123] To illustrate axial and rotational alignment, Figure 22A shows a catheter system comprising a first catheter 2200 and a second catheter 2210, wherein the catheters are axially aligned such that their respective first electrodes 2202, 2212 can form a fistula from opposite sides of the same tissue region (not shown for clarity). Comparatively, Figure 22B illustrates the first catheter 2200 and the second catheter 2210 axially misaligned such that their respective first electrodes 2202 and 2212 contact tissue at two different axial locations along the vessels.

[00124] Both Figures 22C and 22D illustrate a cross-sectional view of the first catheter 2200 and the second catheter 2210 along line A-A of Figure 22A. Figure 22C illustrates the first catheter 2200 and the second catheter 2210 rotationally aligned. By comparison, Figure 22D illustrates the first catheter 2200 rotationally misaligned with the second catheter 2210. One or more magnets, flat junction surfaces, rotation indicators, and cable items may assist in achieving axial and/or rotational alignment, as described in more detail herein. 1. MAGNETS

[00125] As mentioned above, the catheters of the systems described herein may comprise one or more magnets, which may assist in rotational and axial alignment, as well as bringing the catheters together to compress tissue. Generally, the magnets may be configured to be attached to one or more magnetic fields (e.g., produced by one or more magnets on another catheter). The magnets may assist in aligning or otherwise repositioning the catheters when placed in the vasculature. In some cases, a system may comprise a first and a second catheter, each having one or more magnets, such that the magnets on the first catheter may be attracted to the magnets on the second catheter to bring the catheters into close proximity to each other. The one or more magnets may assist in ensuring that the one or more catheters are in proper axial and/or rotational alignment relative to the other catheter or catheters. This axial and/or rotational alignment of catheters may also facilitate the alignment of one or more fistula-forming elements relative to a fistula site.

[00126] In variations in which the catheters of the systems described herein comprise one or more magnets, each catheter may have any number of individual magnets (e.g., one, two, three, four, five, six, seven, or eight or more, etc.). In variations in which a catheter has multiple magnets, these magnets may be grouped into one or more magnet arrays. The magnets may be located inside and/or outside a catheter body. The magnets may be positioned anywhere along the catheter. In some variations in which a catheter comprises an electrode, an alignment portion may include one or more magnets proximal to an electrode. Additionally or alternatively, the first catheter may have one or more magnets distant from the electrode.

[00127] The magnets described throughout this specification may be permanent magnets comprising one or more hard magnetic materials, such as, but not limited to, rare earth element alloys (e.g., samarium cobalt magnets or neodymium magnets, such as N52 magnets) or alnico. In some variations, the magnets may comprise anisotropic magnets; in other variations, the magnets may comprise isotropic magnets. In some variations, the magnets may be formed from compressed powder. In some variations, a portion of the magnets (e.g., a permeable backing) may comprise one or more soft magnetic materials, such as, but not limited to, iron, cobalt, nickel, or ferrite. It should be noted that in some variations of systems comprising two catheters, either of the first catheter and the second catheter may comprise ferromagnetic elements (i.e., elements attracted to, but not generating, a permanent magnetic field). For example, in some variations, the first catheter may include only one or more ferromagnetic elements, while the second catheter may comprise one or more permanent magnets. In other variations, the second catheter may include only one or more ferromagnetic elements, while the first catheter may comprise one or more permanent magnets. However, in other variations, one or both of the first catheter and the second catheter may include any suitable combination of ferromagnetic, permanent magnets, and/or any other suitable types of magnets.

[00128] Generally, the dimensions of the magnets described herein may be selected based on the size of the catheters carrying the magnets, which may in turn be selected based on the anatomical dimensions of the selected blood vessels through which the catheters may be advanced. For example, if the catheter is to be advanced through a blood vessel having an internal diameter of about 3 mm, it may be desirable to configure any magnet to be smaller than about 3 mm at the widest part of its cross-section to reduce the risk of injury to the vessel walls during advancement and manipulation of the catheter. Each magnet may be of any suitable length (e.g., about 5 mm, about 10 mm, about 15 mm, about 20 mm, or the like), although it should be noted that in some cases, longer magnets may limit the flexibility of the catheter for maneuvering through tissue. Accordingly, in some variations, multiple magnets (e.g., square magnets, described in more detail herein) may be arranged in a linear array along the length of a catheter to promote catheter flexibility.

[00129] In view of limitations on magnet size, it may be desirable in some cases to use magnets configured to produce magnetic fields that increase the magnetic force that can be generated with a magnet of a given size. For example, in some variations, the system may comprise one or more of the magnets described in U.S. patent application serial no. 14/214,503, filed March 14, 2014, and entitled "FISTULA FORMULATION DEVICES AND METHODS THEREFOR", and/or in U.S. patent application serial no. 14/657,997, filed March 13, 2015, and entitled "FISTULA FORMATION DEVICES AND METHODS THEREFOR", both of which are fully incorporated by reference herein.

[00130] Each magnet may be attached to a catheter by any suitable method. For example, in some variations, one or more magnets may be embedded, adhered, or friction-fitted within a catheter. 2. PLANE JOINT SURFACES

[00131] In general, proper rotational alignment between two catheters can be promoted by a flat junction surface. For example, both catheters can provide a flat surface configured to align with the flat surface of the opposing catheter, and in some cases, the catheters can have a rectangular or square cross-section for all or part of each catheter length. In some variations, the magnets within the catheters can also have a rectangular or square cross-section. These flat surfaces of opposing catheters can help to naturally align the two catheters with each other. This is similar to the phenomenon that a rigid body placed on an angled surface will automatically align itself to be parallel to the surface due to gravitational forces. For example, when one edge of a box is placed on a flat surface, gravity will produce a moment about the center of gravity of the box, such that the box will fall and align itself with the flat surface to achieve equilibrium. Similarly, magnetic joining forces can bring two flat magnetic surfaces together in parallel to each other when placed in close proximity to each other. As such, flat joining surfaces can bring a pair of catheters into rotational alignment and increase compression on the tissue to enhance fistula formation.

[00132] Figures 23A-23D illustrate a system having a first catheter 2300 and a second catheter 2310, both comprising a flat junction surface. As shown therein, both catheters 2300, 2310 comprise a junction region 2316, 2318 at a distal portion of the catheter, wherein the junction region has a square cross-section. As such, one side of each junction region 2316, 2318 forms a flat junction surface 2306, 2314. When the first catheter 2300 and the second catheter 2310 are properly aligned, as shown in Figures 23A-23D, the flat junction surface 2306 of the first catheter faces the flat junction surface 2314 of the second catheter 2310.

[00133] Each catheter may further comprise magnets within one or more portions of the junction regions. In some variations, the first catheter may comprise one or more magnets proximate to the electrode and one or more magnets remote from the electrode, and the second catheter may comprise one or more magnets proximate to the abutment (or the electrode or the conductive contact in a dual-sided system) and one or more magnets remote from the abutment (or the electrode or the conductive contact in a dual-sided system). Figure 23D is a cross-sectional view of a portion of catheters 2300, 2310 showing a series of square magnets 2320, 2322 in the first and second junction regions 2316, 2318 proximate to electrode 2304 and abutment 2312, respectively. The magnets 2320 at the junction region 2318 of the first catheter 2300 may comprise a lumen for a wire 2308 for electrically coupling a power source to the electrode 2304. The wire 2308 may be covered by insulation 2302 to protect the magnets 2320 from current conducted by the wire 2308.

[00134] In Figures 23A-23D, in which the second catheter 2310 provides a fulcrum 2312, the magnetic forces of attraction are oriented at the fulcrum 2312, which can increase tissue compression between the fulcrum 2312 and the electrode 2304. When the first catheter 2300 is located in a first vessel and the second catheter 2310 is located in a second adjacent vessel, and the two catheters are properly aligned, the magnetic junction forces can compress the electrode 2304 into the vessel tissue (not shown) by placing the two catheters in close proximity to each other, which can improve the ability to make a transmural cut. In addition to placing the catheters in close proximity to each other and compressing the tissue, the flat junction surfaces can provide for the generation of a lateral magnetic junction force. An alignment torque corresponding to a magnetic force of attraction may occur when the first catheter 2300 and the second catheter 2300 are rotationally misaligned, which may promote alignment of the catheters relative to each other. The alignment torque may increase as the catheters are brought into closer proximity to each other. Because the strength of the magnetic forces depends in part on the shape of the surfaces of the catheters (e.g., the alignment torque may be greater between a pair of flat magnetic surfaces than for a pair of magnetic cylinders), each of the two catheters comprises a flat junction surface, which may generate a greater alignment torque for a given degree of rotational misalignment than for each of the two catheters comprising curved junction surfaces. For example, for a pair of catheters each having a square cross-section, forming a flat junction surface, and each having a junction region of magnets of approximately equal dimensions (e.g., diameter and length), the alignment torque generated between the flat magnetic surfaces, at 5 degrees of misalignment, is at least approximately 18 times stronger than that of the alignment torque between magnetic cylinders.

[00135] Figures 19C and 19D show catheters 1900, 1910 each comprising a junction region, with a flat junction surface, and an electrode. The junction region may be located proximal and distal to the electrode, and may have a square cross-section to improve the ability of the catheters to stay in correct rotational alignment due to magnetic alignment torque, as described in more detail herein. The first catheter 1900 comprises a first array of magnets 1905. The second catheter 1910 comprises a second array of magnets 1915. As shown, both catheters comprise arrays of magnets proximal and distal to the electrode. The magnet arrays comprise a plurality of square magnets arranged in a linear array. The magnets of the arrays of magnets proximal to the electrode may comprise a central lumen passing therethrough, which may allow travel of a lead wire to the electrode. The linear arrangement of square magnets within catheters having a square cross-section may allow for improved rotational alignment while still allowing for bending of the catheter. In some variations, a catheter may comprise one or more alignment items to aid in rotational alignment of catheters, as described in more detail herein. For example, Figure 19C shows a first rotation indicator 1907 (described in more detail herein) between the first array of magnets 1905 and the first distal end 1908, and a second rotation indicator 1917 between the second array of magnets 1915 and the second distal end 1918.

[00136] The junction regions may be of any suitable length along the catheters. For example, in a non-limiting variation, the junction regions may extend about 15 mm distal and proximal to an electrode or abutment.

[00137] Once the first catheter and second catheter have been positioned, the force of attraction may also act to maintain the relative positions of the catheters. When the first catheter and second catheter are placed in respective blood vessels, however, tissue positioned between the blood vessels and/or limited compliance of the blood vessels may limit the degree to which the magnets of the first catheter and second catheter direct the first catheter and second catheter toward each other. The size and intensity of the magnets may be configured to provide a desired level of tissue compression, as described herein. 3. VISUAL ALIGNMENT AIDS

[00138] In some variations, the catheters described herein may comprise visual alignment aids to indirectly visualize the alignment of the catheters relative to each other. Fluoroscopically visualized rotation indicators, as described in detail herein, provide improved rotational alignment of the catheters relative to each other, and may also aid in axial alignment.

[00139] A visual alignment aid may be visualized using a technique such as fluoroscopy during positioning and/or alignment of a catheter system. Fluoroscopy is a technique for real-time formation of X-ray images, and may be used to guide insertion and movement of a catheter through blood vessels. Typically, in fluoroscopy, an X-ray beam is emitted from a fluoroscope through an area of interest in a body. The objects to be visualized (e.g., catheters) may be imaged using an image intensifier. A user viewing the real-time images displayed by the image intensifier can then determine the orientation and alignment of the catheters relative to each other. However, due to the two-dimensional nature of the generated X-ray images, some fluoroscopic visualization techniques may not be ideal for determining the three-dimensional orientation of one or more catheters. Thus, a rotation indicator may serve as a visual marker to guide the rotational alignment of two catheters as viewed under fluoroscopy. In some variations, when used in conjunction with a second catheter (not shown) having a second rotation indicator, the rotation indicators on each catheter can be used to rotationally and/or axially position the catheters relative to each other so that one or more fistula-forming elements can be properly positioned to form a fistula. In some variations, a first catheter and a second catheter can both include identical rotation indicators having a radiopaque film. The formation of images of rotation indicators having the same width and shape under fluoroscopy indicates rotation alignment of the first and second catheters relative to each other. Rotation indicators appearing the same under fluoroscopic visualization on adjacent catheters are aligned relative to each other and indicate proper alignment for fistula formation. Consequently, the catheters can be aligned fluoroscopically to each other rather than aligned in an x-ray beam.

[00140] Generally, a rotation indicator may be configured such that its rotation orientation is discernible in a two-dimensional fluoroscopic image. An example is shown in Figures 24A and 24B. Figures 24A and 24B are cross-sectional views of a catheter comprising a rotation indicator, which comprises a thin radiopaque film 2402 in a first rotation orientation parallel to an X-ray beam 2406 (Figure 24A), and in a second rotation orientation rotated relative to the X-ray beam 2406 (Figure 24B). The radiopaque film 2402 may block X-rays emitted from a fluoroscope, to produce a shadowed area when viewed on a user monitor (e.g., an image intensifier). The width of the resulting shaded area, represented by the width W between dashed lines in Figure 24B, may correspond to an angular rotation of the catheter relative to the X-ray beam and is given by 2rsen(θ), where r is the film thickness and θ is the angle between the film and the X-ray beam. The radiopaque film 2402 may be configured to be very sensitive to rotational misalignment to allow for high precision rotary positioning. In some cases, a very thin (e.g., 0.025 mm thick) radiopaque film 2402 parallel to the X-ray beam (Figure 24A) may not be visible under fluoroscopy and may indicate to a user that the catheter 2400 is rotationally aligned with the X-ray beam. As the radiopaque film is rotated, the width of the visualized radiopaque band will increase. Figure 24C illustrates the width of the rotation indicator displayed as a function of the rotation error of a rotation indicator relative to an X-ray beam. Slight angular rotation errors of the rotation indicator 2402 can be easily visualized due to the sensitivity of the radiopaque film 2402.

[00141] Other rotation indicators of various configurations may allow a user to visually match an orientation of a first rotation indicator on a first catheter with an orientation of a second rotation indicator on a second catheter. Figures 25A and 25B show a variation of a rotation indicator 2500 having a cube shape with a corresponding cube-shaped cutout 2502. The cube may have varying thicknesses, as shown in Figure 25C, which illustrates fluoroscopic visualization of rotation indicators, as shown in Figures 25A and 25B, as a function of varying levels of rotation from the normal (e.g., perpendicular to the x-ray beam), for different thicknesses of the indicators. For two catheters having the same rotation indicator, if the rotation indicators appear the same on each of the catheters, despite their misalignment from the normal, then the catheters may be determined to be rotationally aligned relative to each other, and in a fistula-forming position. In another variation, Figures 26A through 26C illustrate a rotation indicator 2600 having a cube shape with a cylindrical cross-section 2602. Figure 26D shows fluoroscopic images of rotation indicators 2600 as shown in Figures 26A through 26C as a function of rotation of the normal for different hole sizes.

[00142] Figure 27 shows the distal portions of a variation of the first catheter 2700 and the second catheter 2710, which may be the rotation indicators of Figures 25A and 25B or of Figures 26A through 26C. Each catheter may comprise a plurality of magnets 2706, 2716, which may be cubic magnets. The rotation indicators 2708, 2718 may be located at the distal ends of the array of cubic magnets as shown, but it should be appreciated that the rotation indicators 2708, 2718 are not limited to the distal ends of the catheters and may be provided anywhere along the catheters. The distal ends 2702, 2712 of the catheters may optionally further include an atraumatic quick-change tip configured to pass a guidewire and allow tracking of the catheter over the guidewire.

[00143] Figures 28A through 28C illustrate fluoroscopic images of another variation of a rotation indicator 2800 viewed fluoroscopically under different angles of rotation. For example, the rotation indicator may have a shape corresponding to an alphanumeric character, such as the letter "H" or any suitable written character. Figure 28A shows a rotation indicator 2800 as imaged by a perpendicular X-ray beam. Figures 28B and 28C show fluoroscopic images 2802, 2804 of the same rotation indicator as that in Figure 28A under increasing angles of deviation from perpendicular to the X-ray beam. For example, the letter may appear thicker 2802 when slightly rotated or appear as a block 2804 after rotation through 90 degrees.

[00144] Figures 29A and 29B illustrate another variation of a rotation indicator 2900, having a cross-sectional shape of the letter "R", as shown in Figure 29A. As the rotation indicator 2900 is rotated (Figure 29B), the contour of the rotation indicator may increase in thickness. Figure 30 is a cross-sectional view of a rotation indicator 3000 having an arrow-shaped cross-section. Figure 31 is a perspective view of a rotation indicator 3100 having a "U"-shaped cross-section.

[00145] The rotation indicator may comprise a radiopaque metal, such as tungsten, a platinum-iridium alloy, stainless steel, titanium, as well as a tungsten-filled polymer, a zirconia ceramic, or any suitable radiopaque material. A visual alignment aid, such as a rotation indicator, may be located at any suitable position on or within the catheter (e.g., on one or more surfaces of the catheter, within the catheter, or the like). In some variations, one or more portions of the catheter may be made of a radiopaque material, or a visual alignment aid may be secured to the catheter by any suitable method, for example, by mechanical attachment (e.g., embedded in a portion of the catheter, circumferential circumscription, or the like), adhesive bonding, welding, white soldering, combinations thereof, or the like. In some variations in which a second catheter has a support, the support itself may include a rotation indicator. For example, a portion of the support may include a radiopaque material, such as a radiopaque zirconia ceramic. The shape of the radiopaque part of the support may comprise any of the configurations described in this descriptive report.

[00146] It should be appreciated that although the figures illustrate rotation indicators having various illustrated cross-sections, in other variations, the rotation indicators may have other shapes, which allow a two-dimensional rotation visualization. For example, in other variations, the rotation indicators may be cylindrical, semi-cylindrical or have a cross-section, which is C-shaped (i.e., a D-shape or semi-cylindrical comprising a channel in the flat surface), rectangular, square, triangular, trapezoidal, ovoid, elliptical or a polygon of the order, or the like.

[00147] A procedure for aligning catheters using the rotation indicators described is provided below. In some variations for positioning a first catheter and a second catheter within two vessels, a first catheter may be advanced into a first blood vessel and a second catheter may be advanced into a second blood vessel. The first catheter may include a first radiopaque portion and the second catheter may include a second radiopaque portion. An X-ray beam emitted from a fluoroscope may form fluoroscopic images of an orientation of the first radiopaque portion and the second radiopaque portion, which may be displayed on a monitor to a user.

[00148] For example, Figures 32A and 32B are illustrative fluoroscopic images of the first catheter 3200 and the second catheter 3210. The first catheter 3200 comprises a first junction region 3204 comprising magnets, a first rotation indicator 3202, and a first electrode 3206. Likewise, a second catheter 3210 comprises a second junction region 3214 comprising magnets, a second rotation indicator 3212, and a second electrode 3216. The junction regions 3204, 3214 and the rotation indicators 3202, 3212 are of equal size and shape to allow comparison between the two under fluoroscopy. The axial alignment of catheters 3200, 3210 is shown in Figure 32A and can be confirmed by junction regions 3204, 3214 that are parallel to their aligned edges.

[00149] The rotational alignment of the first catheter 3200, relative to the second catheter 3210, can be confirmed by the rotation indicators 3202, 3212 appearing substantially identical in shape and thickness. To further illustrate this, Figure 32B shows a detailed image of the first rotation indicator 3220, axially misaligned relative to the second indicator 3224. In particular, the rotation indicators 3220, 3224 have different thicknesses, indicating that they are rotationally offset from each other. The user may rotationally adjust one or both of the catheters 3200, 3210 until the appearance of the rotation indicators 3220, 32224 are substantially indistinguishable, thereby indicating rotational alignment of the first catheter 3200 with the second catheter 3210. It should be appreciated that the rotation indicators and catheter alignment methods described herein need not be based on an (often incorrect) assumption that the X-ray beam is perpendicular to a catheter plane. In other words, the catheters and the rotation indicators described herein may be aligned with each other regardless of the angle of incidence of the X-ray beam with the catheter plane. 4. CABLE

[00150] Generally, a proximal end of a catheter may comprise one or more handles, which a user may utilize to manipulate the catheter as it is advanced through the vasculature. In some variations, a handle may have one or more items to assist with axial and/or rotational alignment with another catheter. For example, a handle may comprise a magnet and/or a flat junction surface. The handle may be coupled to a catheter shaft, which may rotate as the handle is rotated. For example, in variations comprising a first catheter and a second catheter, both having a handle, the handles may include magnets to bring and retain the magnets in close proximity and in alignment with each other. Alignment of the handles may, in turn, bring the distal ends of the catheters into alignment.

[00151] In variations of catheter pairs comprising cables, each having a magnet to attract and align the catheters in a preferred rotational and/or axial position, the cables may all have a flat junction surface to orient the catheters in the correct rotational position relative to each other. In some variations, the catheter cables of the first catheter and the second catheter may be configured to be secured to each other. They may be secured by any suitable method, such as, but not limited to, magnets, latches, spring hooks, adhesive, snap-fits, dovetail fittings, etc. In this way, correct internal orientation (i.e., orientation of the catheters within blood vessels) may be achieved by use of externally visible items (i.e., cables located external to the body).

[00152] In some variations, the ability to orient an internal position of a catheter based on the orientation of an external portion of the catheter (e.g., a handle) may be improved by utilizing a catheter shaft that is kink-resistant (i.e., a braided shaft) such that rotational alignment on the external portion translates directly to rotational alignment of a more distal portion of the catheter. It may be desirable in some cases for different portions of a catheter to have different kink resistances. For example, a proximal portion of the shaft may have a first kink resistance and a distal portion of the shaft may have a second kink resistance greater than the first kink resistance, to facilitate rotation of the shaft along its length. In one variation, a proximal portion of the shaft having a lower kink resistance may be located distant from the handle and proximal to a magnet at a junction region of the catheter.

[00153] As an example, Figure 33 shows a first catheter shaft 3302 and a second catheter shaft 3306 coupled to respective first cable 3300 and second cable 3304. The first cable 3300 and second cable 3304 may include magnets to join the cables together. The catheter shafts 3302, 3306 may be kink resistant such that rotation of the cable provides a corresponding rotation in the catheter shaft by a distal end of the catheter. In this manner, rotational and/or axial alignment of the cables 3300, 3304 may align the catheters as a whole.

[00154] Figures 34A-34C illustrate a variation of a catheter 3404 comprising a handle 3400. Figure 34A shows the handle 3400 comprising a flat surface for junction with a corresponding flat surface of another handle (not shown). Figures 34B and 34C show magnets 3402 within the handle 3400 for attracting and aligning the handle in a preferred rotational and/or axial position. The handle 3400 may thus provide an alignment torque for improved alignment of catheters. It should be appreciated that the handle need not further comprise one or more actuator components for deploying and retracting a fistula-forming element upon user manipulation.

H. SENSORS

[00155] In a variation, systems comprising an electrode in both the first and second catheters may further comprise an impedance measurement circuit, such as a bipolar sensing circuit comprising the tissue removal electrodes. To measure tissue impedance between the electrodes, a low-power direct current or alternating current voltage may be applied to the electrodes. The resulting current and/or phase(s) may be measured to determine impedance. As a user manipulates one or more catheters relative to each other and/or as the fistula is being formed, the measured impedance values may vary to allow determination of a minimum impedance. Impedance values may be dependent on a number of factors, e.g. electrode size, but as a non-limiting example, in one variation, an initial (pre-fistula formation) impedance may be between about 400 Q and about 500 Q, and a post-fistula formation impedance may be between about 0 Q and about 80 Q.

[00156] Additionally or alternatively, four-pole measurements with signal filtering/conditioning may be used to determine impedance. In some variations, one or more impedance measurements may be transmitted to a user as one or more visual and audio feedback. For example, the system may transmit an impedance value on a monitor meter attached to the catheters. The impedance values may be transmitted as audio tones. For example, a predetermined tone may indicate a minimum impedance value corresponding to catheter alignment.

I. EXEMPLARY CATHETER SYSTEMS

[00157] Figures 36A-36G illustrate an exemplary variation of a single-sided electrode catheter system 3600 comprising some of the components described herein. Figure 36A illustrates a distal portion of a first catheter 3602 and a second catheter 3604. Catheters 3602, 3604 may be configured to be advanced through the vasculature in a minimally invasive manner. Catheters may be of any diameter suitable for intravascular use, such as, for example, about 4 on the French scale, about 5.7 on the French scale, about 6.1 on the French scale, about 7 on the French scale, about 8.3 on the French scale, between about 4 and about 9 on the French scale, between about 4 and about 7 on the French scale, between about 4 and about 6 on the French scale, or the like.

[00158] The first catheter 3602 may comprise a fistula-forming element 3606 and the second catheter 3604 may comprise a support 3608. The support 3608 may form and control the removal of tissue performed by the fistula-forming element 3606. The fistula-forming element 3606 may be an electrode and may have the features of the electrode 106 described herein. The electrode may be secured in a housing of the first catheter 3602 and used to remove tissue to form a fistula.

[00159] The electrode 3606 may be configured to have a low profile configuration (not shown) and an extended configuration (shown). The electrodes 3606 may be propelled in the direction of the extended configuration. That is, the electrode may self-expand from the low profile configuration to the extended configuration. In some variations, the electrode may be held in the low profile configuration by the inner surface of a vessel wall during delivery. The electrode may then self-expand in the direction of the extended configuration as the delivery of energy by the electrode results in tissue removal.

[00160] In the extended configuration, the electrode 3606 may be curved such that it forms a convex curve extending from the outer surface of the first catheter 3602. As the electrode 3606 moves from a low-profile configuration to an extended configuration, the radius of curvature of the electrode 3606 may decrease, causing the electrode 3606 to protrude from the first catheter 3606. Conversely, as the electrode 3606 moves from an extended configuration to a low-profile configuration, the radius of curvature of the electrode may increase, causing the electrode to retract into an opening in the catheter body. The electrode 3606 may be configured to slide within the first catheter 3602 as the electrode 3606 moves between the low-profile configuration and the extended configuration. More specifically, as shown in greater detail with respect to Figures 1A-4 , the electrode 3606 may comprise a first end and a second end, wherein both the first end and the second end are located within the first catheter 3602. The first end of the electrode 3606 may be fixed, while a second end of the electrode 3606 may be slidable within a lumen within the first catheter 3602. When the slidable second end of the electrode 3606 moves toward the fixed first end of the electrode, the electrode 3606 may move toward an extended configuration. When the slidable second end of the electrode 3606 moves from the fixed first end of the electrode 3606, the electrode 3606 may move toward a low-profile configuration. Because electrode 3606 is curved, as the second slidable end moves toward the first fixed end of electrode 3606, the radius of curvature of electrode 3606 may decrease, causing electrode 3606 to protrude from the first catheter 3602.

[00161] The electrode 3606 may be coupled to a power source (e.g., an RF generator) by an electrical lead 3622 extending longitudinally through a catheter shaft 3614, which is in turn coupled to an electrical plug 3620, as shown in the cross-sectional view of the first catheter in Figure 36C.

[00162] As best shown in Figure 36D, the support 3608 of the second catheter 3604 may have a protruding support. The support 3608 may comprise a concave shape that is configured to be complementary to that of the electrode 3606 of the first catheter 3602. The concave portion of the support 3608 may correspond to the shape of an intermediate portion of the electrode 3606 when it is in its extended configuration. The support 3608 may be configured to compress tissue in a region located by the electrode 3606 of a first catheter 3602.

[00163] The first catheter 3602 and the second catheter 3624 may further comprise respective junction regions 3610, 3624, which aid in aligning one catheter relative to the other in adjacent blood vessels and/or placing the catheters (and blood vessels) in close proximity relative to each other. Aligning the first catheter 3602 and the second catheter 3604 relative to each other may position the electrode 3606 of the first catheter 3602 in an axial and/or rotational orientation relative to a corresponding mount 3608 of the second catheter 3604. The junction regions 3610, 3624 may comprise one or more magnets, planar junction surfaces, and/or visual alignment aids (e.g., rotation indicators).

[00164] In particular, as best shown in the perspective view of Figure 36E, the catheters may all have square cross-sections at least within the junction regions 3610, 3624. As such, one side of each junction region 3610, 3624 may form a flat junction surface, and the two junction surfaces may face each other when the catheters are properly aligned, as shown in Figure 36E. As described herein, this may aid in rotational alignment of the catheters; the opposing junction surfaces may bring the catheters into rotational alignment with each other and more closely together with the blood vessels.

[00165] Furthermore, catheters 3602, 3604 may comprise magnet arrays comprising a plurality of square magnets 3626 within junction regions 3610, 3624. In the first catheter 3602, square magnets 3626 may be located proximal to and distal from electrode 3606, as shown in Figure 36B, while in the second catheter 3604, square magnets may be located proximal to and distal from support 3608. Magnets 3626 proximal to electrode 3606 in the first catheter 3602 may comprise a lumen for electrical conductor 3622. Electrical conductor 3622 may be covered by insulation to protect magnets 3626 from current conducted by wire 3622. Magnets 3626 of the first catheter 3602 may be attracted to magnets of the second catheter 3604 to place the catheters in close proximity and in rotational alignment with each other. This may join tissue between the flat junction surfaces. In some variations, the catheters 3602, 3604 may comprise one or more rotation indicators within the junction regions 3610, 3626, which may be visualized under fluoroscopy, so that a user may visualize the catheter system 3600 in the blood vessels and manipulate the catheters 3602, 3604 to a desired position and relative orientation with respect to each other.

[00166] The catheter system 3600 may comprise a wrap 3616 configured to cover a portion of a catheter shaft 3614, as shown in Figure 36F. The wrap 3616 may be slidable along the length of the first catheter 3602. In some variations, to be slidable across portions of the catheter having a circular cross-section and portions of the catheter having a square cross-section, the wrap 3616 may comprise a flexible material that allows it to deform between round and square tubular shapes. In some variations, the wrap 3616 may have a length similar to the length of the junction region 3610. In a distal position, the wrap 3616 may cover the junction region 3610, including the electrode 3606 and the areas of the catheter 3602 comprising the magnets 3626, and may additionally cover portions of the remainder of the catheter shaft 3614. When the wrap 3616 covers the electrode 3606, the electrode 3606 may be maintained in a low-profile configuration. In a proximal position, the wrap 3616 may leave the junction region 3610, including the electrode 3606, exposed, thereby allowing the electrode 3606 to return to an extended configuration (assuming no further force squeezes the electrode). Prior to delivery of catheter 3602 into a patient vasculature, wrap 3616 may be distally covering electrode 3606. As discussed herein, when wrap 3616, disposed over catheter 3602, is advanced into a vessel through an access site comprising a homeostasis valve, the distal end of wrap 3616 may, for example, be advanced into the valve while catheter 3606 may be advanced past the valve and wrap 3616 and into the vasculature. As catheter 3602 is advanced, wrap 3616 may slide proximally along the shaft of catheter 3614. In this manner, electrode 3606, in a low-profile configuration, may be covered by wrap 3616 to protect electrode 3606 and catheter 3602 from retention and/or damage as they are advanced through the access site.

[00167] One or both of the catheters 3602, 3604 may comprise a cable 3618, as shown in Figure 36G. The cable 3618 may be located proximate to the junction region 3610 and remote from the electrical plug 3620, and may be coupled to the catheter shaft 3614. The cable 3618 may be used to align at least one proximal end of one catheter relative to at least one proximal end of another catheter. One or more portions of the catheter shaft 3614 may be resistant to twisting, such that rotation of the cable 3618 translates into rotation of more distal portions of the catheter. At their distal ends, the respective distal ends 3612, 3628 of the catheters 3602, 3604 may both include an atraumatic quick-change tip comprising a lumen 3630 configured to pass a guidewire and allow tracking of the catheter over the guidewire.

J. . METHODS

[00168] Also described herein are methods for forming a fistula between two blood vessels using the catheters described herein. The two blood vessels may be closely associated (e.g., adjacent) blood vessels, such as a vein and an artery, two veins, or two arteries. Generally, the methods described herein comprise: accessing a first blood vessel with a first catheter; and advancing the first catheter to a target site within a first blood vessel. A second blood vessel may be accessed with a second catheter, and the second catheter may be advanced to a target site within the second vessel. Once the first catheter and the second catheter have been advanced into their respective blood vessels, the catheters may be adjusted to position the catheters within the blood vessels and/or position the blood vessels relative to each other.

[00169] In some variations, the catheters may comprise alignment items, as described herein, that may be used to face the two vessels and/or align the catheters axially and/or rotationally relative to each other. Methods of aligning a catheter, as described herein, may improve fistula formation with fewer complications. After the vessels are placed facing each other and the catheters are aligned, a fistula formation site may optionally be analyzed by measurements made by one or more catheter electrodes. In some variations, the blood vessels may be modified by the electrodes prior to fistula formation to improve fistula and flow characteristics, as described in more detail herein. In some cases, tissue denaturation may improve fistula patency and blood flow.

[00170] In some variations, one or more fistula-forming elements may be activated to puncture, perforate, or otherwise create a passage between the two blood vessels by removing tissue so that blood may flow between the two adjacent blood vessels. When such a fistula is formed, hemostasis may be created without the need for a separate device or structure (e.g., a suture, stent, shunt, or the like) connecting or joining the blood vessels. During the fistula formation process, the condition of the blood vessels may, in some variations, be monitored, for example, to refine the energy applied to ensure the desired fistula formation. Catheters may be removed from the blood vessels and the body after fistula formation. In some variations, fistula formation may be confirmed by measurements of the catheters.

[00171] Figure 35 is a flowchart generally describing a fistula formation process 3500. It should be appreciated that any of the catheters described herein may be used to form a fistula using the methods described herein as suitable. Generally, the process may begin when one or more catheters are advanced to a target location in one or more blood vessels through an access site 3502. The first catheter and the second catheter may be advanced in the same manner, or they may be advanced in different manners. Once the first catheter and/or the second catheter have been advanced into their respective blood vessel(s), the catheters may be aligned axially and rotationally relative to each other 3504. For example, each catheter may comprise one or more alignment items, such as magnets, flat junction surfaces, visual alignment aids, cables, or the like, that assist in positioning the catheters in a desired alignment. Additionally or alternatively, indirect visualization, such as by fluoroscopy, may be used to align the catheters at a target location. In some variations, impedance measurements may confirm alignment of the catheters with each other.

[00172] In some variations, an analysis of blood vessels may optionally be performed to determine whether the site is suitable for fistula formation 3506. For example, a nerve that is in close proximity to a fistula site may be negatively impacted by the high temperatures experienced during the fistula formation process. In some variations, the proximity of a nerve to a fistula site may be determined using electrode measurements. Based on the fistula site measurements, a determination can be made as to whether the fistula site is suitable for fistula formation 3508. If the fistula site is unsuitable, the user can reposition the catheters 3502 to a new fistula site and repeat the alignment 3504, analysis 3506, and determination 3508 steps. In some variations, the tissue at or adjacent to a fistula site can be optionally modified prior to removal 3510. For example, electrodes can be used to denature tissue without removal, to increase mechanical strength and/or reduce vessel size. For example, a current can be applied to a site in the vessel to denature the adventitia of the blood vessel.

[00173] Tissue may be removed to form a fistula based on patient characteristics and fistula requirements 3512. Plasma may be generated according to energy removal parameters to remove tissue. Removal may be performed by one or more electrodes on one or more catheters. In some variations, a first catheter and a second catheter may form a fistula from opposite sides to provide dual removal of a fistula. In some such variations, synergistic removal may be provided when an electrode of a first catheter removes tissue to activate a conductive portion of a second catheter. In still other variations, a first catheter may form a fistula using an electrode against a fulcrum of a second catheter. Fistula formation may optionally be confirmed, for example, by impedance measurement 3514. Catheters may be removed from blood vessels and body 3516. The steps described above are set forth in more detail below.

A. ADVANCE CATHETER

[00174] The advancement of one or more catheters through a vessel to a target site is not particularly limited. In some variations, a first catheter may be advanced into an artery, and a second catheter may be advanced into a vein. In other variations, a first catheter may be advanced into a vein, and a second catheter may be advanced into an artery. In other variations, a first catheter may be advanced into a first vein, and a second catheter may be advanced into a second vein. In still other variations, a first catheter may be advanced into a first artery, and a second catheter may be advanced into a second artery. The first catheter and/or the second catheter may or may not be advanced by a guidewire, or in any suitable manner, and may or may not occur under indirect visualization (e.g., via fluoroscopy, X-ray, or ultrasound). A catheter's direction of advancement, relative to blood flow, is also not particularly limited - that is, a catheter can be advanced in an antegrade manner (with the blood flow), or in a retrograde manner (against the blood flow).

[00175] In variations in which a catheter is advanced endovascularly into the ulnar artery, access to the ulnar artery may be obtained in any suitable manner. In some variations, the methods described herein comprise: endovascularly advancing a distal portion of a first catheter into an ulnar artery; endovascularly advancing a distal portion of a second catheter into a first deep ulnar vein; and forming a fistula between the ulnar artery and the first deep ulnar vein. The methods may comprise: endovascularly advancing a distal portion of a first catheter into a first proximal ulnar artery; endovascularly advancing a distal portion of a second catheter into a first deep ulnar vein; and forming a fistula between the proximal ulnar artery and the first deep ulnar vein.

[00176] In some variations, the catheter may be advanced along the brachial artery and into the ulnar artery. In some of these methods, the catheter may be introduced into the vasculature via a brachial access site. In some of these methods, the brachial artery may be cannulated with a cannula directed distally into the brachial artery. In other variations, the catheter may be advanced along the brachial artery from an access site upstream of the brachial artery. For example, the catheter may be introduced into the vasculature via a femoral artery access site, and may be advanced to the brachial artery from there. In some variations, the ulnar artery may be accessed directly. In some of these variations, an ulnar artery site may be formed in the ulnar artery (e.g., at a distal site on the wrist or forearm in which the ulnar artery is positioned superficially), and a catheter may be advanced in a retrograde fashion through the ulnar access site. In still other variations, a catheter, or other tool, can be advanced endovascularly into the ulnar artery through an access site in the radial artery.

[00177] In variations in which a catheter, or other tool, is advanced endovascularly into a deep ulnar vein, access to the deep ulnar vein may be obtained in any suitable manner. In some variations, the catheter is introduced into the vascular site via an access site. The venous access site may be in any suitable blood vessel, such as the basilic vein, the cephalic vein, or a brachial vein. In some variations, the catheter may be advanced endovascularly into a deep ulnar vein along the median cubital vein. For example, in some variations, the catheter may be advanced along the basilic vein, into the median cubital vein, and into one of the deep ulnar veins via a perforating branch extending between the median cubital vein and the deep ulnar veins. In cases where the perforating branch extends between the deep ulnar veins and the median antebrachial vein, the catheter can be advanced from the median cubital vein to the median vein, then to one of the deep ulnar veins.

[00178] In other variations, the catheter may be advanced endovascularly to a deep ulnar vein along the median cephalic vein. For example, in some variations, the catheter may be advanced into the vasculature through an access site in the cephalic vein, and may be advanced endovascularly from the cephalic vein to the median cephalic vein, and to one of the deep ulnar veins via a perforating branch (to access the perforating branch, it may be necessary to advance the catheter into the median cubital vein or the median antebrachial vein).

[00179] In still other variations, the catheter may be advanced endovascularly to a deep ulnar vein along a brachial vein. For example, in some variations, the catheter may be advanced to the vasculature through an access site in a brachial vein, and may be advanced endovascularly from the brachial vein to one of the deep ulnar veins in a retrograde fashion.

[00180] Methods for endovascularly advancing a catheter are described in more detail in U.S. patent application Serial No. 14/052,477, filed October 11, 2013, entitled "DEVICES AND METHODS FOR FISTULA FORMATION", which is fully incorporated by reference herein.

[00181] In variations in which one of the catheters is configured for advancement over a guide wire, the catheter may be advanced along a guide wire. In variations in which one of the catheters has a guide wire securely attached to its tip, the guide wire may be advanced through the vasculature to a target site. In other variations, one or more external magnets may assist in advancing or positioning a catheter at a target site. For example, an external magnet may be used to assist in advancing a catheter within a blood vessel and interact with any suitable portion of the catheter to create an attractive force between the catheter and the external magnet. This attractive force may be used to pull, push, or otherwise manipulate the catheter during advancement.

[00182] In variations in which a catheter comprises an electrode having a low profile configuration and an extended configuration, as described in detail herein, the electrode may remain in a low profile configuration as the catheter is advanced. Trauma due to advancement and navigation of the catheter through a blood vessel may be reduced by retraction of the electrode into the housing. For example, in some variations, a distal end may be configured to slide freely within a catheter housing in response to an external force, such as a force from a vessel wall, as described in more detail herein. As the diameter of a vessel through which the catheter is advanced increases, the electrode may extend from the housing by virtue of its elastic force. Consequently, a user does not need to mechanically retract or extend the electrode during advancement of the catheters through a vessel or during the fistula formation process.

B. ALIGN CATHETERS

[00183] In some variations, each of the first catheter and the second catheter may comprise one or more alignment items, such as, but not limited to, magnets, flat junction surfaces, visual alignment aids, and/or cables, as described in detail herein. In some variations, the alignment of the first catheter and the second catheter with each other may comprise axial and/or rotational alignment. For example, the catheters may be oriented such that a fistula-forming element of at least one of the first catheter or the second catheter is positioned to form a fistula at a particular location. In variations in which both the first catheter and the second catheter comprise fistula-forming elements, the catheters may be oriented to align these fistula-forming elements opposite each other. In variations in which a first catheter comprises an electrode and a second catheter comprises a fulcrum, the catheters may be oriented to align the electrode and fulcrum opposite each other.

[00184] In some of the variations of the catheters described herein, magnets within the first catheter and the second catheter may generate an attractive force between the catheters, which may pull the catheters toward each other. The attractive force may also compress tissue between the first catheter and the second catheter. An alignment torque may also pull the catheters together along their respective flat joining surfaces to achieve rotational alignment. In some variations, the catheters may be resistant to twisting, such that a user's rotational alignment of the cables at a proximal end of the first catheter and the second catheter translates into rotational alignment by a distal end of the catheters.

[00185] The catheters may be visualized fluoroscopically if necessary by means of a fistula formation process. In variations of the catheters comprising rotation indicators, the alignment of the axial and rotary catheters may be visualized, such as by fluoroscopy, to assist a user in axially and/or radially aligning the catheters relative to each other. For example, under fluoroscopic imaging, the rotation indicators may be rotated until they appear under fluoroscopic visualization to rotationally align the catheters relative to each other. This ensures that the rotation indicators are aligned relative to each other and not relative to the imaging X-ray beam. Once aligned, a user may place the catheters in close proximity to each other so that the magnets on the catheters hold the aligned catheters together.

[00186] In some variations, confirmation of alignment (e.g., axial and/or rotational) of the first catheter and the second catheter may be based on impedance measurements of the tissue interposed between the two electrodes. For example, the catheters may be aligned both rotationally and axially when the electrodes are at a minimum distance from each other that corresponds to a minimum tissue impedance between the catheters. Confirmation of alignment using impedance measurement may avoid radiation exposure resulting from other methods, such as fluoroscopy.

[00187] In one variation, the system may comprise an impedance measurement circuit, such as a bipolar detector circuit comprising the tissue removal electrodes. To measure impedance, a low-power direct current or alternating current voltage may be applied to the electrodes. The resulting current and/or phase(s) may be measured to determine impedance. As a user manipulates one or more catheters relative to each other, the measured impedance values may vary to allow determination of a minimum impedance. Additionally or alternatively, tetrapolar measurements with signal filtering/conditioning may be used to determine impedance.

[00188] In some variations, one or more impedance measurements may be transmitted to a user as one or more visual and audio feedback. For example, the system may transmit an impedance value on a monitor meter attached to catheters. Additionally or alternatively, the impedance values may be transmitted as audio tones. For example, a predetermined tone may indicate a minimum impedance value corresponding to catheter alignment.

C. ANALYZE VESSEL

[00189] Once a potential fistula site has been identified and one or more catheters have been advanced to the fistula site, the measurement of vessel characteristics may optionally be used to determine the suitability of the site for fistula formation. In some variations, the suitability of a site for fistula formation may be based on the site's proximity to a nerve. For example, it may be undesirable to form a fistula near a nerve because the thermal energies of fistula formation may impinge on one or more nearby nerves.

[00190] In some variations, a system may apply a low-power direct (DC) or alternating (AC) current in a monopolar configuration, via an active electrode of a first catheter, to a ground pad to induce neuromuscular stimulation. The location of the nerve relative to the fistula site may be determined based on visualization of the induced neuromuscular stimulation. The stimulation may be visualized externally (e.g., observation of an arm twitch) and/or internally via techniques such as ultrasound and/or fluoroscopy. A high level of induced neuromuscular stimulation may indicate that the fistula site is in close proximity to a nerve, and may suggest that the fistula site should be relocated by repositioning the catheters. In some variations, a set of stimulation currents may be applied to determine the distance of the nerve from the electrode. For example, a 1 microampere current may be applied and then followed by a 3 microampere current. The difference in neuromuscular stimulation observed may approximate the vicinity of the nerve.

[00191] In some variations, a baseline impedance measurement may be taken prior to fistula formation and used to determine removal parameters. The baseline impedance may also be referenced when performing a second impedance measurement to confirm fistula formation after tissue removal.

D. MODIFY VASE

[00192] In some variations of the methods described herein, portions of the vessels adjacent to a fistula site may be modified to change the characteristics of one or more vessels and the resulting fistula. In some variations, one or more catheters may be configured to denature and/or thermally weld tissue structures at or adjacent to the fistula site. For example, heating tissue to 70°C may result in denaturation. Denaturation or thermal welding may modify the vessel without removing material as occurs when tissue is removed. Furthermore, adhesion of tissue layers may be beneficial in order to increase mechanical strength between vessels.

[00193] In some variations, one or more catheters may be configured to deliver electrical, ultrasonic, or laser energy to at least one of the first and second blood vessels to denature proteins in the vessel walls. For example, a catheter may comprise a fiber optic filament coupled to a laser, such that the catheter may be configured to direct laser energy to heat tissue, denature proteins, and/or weld tissue. As another example, a catheter may comprise a piezoelectric element configured to use ultrasonic vibration to induce heating, denature proteins, and/or weld tissue. In some cases, the tissue may be thermally welded by applying a coagulation current through an electrode to denature connective tissue proteins and thereby increase adhesion between tissue planes. A coagulation current may thermally shrink the vessel and increase the vascular resistance of the vessel. In some cases, the denatured proteins of each blood vessel may intertwine to fuse together. In one variation, denaturation of collagen around a fistula site, without tissue removal, is done before the removal cycle to reinforce the formed fistula. However, a sequence of denaturation may be done before, during, or after fistula creation.

[00194] Additionally or alternatively, impedance may be measured during a period of thermal denaturation by measuring impedance in a bipolar or monopolar circuit. As described herein, the first catheter and the second catheter may comprise an impedance measurement circuit, such as a bipolar detector circuit. In this manner, a single heating cycle may be performed without interrupting the power supply cycle for impedance measurement. Impedances measured, for example, using a first electrode and a second electrode, before and after a denaturation sequence, may determine the level of vessel modification provided.

[00195] In some variations, the systems discussed herein further comprise an electrosurgical controller coupled to one or more electrodes for controlling tissue modification. In some variations, a first impedance may be measured between a first electrode and a second electrode. Tissue modification parameters may be selected based on the first impedance. For example, the measured impedance may correspond to known tissue characteristics. These tissue characteristics may correspond to predetermined tissue modification parameters. The modification parameters may include an energy waveform, an amplitude, a duration, etc. The controller may control one or more electrodes to modify tissue based on the selected modification parameters. After application of one or more pulses of tissue modification energy, a second impedance may be measured, and the process may be repeated until an impedance threshold is reached.

[00196] The controller may control the electrodes to measure a second impedance and determine whether the modification is complete based on the impedance. In some variations, parameters may be selected to complete the modification in a single cycle. In other variations, each cycle may be limited in power and/or duration so as to perform multiple cycles to complete the modification. For example, the thermal effects of denaturation may be dispersed over a longer period of time so as to limit collateral thermal damage to a vessel.

[00197] Devices, systems, and methods for modifying vessels are described in more detail in international patent application Serial No. PCT/US17/13611, filed concurrently with the present application, entitled "SYSTEMS AND METHODS FOR ADHERING VESSELS" and claiming the benefit of U.S. provisional patent application No. 62/279,642, filed January 15, 2016, which is fully incorporated by reference herein.

E. REMOVE FABRIC

[00198] Once one or more catheters are in position, one or more fistula-forming elements may be used to create a fistula between the two blood vessels. For example, in some variations, one of the first catheter and the second catheter may comprise a fistula-forming element (e.g., an electrode), while the other catheter does not comprise a fistula-forming element. In other variations, both catheters may comprise a fistula-forming element. In some such variations, the fistula-forming elements of the first catheter and the second catheter act to form different fistulas. In other variations, the fistula-forming elements of the first catheter and the second catheter interact with each other to form the same fistula. Any suitable combination of electrodes, as described herein, may be used to form the fistula, and, accordingly, current may be delivered in monopolar or bipolar configurations. For example, a fistula-forming element of the first catheter may be activated or otherwise used to form a fistula between a first blood vessel and an adjacent second blood vessel. In still other variations, the catheters may be configured to form the fistula via the first and second blood vessels substantially simultaneously.

[00199] As discussed in detail herein, in variations in which the fistula-forming element of the first catheter is configured to extend or otherwise move through blood vessel tissue during fistula formation in the tissue, a second catheter may contact or otherwise receive the fistula-forming element of the first catheter as it removes tissue therethrough. For example, in some variations, the second catheter may comprise one or more struts, such as those discussed with respect to Figures 13A and 13B. In some such variations, the strut may be configured to receive or otherwise contact an electrode of the first catheter as it passes through vessel tissue. In other variations, current may be passed between an electrode of the first catheter and an electrode of the second catheter during removal of tissue. In some such variations, the electrode of the first catheter may be positioned so that it contacts one or more electrodes or conductive portions of the second catheter.

[00200] For example, in the variations illustrated in Figures 20A-20D, advancement of an active electrode 2008 through the tissue of a blood vessel may contact and energize one or more conductive portions 2002 in a second catheter in a second blood vessel. In these variations, for example, a first blood vessel may be a venous blood vessel, and a second blood vessel may be an arterial blood vessel. Prior to contact of electrode 2008 with conductive portion 2002, only electrode 2008 is energized to remove tissue. Upon contact of the electrode 2008 with the conductive portion 2002, the voltage of the conductive portion 2002 increases to become an extension of the electrode 2008, so that the conductive portion 2002 can remove tissue in contact with the electrode 2008. When contact is broken between the electrode 2008 and the conductive portion 2002, the voltage of the conductive portion 2002 drops and removal by the conductive portion 2002 ends.

[00201] In variations in which a fistula is formed between an artery and a vein, it may be desirable in some cases to begin fistula formation in the vein before an opening forms in the arterial wall. If during fistula formation, the first catheter malfunctions or the procedure is otherwise interrupted so that a complete fistula is not formed, the formation of an opening formed in the artery without a corresponding opening being formed in the vein is prevented. Formation of an opening in a vein without full fistula formation may result in some extravascular bleeding, but the venous pressure may be sufficiently low that significant bleeding does not occur, which may allow the blood vessel itself to warm up. By comparison, when an opening forms in the artery without full fistula formation, arterial pressure may push blood into the extravascular space around the blood vessels, which in some cases may require a surgical procedure to repair. Furthermore, removal of a larger fistula size on the venous side of the fistula versus a smaller size on the arterial side can reduce intrafistula pressure, thereby reducing the likelihood of extravasation. This is done by inducing a greater pressure drop across the arterial fistula opening. Additionally, in some variations, fistula formation using the methods described in this specification can form a larger opening in the first blood vessel than the opening in the second blood vessel. This may be useful in cases where the first blood vessel is a vein and the second blood vessel is an artery. Because a larger opening may have less resistance to blood flow than a smaller opening, forming a larger opening in the vein may promote arterial-to-vein flow, which may reduce the likelihood of extravasation of blood from a fistula into the extravascular space.

[00202] In some variations, radiofrequency energy, such as a radiofrequency alternating current, may be applied to one or more electrodes to generate plasma and remove tissue, as discussed in detail herein. In some variations, parameters may be selected based on tissue properties. For example, an impedance may be measured between a first electrode and a second electrode. Tissue removal parameters may be selected based on the impedance. For example, the measured impedance may correspond to known tissue characteristics. In conjunction with the desired characteristics of the fistula, predetermined tissue removal parameters may be selected. The tissue removal parameters may include an energy waveform, an amplitude, a duration, etc. The controller may control one or more electrodes to remove tissue based on the selected removal parameters.

[00203] In some variations, tissue removal parameters may be selected to form a fistula in a single removal cycle. In other variations, each cycle may be limited in power and/or duration so as to perform multiple cycles to complete removal. In these variations, the thermal effects of removal may be dispersed over a longer period of time so as to limit collateral thermal damage to a vessel.

[00204] By applying a voltage to an electrode, the fluid surrounding the active electrode can be heated to generate a vapor layer. The vapor layer generated expands rapidly and encapsulates the electrode, so that the vapor increases the impedance experienced at the electrode. Plasma is generated from the vapor layer when the voltage applied to the electrode exceeds an ionization threshold. The plasma generates electric arcs with high current density, which heats the tissue and causes rapid dissociation of molecular bonds in organic compounds.

[00205] In variations of a catheter comprising a reservoir formed behind an electrode, a fixed volume of blood may be retained in the reservoir. This isolation creates a bolus of fluid, separate from the free blood stream, and allows for a significant reduction in the time and energy required to generate plasma for removal. For example, when the electrode is energized, the volume of fluid in the reservoir may rapidly vaporize and ionize, thereby rapidly initiating the removal sequence. In configurations comprising a fluid reservoir, the time to achieve a removal plasma may be significantly reduced from about 500 ms to about 15 ms. Reducing the time to plasma ignition may also reduce the total energy applied to the tissue, thereby reducing potential collateral thermal damage to the tissue. Without the reservoir, the electrode may have greater difficulty generating plasma due to convection of the free-flowing blood flow.

[00206] The maximum voltage applied may exceed an ionization threshold of a vapor layer. In some variations, an ionization threshold of a vapor layer may be exceeded by applying a peak voltage of about 180 V. In some variations, a constant voltage square wave may be applied across one or more electrodes to reduce the total amount of energy used to generate and maintain plasma. For example, due to the shape and periodic nature of a sinusoidal waveform, a significant portion of a sinusoidal voltage falls below an ionization threshold. To compensate for this, the maximum voltage is increased, thereby increasing energy utilization and also potentially causing inadvertent damage to a vessel. However, a constant voltage square wave may be above the ionization threshold for a greater percentage of each cycle period compared to a sinusoidal waveform. Consequently, tissue removal may be performed with less energy.

[00207] In some variations, the systems discussed in this specification may further comprise an electrosurgical controller coupled to one or more electrodes for controlling tissue removal. For example, in some variations, the electrosurgical controller may control a first catheter comprising one or more electrodes paired with a second catheter comprising a strut.

[00208] In some variations, the system may comprise a feedback circuit to maintain the voltage at a constant level. By maintaining a constant voltage square wave, a higher current may be supplied during an ionic conduction phase when vapor is not yet formed and the impedance is low. As vapor forms, the impedance increases and the current may be reduced accordingly by constant voltage control of the feedback circuit. The voltage may be maintained at a predetermined level to provide a desired ionization intensity of the plasma. The feedback circuit may comprise any known control scheme, such as P (proportional), PI (proportional and integral), PID (proportional, integral and derivative) control, or the like.

[00209] The size of the fistula formed may vary depending on the thickness of the tissue. For example, if the tissue is thin, limiting the extension of the electrode outside of a catheter housing by means of a protruding support may, for example, produce a smaller fistula. For thicker tissue, the electrode may be designed to extend further outside of the catheter housing, thereby increasing the width of the fistula formed. In this way, a predetermined fistula resistance (e.g., fluid flow rate) may be maintained in the presence of tissue of varying thickness.

[00210] The impedance between the electrodes can be measured before, during and/or after an ablation cycle to measure resistive changes in the tissue caused by the ablation. As tissue is ablated and blood is supplied through the newly formed fistula, an impedance between the electrodes can drop significantly and can indicate successful fistula creation. Conversely, if the impedance has not dropped to a predetermined level, a fistula can be determined not to have formed completely. An impedance measured within a predetermined range can correspond to a created fistula. In a range, an impedance measured at about 150 ohms or less can indicate fistula formation.

[00211] For example, a first impedance may be measured between an electrode in the first vessel and an electrode in the second vessel. After tissue removal, a controller controls the electrodes to measure a second impedance and determine whether a desired fistula has been created using the first impedance as a reference. In one variation, a removal parameter may comprise a tissue removal period of 40 ms, followed by a 20 ms impedance measurement period, with no tissue removal energy being delivered. In another variation, a removal parameter may comprise a tissue removal period of about 500 ms, followed by an impedance measurement period of about 1 second. In some such variations, the measured impedance may be averaged over the measurement period. This cycle of alternating measurement and removal may be repeated until a fistula is created. In some variations, completion of fistula formation may be indicated to a user by means of visual and/or audio feedback, such as a confirmation tone and/or display message. Fistula creation and confirmation may be performed without fluoroscopy. However, it should be considered that, additionally or alternatively, fistula creation may be confirmed by the use of imaging, such as by the use of fluoroscopy and injection of a contrast agent.

[00212] Additionally, in some variations, one or more balloons or expandable elements, such as those described herein, may be used to assist in positioning the first catheter and/or the second catheter, or may act to retain the first catheter and/or the second catheter in place within blood vessels. For example, in some variations, expansion of a balloon or expandable element of one of the catheters may engage the interior of a blood vessel, which may retain the catheter in place within the blood vessel. In other methods, expansion of the balloon or expandable element may urge or otherwise compress a fistula-forming element against blood vessel tissue, which may aid in fistula formation.

[00213] Additionally, one or more balloons may be active to affect blood flow relative to the fistula. For example, in variations in which an arteriovenous fistula is formed, it may be beneficial to dilate one or more portions of the artery and/or veins. Specifically, the portion of the artery upstream of an arteriovenous fistula may be expanded to increase flow through the fistula. Alternatively or additionally, a portion of a vein downstream of a fistula may be dilated to help increase flow through the fistula. In some variations, one or more portions of the expandable elements may comprise an electrode to induce necrosis or swelling in a portion of a blood vessel to decrease flow therethrough. For example, in some variations, a portion of a vein upstream of a fistula may be at least partially occluded to minimize venous hypertension.

[00214] Although the implementations set forth above have, for purposes of clarity and understanding, been described in some detail by way of illustration and example, it will be apparent that certain changes and modifications may be made which are intended to fall within the scope of the appended embodiments. Additionally, it should be understood that the components and features of the devices described in this specification may be used in any combination, and the methods described in this specification may comprise all or a portion of the elements described in this specification. The description of certain elements or features with respect to a specific figure is not intended to be limiting, nor should it be construed as suggesting that the element cannot be used in combination with any of the other elements described.

Claims (35)

1. A system for forming a fistula between two vessels, comprising: a first catheter (100) comprising a housing (102), and an electrode (106) comprising a proximal end (108) and a distal end (112), wherein the proximal end (108) is fixed relative to the housing (102) and the distal end (112) is longitudinally slidable within the housing (102); characterized in that the distal end (112) of the electrode (106) is slidable toward the proximal end (108) in the housing (102) in order to move the electrode (106) to an extended configuration, and wherein the distal end (112) of the electrode (106) is slidable away from the proximal end (108) in the housing (102) in order to move the electrode (106) to a low profile configuration; and where the electrode (106) varies in width along its length. 2. System, according to claim 1, characterized by the fact that the housing (102) comprises an opening (104) and the electrode (106) comprises an intermediate portion (110) between the proximal end (108) and the distal end (112), and in which the intermediate portion (110) extends into and out of the opening (104). 3. System according to claim 1, characterized in that the electrode (106) comprises a leaf spring. 4. System, according to claim 1, characterized by the fact that the first catheter (100) comprises a fluid seal, to prevent fluid from entering the interior of the first catheter (100) at the proximal end (108) of the electrode (106). 5. System according to claim 1, characterized in that the housing (102) comprises a thermally insulating portion adjacent to at least the proximal end (108) of the electrode (106). 6. System according to claim 1, characterized in that the electrode (106) varies in height along its length. 7. System, according to claim 2, characterized by the fact that the intermediate portion (110) of the electrode (106) comprises a plurality of folds. 8. System according to claim 2, characterized in that the intermediate portion (110) comprises a bend of less than about 40 degrees. 9. The system of claim 1, wherein the electrode (106) comprises a low-profile configuration in which the electrode (106) is retracted within the housing (102). 10. The system of claim 1, wherein the housing (102) comprises a reservoir between a proximal end (108) of the electrode (106) and a distal end (112) of the first catheter (100), the reservoir configured to hold fluid, and wherein the electrode (106) is configured to generate plasma from fluid in the reservoir. 11. The system of claim 1, further comprising a feedback circuit configured to apply a constant square wave voltage to the electrode (106). 12. System according to claim 1, characterized in that it further comprises a second catheter comprising a second housing (102) and a projecting support. 13. The system of claim 12, wherein the second catheter comprises a retracted portion opposite the projecting support, wherein the retracted portion has a shape complementary to the projecting support. 14. The system of claim 13, further comprising an intubating sheath, wherein the projecting support and the retracted portion are within the intubating sheath. 15. The system of claim 1, further comprising a second catheter comprising a protruding support, wherein the protruding support has a shape that is complementary to a portion of the electrode (106). 16. System, according to claim 15, characterized by the fact that the electrode (106) comprises an intermediate portion (110) between the proximal end (108) and the distal end (112), and in which the projecting support has a shape that is complementary to the intermediate portion (110) of the electrode (106). 17. The system of claim 16, wherein the electrode (106) comprises an extended configuration in which the electrode (106) is extended so as to extend away from the housing (102), and wherein the complementary shape corresponds to a shape of the electrode (106) in the extended configuration. 18. System, according to claim 15, characterized by the fact that the shape comprises a concave portion comprising an opening (104) configured to receive the electrode (106). 19. The system of claim 1, wherein the first catheter (100) comprises a first junction region comprising a flat junction surface. 20. The system of claim 19, wherein the first junction region has a square or rectangular cross-section. 21. The system of claim 19, further comprising a second catheter comprising a second junction region comprising a planar junction surface. 22. The system of claim 21, wherein the second junction region has a square or rectangular cross-section. 23. The system of claim 21, wherein the first junction region comprises a first magnet and the second junction region comprises a second magnet. 24. The system of claim 1, wherein the first catheter (100) comprises a first cable, and the system further comprises a second catheter comprising a second cable, wherein the first cable and the second cable each comprise a flat surface. 25. The system of claim 24, wherein the first catheter (100) comprises a first rod and the second catheter comprises a second rod, and wherein the first rod and the second rod each comprise an interlace configured to improve resistance to torsion. 26. The system of claim 24, wherein the first cable comprises a first magnet and the second cable comprises a second magnet. 27. The system of claim 1, wherein the first catheter (100) comprises a rotation indicator comprising a radiopaque material. 28. The system of claim 27, wherein the rotation indicator comprises a radiopaque film. 29. The system of claim 28, wherein the radiopaque film has a thickness of about 0.025 mm. 30. The system of claim 27, wherein the rotation indicator has a square or rectangular cross-section. 31. The system of claim 27, wherein the rotation indicator has a cross-section having a written character shape. 32. The system of claim 27, wherein the rotation indicator comprises a hub comprising a cylindrical cutout. 33. The system of claim 27, wherein the rotation indicator comprises an arrow-shaped cutout. 34. The system of claim 1, wherein the electrode (106) is configured to self-expand away from the housing (102). 35. The system of claim 17, wherein the electrode (106) is configured to tend toward the extended configuration.