GB2281863A - Medical diathermy apparatus - Google Patents

Abstract

An electrical generator for the purpose of diathermy, that is to say passing high frequency electric currents through living tissue in order to generate heat, is provided with multiple outputs comprising polyphase or time multiplexed voltages. These are fed to electrodes capable of being inserted into tissue, with mechanical means for predetermined alignment, and further provision for inbuilt temperature sensing and the supply of a cooling medium under closed loop control to achieve a required temperature uniformity and variation with time. At least three electrodes are used, so that high frequency current flow in the tissue varies in magnitude and angular direction over a repetitive cycle. The apparatus may be used to destroy tumour cells.

Description

Provisional Patent Specification IMPROVEMENTS IN RELATION TO MEDICAL DIATHERMY In medicine and the practice of surgery, the technique of diathermy is known in the prior art. While electric currents flowing through living tissue, in particular the human body, are in general known to cause nerve stimulation and disturbance of function this is not true of alternating currents at a frequency in excess of 200KHz which cause only heating.

If a patient on an operating table is connected by means of an extensive conducting (normally metal) pad to one pole of a high frequency current generator, that is to say alternating current generator operating at a frequency exceeding 200KHz and typically at least 500KHz, while the other pole of the generator is connected to a needle-type electrode which is then applied to some accessible piece of tissue, then provided that the levels of voltage and current are suitable a local heating will take place in the region of the needle point which upon movement of this electrode will serve to cut through the tissue. Such cutting action is preferable to that of a normal knife it that it tends to coagulate blood and other body fluids and thus minimise the bleeding from severed blood vessels. It also tends to prevent infection. However a danger exists that if the said conducting pad makes imperfect contact, local burning may occur at a point remote from the operating site through which said current finds a return path.

A variant of this technique, known as Bipolar Diathermy, is used when the heating action of the high frequency current is required to be exercised not at a single point but uniformly along a line, as for example when sealing the end of a severed blood vessel to prevent destructive bleeding. Here no connection needs to be made to the body of the patient, but the high frequency generator is connected between the two arms of a pair of tweezers, the arms being insulated from one another and used to grasp the area of of tissue which requires to be heated. In this case the current flow becomes localised so as virtually to eliminate the danger of burning the patient at a point remote from the operation site.

However neither of these arrangements is completely suitable for a third class of application, which is where a relatively uniform heating is required within a defined volume of tissue - such as a tumour - for the purpose of destroying it or of limiting its infectivity without causing burns or other damage to tissue elsewhere.

Controlled heating of a volume of tissue may be required in connection with the Hyperthermia treatment method, whereby a temperature in the order of 45 degrees Centigrade has a selectively destructive effect upon tumour cells.

This invention is to provide an improved apparatus and means for achieving such a purpose.

According to this invention I provide firstly a set of three or more cylindrical or needle type electrodes which may be fixed to a common insulated plate or holder, and may further provide means for adjusting their relative spacing prior to insertion into tissue. These electrodes are wired to a polyphase or time multiplexed high frequency current generator.

Thereby within any one period of a repetitive cycle a high frequency current may be caused to flow through the said tissue between all possible pairs of said electrodes, said currents because of all their differences in phase being additive in their heating effects while avoiding the spatial pattern of cancellation and reinforcement of the heating effects which would occur were only two driving poles to be connected to multiple electrodes, with heating effects determined by a single resultant current flow at any given point.

Preferably further means are provided for monitoring temperatures within the tissue and/or voltage levels and current flows.

Preferably the electrodes are wired to a transformer having isolated output windings which will cause current flow from any one of said electrodes only to another electrode of the said set, and not to equipment earth.

For economy in components, a three phase output from said current generator may be produced by series connection of one secondary winding on each of two transformers, which if ferromagnetic cored may have a common centre limb, each excited by an oscillator or converter running from a common power supply but constrained to provide a relative phase difference of 90 degrees between said secondary windings.

This phase difference may be achieved by providing an oscillator running at twice or four times the required output frequency, which drives a scale-of-two binary counter, and two separate converters each of which can draw power from the supply only during one output state of the binary counter.

The "Scott" method of interconnection may be used as known in the prior art, where the first of said secondary windings is centre tapped and generates 1.154 times the voltage of the second secondary winding, one extremity of said second secondary winding being linked to the centre tap of said first secondary winding and the remaining extremities of said secondary windings being wired respectively to the said three electrodes which will be inserted into the said tissue. The set of windings may be duplicated in the reverse sense to provide six phases and therefore allow of six electrodes. It will be obvious that any reasonable number of separate electrodes may be accommodated by suitable combinations of windings, on the basis that each electrode will tend to be placed where its corresponding supply phase would appear on a vector diagram.

Alternatively the power source supplies pulses of energy at different times between successive pairs of electrodes. This may be achieved by a set of at least six switchable semiconductor elements of which series connected pairs are connected in parallel across a power supply which is centre tapped to earth.

An electrode is connected to each junction of said series connected pairs. All of the required permutations of switching which do not allow both members of a series connected pair to conduct simultaneously are then generated by an electronic counter counting at an appropriate multiple of the desired operating frequency.

Steps are taken, such as an intervening transformer and/or series connected capacitors, to ensure that said pulses are of alternating polarity and that no electrode can sustain a flow of direct or low frequency current.

In a simple preferred embodiment of this invention, a temperature sensor is incorporated in a sensing needle placed within the pattern of electrodes and the power applied to said electrodes is made responsive to the temperature indicated by said sensor: for example being cut off when said temperature reaches a predetermined limit or has exceeded this limit for a predetermined time, or the current may be controlled to produce a predetermined rate of temperature rise.

In case the electrode assembly has to be inserted through a narrow aperture in other tissue which should remain unaffected, an insulated conductive cylinder is placed surrounding said electrodes so that whether or not an "earth" connection is made to said cylinder the net power induced by stray capacitances to said electrodes, and capable of inducing a current flow to earth, will tend to zero and the electric field between said electrodes will not for practical purposes extend to a radius greater than that determined by said cylinder.

Further electrodes connected to earth, or to other electrodes as required, may be added to limit or modify the pattern of current flow.

Further embodiments of this invention relate to overcoming some remaining practical problems, in achieving an adequately uniform temperature distribution from needle shaped electrodes implanted in living tissue to convey a high frequency electric current, said electrodes serving to generate heat in the said tissue.

Within these embodiments I firstly provide that said electrodes are at least partly hollow or attached to tubular members.

Secondly I provide that said electrodes or tubular members may contain inbuilt temperature sensing elements, such as negative temperature coefficient thermistors or thermocouples known in the prior art.

Thirdly, the said electrodes may be linked to a high frequency current generator, preferably of the polyphase type, by individual twisted or coaxial cables each having a main or outer conductor connected to one of said electrodes, either directly or via a capacitor, and one or more inner conductors connected to those of said sensing elements contained within said one of said electrodes, said cables being extended to constitute the secondary windings of one or more power transformers which convey high frequency electrical power to said electrodes. The purpose is for the sensing elements with their connected wires to be electrically screened by said electrodes and said outer conductors from all external conducting objects.

Fourthly, said inner conductors branch or terminate within one or more screened enclosures which may be at points of minimum electrical potential of said outer conductors, and which are provided with an aperture in said screening which allows of connection via an isolating filter or transformer, or via an optically isolated link, to a controller. Alternatively said inner conductors may be partly or wholly supplanted by optical fibres.

Fifthly, I provide that the said cables or optical fibres may be placed within loose fitting flexible tubes, constructed of insulating material such as PTFE, the whole being preferably capable of biological sterilisation. One extremity of each of said flexible tubes is firmly attached to one of said electrodes, allowing gas or fluid conveyed by said flexible tube to escape after passing in good thermal contact with said electrode preferably via an annular space within said electrode and surrounding said tube.

Sixthly, the said flexible tubes pass through one or more enclosed chambers in which the said gas or fluid is maintained at a variable positive pressure and predetermined temperature, and are cut so that said gas or fluid is conveyed from said chamber(s) along said tubes to said electrodes.

Seventhly, I provide valves actuated by said controller, with a supply of compressed gas or fluid, in order to make the pressure within each of the said chambers responsive to one or more of the temperatures sensed within the system.

Eighthly, the said controller is adapted to respond to the temperatures registered by said sensors, preferably in combination with at least one "passive" temperature sensor as previously described which is not contained within an electrode energised by high frequency electrical power, in order by negative feedbacks to create an optimal correspondence between desired and actual temperatures This may be achieved by using principles such as those disclosed in my British patent no. 1224871, whereby signals at the output of said controller, increasing magnitudes of which serve to reduce electrical power levels, and similar signals increasing magnitudes of which which serve to initiate proportionate rates of air flow, become the variables in a Linear Programme model with temperature limitations as constraints, and some measure of unevenness of temperature serving if so required as an objective function.

Ninthly, said "passive" temperature sensor may be comprised within an electrode provided with voltage measurement means, to confirm whether a predicted pattern of electrical voltage exists within said tissue, or alternatively that a fault is probable such as a disconnected or missing electrode. Said measurement means will provide an audible or visual indication to the equipment operator, and/or effect a modification of the levels of electrical power supplied to said tissue.

Preferably, said controller is so configured that the temperatures registered by said inbuilt sensing elements cause an immediate increase in coolant flow as soon as any of them exceed a predetermined upper limit, so that excess temperature will be limited even though the electrical power which has caused it continues to be applied, producing a desired heating effect at points remote from the said electrode.

Claims (15)

1. A diathermy apparatus comprising three or more electrodes capable of being inserted into or surrounding a chosen volume of living tissue, said electrodes being energised by a polyphase or time multiplexed high frequency electrical generator so that high frequency currents will flow between them through said tissue and both the resultant magnitude and angular direction of current flow at points within said tissue vary with time over a repetitive cycle.

2. Apparatus according to claim 1 wherein said electrodes are energised by isolated transformer windings whereby there is no tendency for electric currents to flow from any of said electrodes to other electrodes which may be present in adjacent tissues, or to operating tables or equipment earth.

3. Apparatus according to any preceding claim wherein two high frequency electrical generators are constrained to operate in quadrature (i.e. with a constant 90 degrees phase difference) and the outputs of said generators are summed in different ratios, which may be positive or negative1 to provide the voltages energising the said electrodes.

4. Apparatus according to claim 3 in which the said generators feed transformer primary windings surrounding a common magnetic core, and the electrodes are energised by series combinations of winding turns positioned to have different mutual inductance couplings to one or both of said primary windings.

5. Apparatus according to claim 1 or claim 2 wherein the said electrodes are connected sequentially, at different times, to sources of high frequency electrical power.

6. Appaatus according to claim 1 wherein the said electrodes are connected sequentially to the positive and negative power rails of a direct current supply for equal proportions of the total time, with intervening means to ensure that no electrode can sustain a flow of direct or low frequency current.

7. Apparatus according to claim 1 wherein one or more of said electrodes contains temperature sensing means.

8. Apparatus according to claim 2 and claim 6 whereby said isolated transformer windings consist of co-axial or multi-stranded conductors, at least one pair of which allows connection between measurement or control apparatus and a temperature sensor mounted within one of said electrodes.

9. Apparatus according to claim 6 whereby the connections to said electrodes and/or said isolated transformer windings comprise both electrically conducting elements and one or more optical fibre links capable of communicating the output of a temperature sensor to measurement or control apparatus.

10. Apparatus according to any preceding claim in which the electrical connections to one or more of said electrodes are within a flexible insulating tube, connected to said electrode so that a flow of coolant gas passing along said tube discharges via an annular space in good thermal contact with the electrically conducting portion of said electrode.

11. Apparatus according to claim 9 in which said flexible insulating tube passes through an enclosed chamber which may be maintained at a positive pressure and predeterminable temperature said tube having an aperture so that coolant gas or fluid will pass to said electrode(s) in such volume as may be determined by a control valve.

12. Apparatus according to any preceding claim wherein the power output of a high frequency generator and/or a flow of coolant gas is responsive to the difference been a measured and a desired temperature within said tissue.

13. Apparatus according to claim 12 wherein said power outputs and/or gas flows are automatically controlled to constitute a composite function of the deviations between a pattern of desired and corresponding measured temperatures, with a view to minimising the aggregate disadvantage of said deviations in achieving a desired end such as reduction of a tumour: said composite function being implemented by the method of Linear Programming or Least Squares.

14. Apparatus according to any preceding claim wherein an additional electrode, unconnected to a source of high frequency electrical power, is connected to electrical measurement apparatus1 whereby if the voltage measured on said electrode with respect to other electrodes, or to an earth connection, exceeds a preset limit the power will be disconnected or other corrective action taken.

15. Apparatus according to any preceding claim wherein the source of high frequency power is derived from a storage battery, the stored energy of which determines the total amount of heat generated over the period of a single treatment.