GB2534490B

GB2534490B - Catheter drainage systems

Info

Publication number: GB2534490B
Application number: GB1603557.8A
Authority: GB
Inventors: Benjamin Hopley Ian
Original assignee: ALTERNATIVE UROLOGICAL CATHETER SYSTEMS Ltd
Current assignee: ALTERNATIVE UROLOGICAL CATHETER SYSTEMS Ltd
Priority date: 2013-08-07
Filing date: 2014-07-25
Publication date: 2019-10-30
Anticipated expiration: 2034-07-25
Also published as: GB2534490A; GB201603557D0; GB201314140D0; WO2015019056A1

Description

Catheter drainage systems

Background of the Invention

The present invention relates to catheter drainage systems for drainage of the urinary bladder. In particular, the present invention relates to catheter drainage systems for intermittent bladder drainage.

Summary of the Prior Art

The catheter is a medical device used to drain urine from the bladder. One out of four hospitalized patients receives an indwelling catheter [1] . However, catheters provide a conduit for bacteria to enter the body, and catheter-associated urinary tract infections present the most common nosocomial infections, developing at an estimated rate of 5% per day and increasing the risk of bacteraemia and mortality [2]. Financial estimates in 2000 suggested that a symptomatic urinary tract infection raises the cost of care by around £420 and bacteraemia to at least £1750 by lengthening the stay in hospital and antibiotic therapy [3]. In developing countries the rates of healthcare-associated infections are 3 to 5 times higher than international standards [4].

The self-retaining urinary catheter introduced by Dr

Foley in 1937 has been the standard product in routine use for over 70 years [5]. A Foley-type catheter is usually positioned in a patient by passing it up through the urethra, and retained in the bladder by means of an inflatable balloon. Such a catheter is known as a urethral catheter. However, catheters may alternatively be inserted into the bladder through the abdominal wall just above the pubic bone i.e. along a suprapubic track.

The latter is known as a suprapubic catheter.

In addition to infection, other common catheter problems such as encrustation and blockage of the catheter, urinary leakages and discomfort from the catheter have an immense impact not only on the individual's quality of

life but also on the cost to healthcare services [6]. A system that mimics the cyclical filling and emptying of the bladder has been advocated to reduce the risk of urinary infections and the incidence of catheter blockages [7]. To achieve this, a pinch valve can be applied to the drainage system of a conventional catheter. While the pinch valve is closed, a volume of urine accumulates in the bladder and upstream portion of the catheter. The pinch valve may then be opened at intervals to drain the bladder, mimicking a natural urinary cycle .

Summary of the Invention

However, the high flow rates associated with catheter systems currently used for intermittent bladder drainage can cause bladder damage and other adverse effects owing to high pressure gradients between the bladder and point of drainage, and rapid pressure changes within the bladder. The pressure gradient is a result of the both the pressure exerted on the urine by the bladder wall and the difference in height between the upper surface of the urine in the bladder and the point of drainage. The pressure exerted by the bladder is a consequence of the combined action of the elasticity of the bladder wall and the pressure exerted externally on the bladder by the intra-abdominal pressure, and is typically about 18 cm of water above atmospheric pressure.

On rapid draining of the bladder, the resultant pressure is the sum of the bladder pressure and the pressure created by the difference in height between the upper surface of the urine in the bladder and the point of drainage. The typically smooth, fixed diameter tubes of the catheter and drainage system offer little viscous drag to the urine, with the result that the generated pressure gradient accumulates particularly in the location of the catheter eye holes located in the bladder. If the difference is large there is a risk of serious damage to the bladder resulting from rapid drainage, and in particular to the bladder mucosa, which may be drawn into the catheter eye holes and, in extreme cases, may result in the formation of pseudopolyps and even block the catheter flowpath.

During drainage of these systems, significant kinetic energy can accumulate. As the flow ceases, a negative pressure wave is generated to arrest the flow and absorb the kinetic energy, placing a strain on the bladder. If mucosa is drawn into or towards the eye holes, this effect may be repeated multiple times, resulting in a stuttering effect in the flow and generating alternating waves of high and low pressure, risking serious bladder damage .

The present invention aims to provide an improved system for intermittent bladder drainage via a catheter that is better able to mimic the natural bladder emptying process while reducing the risk of serious bladder damage during drainage. The present invention may provide improved drainage systems for urinary drainage using, for example, a Foley catheter. Foley catheters are often inserted into the patient via the urethra and maintained in the bladder by an inflated balloon, and are known as urethral

Foley catheters. However, systems of the present invention may also suitable for use in suprapubic drainage; that is, drainage systems for catheters that are positioned in a patient via a suprapubic track, and may be used with catheters as described in EP2470249 (European application No. 10807469.1).

The present invention has been developed especially for use in patients requiring long term catheterisation and/or for whom intermittent bladder drainage is especially desirable.

At its most general, the first aspect of the present invention relates to a catheter drainage system for draining urine from the bladder of a patient, the drainage system having a one-way valve that opens to allow air into the system when pressure inside the flowpath at the location of the valve drops below atmospheric pressure. While this valve is configured to allow air to enter the flowpath, leakage of fluid out of the system through the valve is prevented. The drainage system has a drainage conduit through which urine flows when drained. In use, the drainage conduit is in fluid communication with the interior of the bladder.

As used herein, the downstream direction refers to positions relatively further away from the bladder, while upstream refers a position closer to the bladder. In use of a typical urethral Foley catheter, the upstream end of the catheter tube is maintained in the bladder by an inflated balloon. The catheter tube then continues along the urethra, emerges from the patient and is connected to a bag or other suitable reservoir at an outlet at the downstream end (drainage point). The present invention, by providing a drainage system having a suitable valve between the bladder and the drainage point, allows the drainage pressure to depend on the height difference between the valve and the urine level of the bladder, rather than depending on the height difference between the drainage point and the urine level of the bladder.

This advantageously means that the drainage pressure is independent of the location of the drainage point, resulting in greater versatility of catheter system arrangement.

Accordingly, the first aspect may provide a catheter drainage system for draining the urinary bladder, the drainage system having a drainage conduit, the drainage conduit having a urinary catheter end part for insertion into the bladder of a patient, and a flow conduit in fluid communication with the catheter end part to permit urine to flow from the urinary catheter end part to an outlet, the flow conduit having a pressure relief valve, wherein the pressure relief valve is configured to open when pressure inside the flow conduit at the location of the pressure relief valve drops below atmospheric pressure to allow air to enter the flow conduit.

In other words, the first aspect may provide a urinary drainage system that, in use, has a pressure relief valve acting on the system between the point at which the urine contained in the system emerges from the body and the drainage point. A reservoir or drain is typically located at the drainage point. Examples of suitable reservoirs include a bag strapped to the patient's leg or hospital bed, or a bag or other suitable container which may, for example, rest on the floor beneath a bed, and which may be located at a relative height of 60 cm or even greater below the level of urine in the bladder.

The pressure relief valve provided by the first aspect permits the pressure gradient generated during intermittent bladder drainage of catheterised patients to be reduced and more easily adjusted through judicious placement of the portion of the system in which the pressure relief valve is located, irrespective of where the drainage point is located. For example, if the pressure relief valve is located at a height 10 cm below the level of urine in the bladder, the pressure driving the drainage is the bladder pressure plus 10 cm of water above atmospheric pressure. The position of this pressure relief valve relative to the patient's bladder may be adjusted for example by taping the drainage conduit proximal to the pressure relief valve to the patient, for example to the abdomen, hip or upper leg, to moderate the pressure difference that occurs during draining.

The reduction in drainage pressure owing to the inclusion of the pressure relief valve in drainage systems of the first aspect of the invention reduces the risk of damage to the bladder caused by both the initial steady state flow and by the destruction of the kinetic energy of the escaping urine as the drainage is completed.

However, in some circumstances, the inclusion of a pressure relief valve as described herein may not be sufficient to prevent bladder damage. At the end of the drainage process the kinetic energy of the escaping urine must be absorbed, and the smooth, fixed diameter tubing of the catheter and remainder of the drainage system does not sufficiently dampen this. Furthermore, successive negative and positive pressure waves can form. These oscillations may result in the re-introduction of urine into the bladder, thereby increasing the risk of bladder infection.

To address this problem, and to reduce further the risk of bladder damage during intermittent drainage, the flow conduit of the drainage system of the first aspect may further have a tubular dampening section, the dampening section being configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure. This dampening section is configured to collapse under negative pressure waves generated during the final stages of drainage, absorbing the residual kinetic energy and thereby reducing the effect of these negative pressure waves on the bladder itself. This mimics the natural action of the urethra during normal urination, and may reduce the risk of bladder damage and/or reintroduction of drained urine.

The flexibility of the dampening section may be selected to produce the required dampening without restricting the process of drainage, for example by using a dampening section made of a material of sufficient flexibility.

In order to protect the dampening section from accidental damage, the dampening section may be encased in an outer tube which is more resistant to inward collapse. To ensure that atmospheric pressure acts on the outside of the dampening section, the outer tube encases the dampening section in such a way that the dampening section is subject to atmospheric pressure. For example, the outer tube may encase the dampening section with an upstream sealed connection and a downstream sealed connection, so that it is sealed to the flow conduit on either side of the dampening section. In this arrangement, the outer tube may have, for example, an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section, although other arrangements may be envisaged.

Suitable materials for the dampening section may include silicone (as the material more resistant to collapse) and/or latex with or without coating with other polymers such as polyvinylchloride or polyethylene. The dampening section may be located at any point in the drainage conduit that is not situated intracorporeally during use, but is preferably located upstream of the pressure relief valve .

In some drainage systems of the first aspect, the drainage conduit is separable into an upstream portion and a downstream portion, these being the urinary catheter end part and the flow conduit, respectively, and being joined by a connector. In drainage systems comprising a urinary catheter end part and a flow conduit joined by a connector, the dampening section is preferably located in the flow conduit between the connector and the pressure relief valve. In drainage systems in which the entire length of the drainage conduit is a single length (that is, not separable) and there is no connector, the dampening section is located at a point that is outside of the body when in use, preferably between the point at which the drainage conduit emerges from the body and the location of the pressure relief valve. Location of the dampening section outside of the body during use avoids the need to attempt to introduce a plastic tube, modelled on the urethra, into the body. The inventor believes that any such attempt may present formidable problems.

In use, a preferred arrangement along the drainage conduit has, moving in downstream direction, sequentially, a dampening section, followed by the fluid regulating valve, followed by a pressure relief valve.

In some preferred arrangements, the drainage conduit has, moving in downstream direction, sequentially, a connector, followed by a dampening section, followed by a fluid regulating valve, followed by a pressure relief valve .

The pressure relief valve may be a reed valve and may be formed of, for example, a valve eye hole in a wall of the flow conduit and a cover. To prevent fluid leakage, it is preferable for the cover to be connected to the interior of the wall of the flow conduit upstream of the valve eye hole and to extend beyond the eye hole in the downstream direction. Thus, when pressure inside the flow conduit in the location of the pressure relief valve drops sufficiently below atmospheric pressure, the downstream portion of the cover moves away from the wall of the flow conduit to permit air to enter and equalise the pressure. Preferably, the cover is an inner sleeve arranged to be close fitting with the wall of the flow conduit. Preferably, the sleeve is sealed entirely around the inner wall of the flow conduit upstream of the valve eye hole and free-hanging downstream of the eye hole. The cover may have the same degree of flexibility as the wall of the flow conduit and may, for example, be made of the same material.

The drainage system of the first aspect may further comprise a fluid regulating valve adapted to open and close the flowpath through the drainage conduit to permit intermittent bladder drainage. The fluid regulating valve may be an integral part of the system of the first aspect, or may be attached during the catheterisation procedure to the outside of the drainage conduit. In systems of the first aspect, the fluid regulating valve is located at a position upstream of the pressure relief valve. The fluid regulating valve may be, for example, a pinch valve. Other suitable valves for medical tubing are known in the art.

The advantages of incorporating means to dampen the destruction of the kinetic energy of the emerging urine at the end of drainage are described herein.

Accordingly, a first non-claimed example relates to catheter drainage systems for draining the urinary bladder of the patient that are able to mimic the natural dampening effect provided by the urethra during normal urination. This may be achieved by the inclusion of a dampening section within the drainage conduit that is configured to absorb kinetic energy and prevent or reduce pressure oscillation, reducing the risk of damage to the bladder, stuttering drainage and the possible reintroduction of expelled urine back into the bladder.

Consequently, a first non-claimed example may provide a catheter drainage system for draining the urinary bladder, the drainage system having a drainage conduit, the drainage conduit having a urinary catheter end part suitable for insertion into the bladder of a patient, and a flow conduit in fluid communication with the catheter end part to permit urine to flow from the urinary catheter end part to an outlet, the flow conduit having a tubular dampening section, wherein the dampening section is configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure.

As in the first aspect, the flexibility of the dampening section may be selected to produce the required dampening without restricting the process of drainage, and preferably to mimic the natural dampening effect of the urethra, for example, by selecting a sufficiently flexible material for the dampening section, so that the material of the dampening section is more flexible than the material of the rest of the flow conduit.

As in the first aspect, the dampening section of the first )n-claimed example may be enclosed in an outer tube which is more resistant to collapse. To ensure that atmospheric pressure acts on the outside of the dampening section, the outer tube encases the dampening section in such a way that the dampening section is subject to atmospheric pressure, for example, the outer tube may encase the dampening section with an upstream sealed connection and a downstream sealed connection, so that it is sealed to the flow conduit on either side of the dampening section.

In this arrangement, the outer tube may have, for example, an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section, although other arrangements may be envisaged. Suitable materials for the dampening section may include silicone (as the material more resistant to collapse) and/or latex coated with other polymers such as polyvinylchloride or polyethylene .

Like the drainage system of the first aspect, the drainage system of the first non-claimed example may further comprise a fluid regulating valve. In drainage systems of the first non-claimed example, the fluid regulating valve is located at a position upstream of the pressure relief valve. The fluid regulating valve may be, for example, a pinch valve. Other suitable valves for medical tubing are known in the art. The dampening section may be located at any point in the drainage system that is outside of the body in use, but is preferable located upstream of the position of the fluid regulating valve.

It will be appreciated that the advantageous inclusion of a pressure relief valve and/or a tubular dampening section, as described for the first aspect of the invention and the first 3n-claimed example, within urinary drainage systems may be of use in the catheterisation of a wide variety of patients and in a variety of clinical situations. For this reason, it will be understood that in some circumstances, drainage systems comprising a separable urinary catheter end part and flow conduit, these being supplied separately and connectable by a connector, may be preferred. The connector may comprise a first connecting means and a second connecting means, with the urinary catheter end part and the flow conduit being connected at the connector by engagement of the first connecting means located at the downstream end of the urinary catheter end part with the second connecting means located at the upstream end of the flow conduit. The urinary catheter end part may be, for example, part of a urethral or suprapubic catheter, which may be supplied separately and may be selected to suit the patient (for example, in length to suit the gender of the patient).

According to a second aspect of the invention there is provided a kit of parts for assembling a catheter drainage system, the kit comprising: a urinary catheter having a urinary catheter end part for insertion into the bladder of a patient, and a first connecting means in series, and a drainage portion having a flow conduit and a second connecting means in series, the first and second connecting means being interconnectable to connect the urinary catheter and the flow conduit in fluid communication, wherein the flow conduit has a pressure relief valve, the pressure relief valve being configured to open when pressure inside the flow conduit at the location of the pressure relief valve drops below atmospheric pressure to allow air to enter the flow conduit.

The flow conduit may further may have a tubular dampening section, the dampening section being configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure, and preferably this is located upstream of the pressure relief valve.

The dampening section may be made of a material more flexible than the material of the rest of the flow conduit, so that it is sufficiently flexible to collapse inwards when the pressure within the dampening section falls below atmospheric pressure, and may be encased by an outer tube, the outer tube being more resistant to inward collapse than the dampening section and encasing the dampening section in such a way that the dampening section is subject to atmospheric pressure.

The outer tube may encase the dampening section and be sealed to the flow conduit on either side of the dampening section by an upstream sealed connection and a downstream sealed connection, and the outer tube may have an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section.

The pressure relief valve may be a reed valve and may be formed of, for example, a valve eye hole in a wall of the flow conduit and a cover. To prevent fluid leakage, it is preferable for the cover to be connected to the interior of the wall of the flow conduit upstream of the valve eye hole and to extend beyond the eye hole in the downstream direction. Thus, when pressure inside the flow conduit in the location of the pressure relief valve drops sufficiently below atmospheric pressure, the downstream portion of the cover moves away from the wall of the flow conduit to permit air to enter and equalise the pressure. Preferably, the cover is an inner sleeve arranged to be close fitting with the wall of the flow conduit. Preferably, the sleeve is sealed entirely around the inner wall of the flow conduit upstream of the valve eye hole and free-hanging downstream of the eye hole. The cover may have the same degree of flexibility as the wall of the flow conduit and may, for example, be made of the same material. A second non-claimed example provides a kit of parts for assembling a catheter drainage system, the kit comprising: a urinary catheter having a urinary catheter end part for insertion into the bladder of a patient, and a first connecting means in series, and a drainage portion having a flow conduit and a second connecting means in series, the first and second connecting means being interconnectable to connect the urinary catheter and the flow conduit in fluid communication, wherein the flow conduit has a tubular dampening section, the dampening section being configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure.

As described above, the dampening section may be made of a material more flexible than the material of the rest of the flow conduit so that it is sufficiently flexible to collapse inwards when the pressure within the dampening section falls below atmospheric pressure, and may be encased by an outer tube, the outer tube being more resistant to inward collapse than the dampening section and encasing the dampening section in such a way that the dampening section is subject to atmospheric pressure. The outer tube may encase the dampening section and be sealed to the flow conduit on either side of the dampening section by an upstream sealed connection and a downstream sealed connection, and the outer tube may have an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section.

The kits of parts of the second non-claimed example may further include a fluid regulating valve and/or a collection chamber as described elsewhere in the application.

Brief Description the Drawings

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows the diagrammatic arrangement of an embodiment of a drainage system of the present invention fitted within a patient.

Figure 2 shows a cross-sectional view of a pressure relief valve according to an embodiment of the present invention.

Detailed Description

The following detailed description of some embodiments of the invention should be read with reference to the drawings, wherein like reference numerals indicate like elements throughout the several views.

Referring to Figure 1, there is illustrated the diagrammatic arrangement of the components of a drainage system according to an embodiment of the first aspect of the present invention in place in the body.

The drainage system comprises a drainage conduit 1 in fluid communication with the bladder 2. In this embodiment, the urinary catheter end part is a Foley catheter 3, with the upstream end of the Foley catheter inserted into the bladder via the sphincter 4 and retained in place by an inflated balloon 5. The balloon 5 of the Foley catheter 3 is inflated with sterile water introduced via a second tube (not shown). The upstream end of the Foley catheter has one or more eye holes 6 (one shown) to allow urine to exit the bladder and enter the drainage conduit. The remainder of the Foley catheter then lies within the urethra and emerges from the body as shown.

In this embodiment, the drainage conduit is separable into a urinary catheter end part which in this case is the Foley catheter 3, and a drainage portion having a flow conduit 7. The downstream end of the Foley catheter 3 is connected to the upstream end of the flow conduit by a connector 8. The connector 8 may be formed of two complementary portions, one located on the catheter, the other on the upstream end of the flow conduit, for example, a snap fit interaction or male and female screw threads, although other fluid-tight connections may be envisaged and indeed the catheter and flow conduit may be integral. In this embodiment, urine may exit the bladder through eye holes 6, drain down the catheter 3, pass through the connector 8 and enter the flow conduit 7.

This system advantageously allows the catheter 3 to be inserted into the patient without the encumbrance of the remainder of the drainage system being attached. It further allows the drainage system downstream of connector 8 to be decoupled and changed without necessitating removal and reinsertion of a catheter into the urethra. However, in an alternative embodiment, the drainage conduit may be a single length, obviating the need for a connector 8. A pinch valve 9 is located on the flow conduit. This closes the flowpath through the drainage conduit, allowing urine to build up in the bladder. The valve may be opened to drain the bladder as desirable/necessary.

Other valves suitable to open and close the flowpath through the drainage conduit to permit intermittent bladder drainage may also be envisaged. The valve may be built into the housing of the flow conduit, or may be a detachable clip supplied with the system or separately.

The length of the flow conduit between connector 8 and pinch valve 9 has a dampening section (represented by 10). This dampening section is configured to collapse inwardly when the pressure within this dampening section drops below atmospheric pressure, thus reducing or preventing pressure oscillation within the bladder. The dampening section is made of a material sufficiently flexible to collapse under the desired pressure conditions, although other arrangements, for example, longitudinally alternating strips of comparatively very flexible and more rigid materials may be envisaged.

The dampening section is housed within a more rigid outer casing to protect it from accidental damage. This outer casing may be the tube forming the flow conduit of the system, such that the dampening section is a more flexible inner sleeve, sealed entirely at its upstream and downstream ends, housed within the tube that forms the flow conduit. In order to facilitate action of atmospheric pressure on the outside of the dampening section, the outer tube has at least one eye hole (not shown) to allow air to enter the area between the outer tube and the dampening section, facilitating inward collapse of the dampening section when the pressure within the dampening section falls below atmospheric pressure. Accordingly, in this embodiment, the drainage flowpath of the system is initially defined by the hollow tube, then by the dampening section, that is, an inner sleeve within a hollow tube, sealed to the walls of the inner tube. At the downstream end of the dampening section, the inner sleeve is sealed to the walls of the hollow tube. Downstream of this point, the wall of the hollow tube itself again defines the drainage flowpath.

Downstream of the pinch valve 9 along the flow conduit, there is a pressure relief valve 11. A diagrammatic representation of a cross-section of this pressure relief valve is shown in Figure 2. The pressure relief valve 11 is formed of an inner sleeve 111 that is close fitting to the inner wall 112 of the hollow tube forming the flow conduit. The inner sleeve 111 is sealed to the inner wall of the hollow tube around its circumference at the upstream end 113 and free to move away from the inner wall at the downstream end 114. An eye hole 115 in the wall of the hollow tube allows atmospheric pressure to act on the inner sleeve of the pressure relief valve. In this embodiment, the inner sleeve 111 is of the same degree of flexibility as the hollow tube, the inner sleeve being formed of the same material as the hollow tube .

The operation of the pressure relief valve 11 within the drainage system will now be described. When the pinch valve 9 is closed, there is no flow of urine though the flow conduit 1 through the pressure relief valve 11.

When the pinch valve 9 is opened, urine drains from the bladder 2 along the drainage conduit 1 through the pressure relief valve. As a pressure gradient is created within the drainage conduit, the pressure in the flow conduit at point 116 in the vicinity of the pressure relief valve 11 may drop below atmospheric pressure. The action of atmospheric pressure on the inner sleeve 111 through eye hole 115 then forces the downstream portion of inner sleeve 111 away from the inner wall 112 of the hollow tube to allow air to enter and maintain the system around point 116 at atmospheric pressure. When the pressure at point 116 is not less than atmospheric pressure, the valve remains shut. Pressure relief valve 11 therefore permits air to enter the drainage system but does not allow fluid to leak out of the system.

As a result of pressure relief valve 11, the drainage pressure depends only of the distance of point 116 below the level of urine in the bladder irrespective of the position of any collection chamber. Consequently if, for example, the pressure relief valve 11 is 10 cm below the level of urine in the bladder, the pressure driving the drainage will be the bladder pressure plus 10 cm of water. It can be shown that a reduction in this height to 10 cm from 60 cm results in a decrease in kinetic energy of the emerging stream of urine by a factor of 36.

Most of this kinetic energy is absorbed by the bladder, with higher levels more likely to cause bladder damage.

To facilitate collection and storage of the urine drained from the bladder, a collection chamber 12 is provided in fluid communication with the outlet of the drainage conduit 1. The collection chamber and outlet are connected immediately downstream of the pressure relief valve by connector 13 and a length of tubing 14. A nonreturn valve 15 prevents urine re-entering the system.

It will be appreciated that the drainage conduit may be of variable length downstream of the pressure relief valve. It will also be appreciated that the length of tubing 14 is not essential, and that the connection between the outlet of the drainage conduit and the collection chamber may be at the location of the nonreturn valve 15. The collection chamber may be a bag or rigid tank as preferred.

The action and advantages of the dampening section will now be described. In the case of normal urethral drainage, the collapse of the walls of the urethra at the end of the drainage process provides sufficient viscous drag to produce effective viscous dampening. This is because the urethra is in the form of a flattened tube, the walls of which are pushed apart by the passage of urine and move together again as the flow rate is reduced, thereby creating viscous drag along the entire length of the urethra. By contrast, the wide, smooth fixed diameter tubes of typical urinary or Foley catheter drainage systems offer little viscous drag to the flow of urine .

At the end of the drainage process, the emerging urine has significant kinetic energy. It can be shown, from hydrodynamic considerations, that the kinetic energy on cessation of urinary catheter drainage is proportional to the square of the distance between the bladder sphincter 4 and the pressure relief valve 11.

The derivation is as follows (no dampening section in place):

Immediately before completion of the drainage of the bladder, the pressure on the urine within it differs little from atmospheric and the viscous drag exerted by the drainage tube is small. Consequently the pressures and velocities are similar to those in an open vessel attached to a vertical tube.

The kinetic energy of the emerging urine is:

where m is the mass of urine in the drainage tube and v is its velocity.

If A is the area of cross-section of the tube, 1 its length, p the density of the urine and g the acceleration of gravity, the above equation becomes:

From Bernoulli's equation for streamline flow:

Consequently, the kinetic energy of the fluid in the drainage tube is:

Therefore, if the pressure relief valve is not present, the kinetic energy on cessation of urinary catheter drainage is proportional to the square of the distance between the bladder sphincter 4 and the outlet to the collection chamber (at non-return valve 15).

Without a dampening effect, negative pressure waves may be formed as a result of the kinetic energy of the urine being absorbed by the bladder and not by the walls of the urethra. This may manifest itself in an oscillation

between successive positive and negative pressure waves.

While judicious placement of the pressure relief valve 11 reduces the drainage pressure and consequently reduces the risk of damage to the bladder caused by both the initial steady state flow and by the destruction of the kinetic energy of the escaping urine as drainage is completed, the inclusion of a dampening section is advantageous to dampen the effect of the destruction of kinetic energy and because the possibility of oscillation within the system remains.

By careful tuning of the flexibility of the dampening section, it is possible to mimic the natural urethra, such that the process of drainage is not restricted with the dampening section only beginning to collapse as the flow peters out and eventually stops.

References 1. Saint S, Elmore JG, Sullivan SD, Emerson SS,

Koepsell TD. The efficacy of silver alloy-coated urinary catheters in preventing urinary tract infection: a metaanalysis. Am J Med 1998:105:236-241. 2. Maki DG, Tambyah PA. Engineering Out the Risk for infection with Urinary Catheters . Emerging Infectious

Diseases 2001:7(2):342-347. 3. Saint S. Clinical and economic consequences of nosocomial catheter-related bacteriuria. Am J Infect

Control 2000: 28(1):68-73. 4. Rosenthal VD. Device-associated nosocomial infections in limited-resources countries: findings of the International Nosocomial Infection Control Consortium (INICC) . Am J Infect Control 2008:36 (10) :S171-12. 5. Foley FEB. A Self-Retaining Bag Catheter. J Urol 1937: 38:140-143. 6. Kohler-Ockmore J. and Feneley RCL. Long-term catheterisation of the bladder: prevalence and morbidity.

British Journal of Urology 1996:77:347-351. 7. Kunin, CM. Can we build a better urinary catheter? N Eng J Med 1988:319(6):365-366.

Claims

1. A catheter drainage system for draining the urinary bladder, the drainage system having a drainage conduit, the drainage conduit having: a urinary catheter end part for insertion into the bladder of a patient, and a flow conduit in fluid communication with the urinary catheter end part to permit urine to flow from the urinary catheter end part to an outlet, the flow conduit having a pressure relief valve, wherein the pressure relief valve is configured to open when pressure inside the flow conduit at the location of the pressure relief valve drops below atmospheric pressure to allow air to enter the flow conduit.

2. The catheter drainage system of claim 1, wherein the flow conduit further has a tubular dampening section, the dampening section being configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure.

3. The catheter drainage system of claim 2, wherein the dampening section is located between the pressure relief valve and the urinary catheter end part.

4. The catheter drainage system of claim 2 or claim 3, wherein the dampening section is made of a material more flexible than the material of the rest of the flow conduit.

5. The catheter drainage system of claim 2, wherein the dampening section is encased by an outer tube, the outer tube being more resistant to inward collapse than the dampening section, and wherein the outer tube encases the dampening section in such a way that the dampening section is subject to atmospheric pressure.

6. The catheter drainage system of claim 5, wherein the outer tube is sealed to the flow conduit on either side of the dampening section, and wherein the outer tube has an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section.

7. The catheter drainage system of any one of claims 1 to 6, wherein the pressure relief valve is a reed valve comprising a valve eye hole in a wall of the flow conduit and a cover, the cover being connected to the interior of the wall of the flow conduit upstream of the valve eye hole .

8. The catheter drainage system of claim 7, wherein the cover is an inner sleeve arranged to be close-fitting with the wall of the flow conduit.

9. The catheter drainage system of claim 7 or claim 8, wherein the cover has the same degree of flexibility as the wall of the flow conduit.

10. The catheter drainage system of any one of the preceding claims, wherein the drainage system further has a connector, the connector comprising a first connecting means in series with the catheter end part and a second connecting means in series with the flow conduit, and wherein the urinary catheter end part and flow conduit are seperably connected by engagement of the first connecting means with the second connecting means.

11. The catheter drainage system of claim 10, having a urethral catheter, the urethral catheter including said urinary catheter end part and being connected to said flow conduit.

12. The drainage system of claim 10, having a suprapubic catheter, the suprapubic catheter including said urinary catheter end part and being connected to said flow conduit.

13. A kit of parts for assembling a catheter drainage system, the kit comprising: a urinary catheter having a urinary catheter end part for insertion into the bladder of a patient, and a first connecting means in series, and a drainage portion having a flow conduit and a second connecting means in series, the first and second connecting means being interconnectable to connect the urinary catheter and the flow conduit in fluid communication, wherein the flow conduit has a pressure relief valve, the pressure relief valve being configured to open when pressure inside the flow conduit at the location of the pressure relief valve drops below atmospheric pressure to allow air to enter the flow conduit.

14. The kit of parts of claim 13, wherein the flow conduit further has a tubular dampening section, the dampening section being configured to collapse inwards when the pressure within the dampening section falls below atmospheric pressure.

15. The kit of parts of claim 14, wherein the dampening section is located between the pressure relief valve and the second connecting means.

16. The kit of parts of claim 14 or claim 15, wherein the dampening section is made of a material more flexible than the material of the rest of the flow conduit.

17. The kit of parts of any one of claims 14 to 16, wherein the dampening section is encased by an outer tube, the outer tube being more resistant to inward collapse than the dampening section, and wherein the outer tube encases the dampening section in such a way that the dampening section is subject to atmospheric pressure .

18. The kit of parts of claim 17, wherein the outer tube is sealed to the flow conduit on either side of the dampening section, and wherein the outer tube has an eye hole to permit ingress of air to maintain atmospheric pressure within the outer tube and facilitate collapse of the dampening section.

19. The kit of parts of any one of claims 13 to 18, wherein the pressure relief valve is a reed valve comprising a valve eye hole in a wall of the flow conduit and a cover, the cover being connected to the interior of the wall of the flow conduit upstream of the valve eye hole .

20. The kit of parts of claim 19, wherein the cover is an inner sleeve arranged to be close-fitting with the wall of the flow conduit.

21. The kit of parts of claim 19 or claim 20, wherein the cover has the same degree of flexibility as the wall of the flow conduit.