Electrosurgical instrument

• US 6,004,319 A
• Filed: 08/26/1996
• Issued: 12/21/1999
• Est. Priority Date: 06/23/1995
• Status: Expired due to Term

• Current Assignee: Gyrus Medical Incorporated (Olympus Corporation)
• Original Assignee: Gyrus Medical Incorporated (Olympus Corporation)
• Inventors: Goble, Colin Charles Owen, Goble, Nigel Mark
• Primary Examiner(s): PEFFLEY, MICHAEL F
• Application Number:
  US08/702,512
• Time in Patent Office: 1,212 Days
• Field of Search: 606/32-35, 606/37-42, 606/45-50, 607/100-105, 607/115, 607/116, 128/642, 600/374, 604/21, 604/22, 604/114
• US Class Current: 606/48
• CPC Class Codes: A61B 18/1206 Generators therefor
• CPC Class Codes: A61B 18/14 Probes or electrodes therefor
• CPC Class Codes: A61B 18/1482 having a long rigid shaft for accessing the inner body transcutaneously in minimal invasive surgery, e.g. laparoscopy
• CPC Class Codes: A61B 18/1485 having a short rigid shaft for accessing the inner body through natural openings
• CPC Class Codes: A61B 18/1492 having a flexible, catheter-like structure, e.g. for heart ablation A61B18/1477 takes precedence
• CPC Class Codes: A61B 2018/126 bipolar
• CPC Class Codes: A61B 2018/1472 for use with liquid electrolyte, e.g. virtual electrodes

Abstract

In an electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium a bipolar electrode assembly has an active electrode with an exposed tissue treatment portion, a return electrode having an exposed fluid contact surface, and an insulating member positioned between and electrically insulating the active electrode in the return electrode. The insulating member serves to space apart the exposed active electrode treatment portion and the exposed fluid contact portion of the return electrode. The dimensions and configurations of the exposed portions of the electrodes and of the insulating member are such that when the electrode assembly is immersed in a conductive fluid medium, the ratio between the longest and shortest conduction path lengths between the active and return electrodes is less than or equal to 2:1. The invention also includes a combination of an electrosurgical instrument and a radio frequency generator.

Claims

1. An electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium, comprising an instrument shaft and an electrode assembly at a distal end of the shaft, wherein the electrode assembly comprises:

a single active electrode having an exposed tissue treatment portion,

a return electrode having an exposed fluid contact surface, the fluid contact surface being substantially parallel to the electrode assembly'"'"'s longitudinal axis, such that the fluid contact surface faces substantially perpendicular to said longitudinal axis, and

an insulating member positioned between and electrically insulating the active electrode and the return electrode and axially spacing apart the exposed treatment portion of the active electrode and the exposed fluid contact surface of the return electrode such that, when the treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the fluid medium completes a conduction path between the active electrode and the return electrode,

and wherein the exposed treatment portion is dimensioned and configured in a fixed relationship to the exposed fluid contact surface and the insulation member, such that, when the electrode assembly is immersed in the conductive fluid medium, the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1.

2. An instrument according to claim 1, wherein the return electrode comprises a conductive sleeve located around the insulation member behind the treatment portion of the active electrode.

3. An instrument according to claim 1, wherein the treatment portion of the active electrode is located at an extreme distal end of the assembly and the fluid contact surface of the return electrode is spaced proximally from the active electrode treatment portion, and wherein the exposed portion of the active electrode has a length and a width, the length being greater than at least one half of the width.

4. An instrument according to claim 3, wherein the longitudinal spacing of the active electrode exposed portion and the return electrode fluid contact surface is at least 1 mm.

5. An instrument according to claim 4, wherein the ratio of (i) the longitudinal distance between a distal end of the active electrode exposed portion and a most distal part of the return electrode, to (ii) the shortest longitudinal distance between the active electrode exposed portion and the most distal part of the return electrode, is less than or equal to 2 to 1.

6. An instrument according to claim 3, wherein the return electrode has a fluid contact surface encircling the insulation member and wherein the ratio of (i) the longitudinal distance between a distal end of the active electrode exposed portion and a distal edge of the fluid contact surface of the return electrode to (ii) a circumference of the fluid contact surface adjacent to its distal edge is less than or equal to 1.43:

1.

7. An instrument according to claim 1, wherein the instrument shaft comprises a metallic tube as its main structural element, and the return electrode is an integrally formed distal end portion of the tube.

8. An instrument according to claim 1, wherein the exposed portion of the active electrode extends longitudinally from the distal end of the shaft.

9. An instrument according to claim 8, wherein the insulation member comprises a generally conic member that tapers towards the distal end of the instrument.

10. An instrument according to claim 1, wherein the insulation member comprises a generally cylindrical sleeve and the return electrode is located on the outside of the sleeve longitudinally spaced from the exposed portion of the active electrode by a distance of at least 1 mm.

11. An instrument according to claim 10, wherein the insulation member has an annular distal end face defining a shoulder, and the active electrode exposed portion is centrally located with respect to and projects from the insulation member end face, the shoulder having a depth in a direction laterally away from the active electrode being between 0.05 l and 0.5 l, where l is the length of the central active electrode exposed portion.

12. An instrument according to claim 10, wherein the shortest conduction path through the fluid medium between the return electrode fluid contact surface and the active electrode exposed portion is at least 1 mm in length.

13. An instrument according to claim 1, wherein the return electrode fluid contact surface is annular and has a length and a diameter, the length of the fluid contact surface being at least as great as its diameter, and wherein the ratio of (i) the shortest conduction path through the fluid medium between the return electrode fluid contact surface and that part of the active electrode exposed portion which is furthest from the fluid contact surface, to (ii) the fluid contact surface diameter is at most 4.5 to 1.

14. An instrument according to claim 1, wherein the exposed treatment portion of the active electrode extends along said instrument'"'"'s longitudinal axis and said longitudinal axis corresponds to a treatment axis.

15. An instrument according to claim 1, wherein the exposed treatment portion of the active electrode extends in a direction perpendicular to said instrument'"'"'s longitudinal axis and said direction of extension corresponds to a treatment axis.

16. An instrument according to claim 1, wherein the exposed treatment portion of the active electrode extends in a direction that does not correspond to said instrument'"'"'s longitudinal axis and said non-corresponding direction of extension corresponds to a treatment axis.

17. An electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium, comprising an instrument shaft and an electrode assembly at a distal end of the shaft, wherein the electrode assembly comprises:

a single active electrode fixedly positioned in the electrode assembly and having an exposed tissue treatment portion,

a return electrode having an exposed fluid contact surface, and

an insulating member positioned between and electrically insulating the active electrode and the return electrode and axially spacing apart the exposed treatment portion of the active electrode and the exposed fluid contact portion of the return electrode such that, when the treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the fluid medium completes a conduction path between the active electrode and the return electrode,

and wherein the exposed treatment portion is dimensioned and configured in relation to the exposed fluid contact surface and the insulation member such that, when the electrode assembly is immersed in the conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1.

18. An instrument according to claim 17, wherein the exposed treatment portion of the active electrode projects in a first direction from the insulation member, the fluid contact surface of the return electrode is set back from the active electrode treatment portion, and the insulating member surrounds the active electrode and, between the active electrode exposed portion and the return electrode fluid contact surface, projects outwardly in a second direction perpendicular to the first direction to define an insulation barrier to divert electrical current flow through the fluid medium thereby to increase said shortest conduction path length (P₂) between the exposed fluid contact surface and the exposed treatment portion.

19. An instrument according to claim 18, wherein the first direction defines a treatment axis along which the exposed treatment portion and the exposed fluid contact portions are spaced apart, and said two shortest conduction paths (P₁, P₂) lie in a common plane containing the treatment axis.

20. An instrument according to claim 17, wherein the length of said shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion is at least 1 mm.

21. An instrument according to claim 17, wherein the exposed fluid contact surface is generally cylindrical and has a length and a diameter, the length of the fluid contact surface being at least as great as its diameter and wherein the ratio of (i) the shortest conduction path (P₁) through the fluid medium between the fluid contact surface and that part of the exposed treatment portion which is furthest from the fluid contact surface, to (ii) the fluid contact surface diameter, is at most 4.5 to 1.

22. An instrument according to claim 17, wherein the exposed treatment portion of the active electrode extends along said instrument'"'"'s longitudinal axis and said longitudinal axis corresponds to a treatment axis.

23. An instrument according to claim 17, wherein the exposed treatment portion of the active electrode extends in a direction perpendicular to said instrument'"'"'s longitudinal axis and said direction of extension corresponds to a treatment axis.

24. An instrument according to claim 17, wherein the exposed treatment portion of the active electrode extends in a direction that does not correspond to said instrument'"'"'s longitudinal axis and said non-corresponding direction of extension corresponds to a treatment axis.

25. An electrosurgical system comprising an electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium comprising an instrument shaft and an electrode assembly at a distal end of the shaft and an electrosurgical generator for supplying radio frequency power to the instrument,

the electrode assembly comprising:

a single active electrode fixedly positioned in the electrode assembly and having an exposed tissue treatment portion,

a return electrode having an exposed fluid contact surface, and

an insulating member positioned between and electrically insulating the active electrode and the return electrode and axially spacing apart the exposed treatment portion of the active electrode and the exposed fluid contact portion of the return electrode such that, when the treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the fluid medium completes a conduction path between the active electrode and the return electrode,

and wherein the exposed treatment portion is dimensioned and configured in relation to the exposed fluid contact surface and the insulation member such that, when the electrode assembly is immersed in the conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1,

the generator including an output stage having an output voltage applied to output connections connected respectively to the active electrode and the return electrode of the instrument, a sensing circuit for deriving a sensing signal representative of the peak radio frequency output voltage developed between the output connection, and a power adjustment circuit for automatically causing a reduction in delivered output power when the sensing signal is indicative of a predetermined peak radio frequency output voltage having been reached.

26. A system according to claim 25, wherein the power adjustment circuit is operable to cause at least a 50% reduction in delivered output power when the sensing signal is indicative of said predetermined peak radio frequency output voltage having been reached, said reduction being effected with a period of conduction of 100 μ

s or less.

27. A system according to claim 26, wherein the power adjustment circuit is operable to effect said reduction in a period of 20 μ

s or less.

28. A system according to claim 26, wherein the output stage includes at least one radio frequency power device and wherein the control circuitry is arranged such that the at least 50% reduction in output power is effected by reducing the period of conduction of the device during individual cycles of radio frequency oscillation independently of a supply voltage to the device.

29. A system according to claim 28, wherein the sensing circuit and the power adjustment circuit are operable repeatedly to effect a rapid reduction in the period of conduction of the device from a peak level to a trough level followed by a less rapid progressive increase in the conduction period until the conduction period again reaches its peak level, the rapid reduction and progressive increase sequence being repeated while simultaneously reducing the supply voltage to said output stage until said peak conduction period level can be reached without the output voltage exceeding said predetermined threshold.

30. An electrosurgical apparatus for the treatment of tissue in the presence of an electrically conductive fluid medium, comprising an electrosurgical instrument and an electrosurgical generator, the electrosurgical instrument comprising an instrument shaft and an electrode assembly at a distal end of the shaft, wherein the electrode assembly comprises:

a single active electrode having an exposed tissue treatment portion,

a return electrode having an exposed fluid contact surface, and

an insulating member positioned between and electrically insulating the active electrode and the return electrode and axially spacing apart the exposed treatment portion of the active electrode and the exposed fluid contact surface of the return electrode such that, when the treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the fluid medium completes a conduction path between the active electrode and the return electrode,

the generator applying sufficient radio frequency output power to the active and return electrodes in a first voltage range to increase the temperature of the conductive fluid adjacent the active electrode treatment portion to near boiling without creating a vapour pocket surrounding the treatment portion when the electrosurgical instrument is used for desiccation and in a second voltage range to thermally evaporate conductive fluid adjacent the active electrode treatment portion to create a vapour pocket surround the treatment portion when the electrosurgical instrument is used for cutting or ablation; and

wherein the exposed treatment portion is dimensioned and configured in a fixed relationship to the exposed fluid contact surface and the insulation member, such that, when the electrode assembly is immersed in the conductive fluid medium, the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1.

31. An electrosurgical instrument for the treatment of tissue in the presence of an electrically conductive fluid medium, comprising an electrosurgical instrument and an electrosurgical generator, the electrosurgical instrument comprising an instrument shaft and an electrode assembly at a distal end of the shaft, wherein the electrode assembly comprises:

a single active electrode having an exposed tissue treatment portion,

a return electrode having an exposed fluid contact surface, and

an insulating member positioned between and electrically insulating the active electrode and the return electrode axially spacing apart the exposed treatment portion of the active electrode and the exposed fluid contact surface of the return electrode such that, when the treatment portion is brought adjacent to a tissue surface immersed in the fluid medium, the fluid medium completes a conduction path between the active electrode and the return electrode,

the generator applying sufficient radio frequency output power to the active and return electrodes in a first voltage range to increase the temperature of the conductive fluid adjacent the active electrode treatment portion to near boiling without creating a vapour pocket surrounding the treatment portion when the electrosurgical instrument is used for desiccation and in a second voltage range to thermally evaporate conductive fluid adjacent the active electrode treatment portion to create a vapour pocket when the electrosurgical instrument is used for cutting or ablation and;

wherein the exposed treatment portion is dimensioned and configured in a fixed relationship to the exposed fluid contact surface and the insulation member, such that, when the electrode assembly is immersed in the conductive fluid medium, the length of the shortest conduction path through the fluid medium between the exposed fluid contact surface and the exposed treatment portion is large enough to prevent direct arcing between the exposed treatment portion and the fluid contact surface of the return electrode, and

wherein the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1.

32. A method of desiccating tissue using a bipolar electrode assembly, the assembly including a active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface spaced and set back axially from the exposed treatment portion, the exposed treatment portion being dimensioned and configured in a fixed relationship to the exposed fluid contact surface, such that, when the electrode assembly is immersed in a conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium, between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is greater than or equal to a ratio of 1.25 to 1, but less than or equal to a ratio of 2 to 1 to ensure that power applied to the exposed treatment portion is controlled without the formation of a vapor pocket surrounding the exposed treatment portion, the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a conductive fluid so that the conductive fluid defines an electrical path between the active and return electrodes;

(c) applying sufficient radio frequency output power to the electrode assembly to increase the temperature of the conductive fluid adjacent the active electrode treatment portion without creating a vapour pocket surrounding the treatment portion, said output power being subsequently reduced by at least 50% within a predetermined period of time to maintain a desiccating mode of operation; and

(d) contacting the treatment portion to tissue while maintaining the return electrode fluid contact surface out of contact with the tissue.

33. The method of claim 32, wherein step (d) includes maintaining a part of the exposed treatment portion of the active electrode out of contact with the tissue.

34. The method of claim 33, wherein step (d) includes the further step of:

(e) moving the active electrode across a surface of the tissue.

35. The method of claim 34, wherein step (e) includes moving the electrode across the tissue surface in a side-to-side motion.

36. The method of claim 32, wherein step (c) includes maintaining the temperature of the conductive fluid adjacent to the active electrode treatment portion substantially at the boiling point of the conductive fluid.

37. The method of claim 32, wherein the conductive fluid comprises a saline solution.

38. The method of claim 32, wherein the conductive fluid comprises a compound sodium lactate solution.

39. The method of claim 32, wherein the step of applying power includes applying sufficient radio frequency power to the electrode assembly to ensure maximum power distribution and the greatest wetted area of the active electrode treatment portion.

40. A method of vaporising tissue using a bipolar electrode assembly, the bipolar electrode assembly including an active electrode fixedly mounted in the electrode assembly and a return electrode axially spaced from the active electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface, the exposed treatment portion being dimensioned and configured in relation to the exposed fluid contact surface such that, when the electrode assembly is immersed in a conductive fluid medium, the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to a ratio of 2 to 1, but greater than or equal to a ratio of 1.25 to 1 to ensure that the exposed treatment portion is enveloped in a vapor pocket, the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a conductive fluid;

(c) applying sufficient radio frequency output power to the electrode assembly to vaporise the conductive fluid adjacent the active electrode treatment portion to create a vapour pocket surrounding the treatment portion, said output power being reduced by at least 50% with a predetermined period of time to maintain a vaporising mode of operation; and

(d) positioning the treatment portion of the active electrode adjacent the tissue with the vapour pocket in contact with the tissue while maintaining the return electrode out of contact with the tissue.

41. The method of claim 40, wherein step (d) includes the further step of:

(e) moving the active electrode treatment portion over a surface of the tissue.

42. The method of claim 41, wherein step (e) includes moving the electrode over the tissue surface in a side-to-side motion.

43. The method of claim 41, wherein step (e) includes moving the active electrode over the surface of the tissue to contour the tissue.

44. The method of claim 40, wherein the conductive fluid comprises a saline solution.

45. The method of claim 40, wherein the conductive fluid comprises a compound sodium lactate solution.

46. The method of claim 40, wherein the treatment portion of the active electrode is a distal end portion and the exposed fluid contact surface of the return electrode is proximally spaced from the treatment portion, and wherein step (a) includes positioning a part of the exposed treatment portion adjacent and from time to time in contact with the tissue.

47. A method of desiccating tissue using a bipolar electrode assembly, the assembly including an active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface radially spaced and axially set back from the exposed treatment portion, the active electrode and return electrode being in a fixed relationship during use, such that, when the electrode assembly is immersed in a conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1, the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a conductive fluid so that the conductive fluid defines an electrical path between the active and return electrodes;

(c) applying sufficient radio frequency output power to the electrode assembly to increase the temperature of the conductive fluid adjacent the active electrode treatment portion without creating a vapour pocket surrounding the treatment portion;

(d) contacting the treatment portion to tissue while maintaining the return electrode fluid contact surface out of contact with the tissue.

48. The method of claim 47, wherein step (d) includes maintaining a part of the exposed treatment portion of the active electrode out of contact with the tissue.

49. The method of claim 48, wherein step (d) includes the further step of:

(e) moving the active electrode across a surface of the tissue.

50. The method of claim 49, wherein step (e) includes moving the electrode across the tissue surface in a side-to-side motion.

51. The method of claim 47, wherein step (c) includes maintaining the temperature of the conductive fluid adjacent to the active electrode treatment portion substantially at the boiling point of the conductive fluid.

52. The method of claim 47, wherein the conductive fluid comprises a compound saline solution.

53. The method of claim 47, wherein the conductive fluid comprises a compound sodium lactate solution.

54. The method of claim 47, wherein the step of applying power includes applying sufficient radio frequency power to the electrode assembly to ensure maximum power distribution and the greatest wetted area of the active electrode treatment portion.

55. A method of vaporising tissue using a bipolar electrode assembly, the assembly including an active electrode and a return electrode, the active electrode having an exposed treatment potion, and the return electrode having an exposed fluid contact surface radially spaced and axially set back from the exposed treatment portion, the active electrode and return electrode being in a fixed relationship during use, such that, when the electrode assembly is immersed in a conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1, the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a conductive fluid;

(c) applying sufficient radio frequency output power to the electrode assembly to vaporise the conductive fluid adjacent the active electrode treatment portion to create a vapour pocket surrounding the treatment portion;

(d) positioning the treatment portion of the active electrode adjacent the tissue with the vapour pocket in contact with the tissue while maintaining the return electrode out of contact with the tissue.

56. The method of claim 55, wherein step (d) includes the further step of:

(e) moving the active electrode treatment portion over a surface of the tissue.

57. The method of claim 56, wherein step (e) includes moving the active electrode over the surface of the tissue in a side-to-side motion.

58. The method of claim 56, wherein step (e) includes moving the active electrode over the surface of the tissue to contour the tissue.

59. The method of claim 55, wherein the conductive fluid comprises a saline solution.

60. The method of claim 55, wherein the conducive fluid comprises a compound sodium lactate solution.

61. The method of claim 55, wherein the treatment portion of the active electrode is a distal end portion and the exposed fluid contact surface of the return electrode is proximally spaced from the treatment portion, and wherein step (a) includes positioning a part of the exposed treatment portion adjacent and from time to time in contact with the tissue.

62. A method of operating a bipolar electrode assembly in two output voltage ranges, the assembly including an active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface radially spaced and axially set back from the exposed treatment option, the active electrode and return electrode being in a fixed relationship during use such that, when the electrode assembly is immersed in a conductive fluid medium the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1 the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a first fluid constituting a conductive liquid;

(c) applying sufficient radio frequency output power in a first voltage range to the electrode assembly to thermally evaporate the first fluid adjacent the active electrode treatment portion to create a second fluid, the second fluid constituting a vapour pocket;

(d) applying sufficient radio frequency output power in a second voltage range to the electrode assembly to maintain the second fluid adjacent the active electrode treatment portion without producing thermal disintegration of said active electrode;

(e) positioning the treatment portion of the active electrode adjacent the tissue with the vapour pocket in contact with the tissue while maintaining the return electrode out of contact with the tissue.

63. A method of operating a bipolar electrode assembly in two output voltage ranges, the assembly including an active electrode and a return electrode, the active electrode having an exposed treatment portion, and the return electrode having an exposed fluid contact surface radially spaced and axially set back from the exposed treatment portion in a fixed relationship during use, such that, the length of the shortest conduction path through a fluid medium between the exposed fluid contact surface and the exposed treatment portion is large enough to prevent direct arcing between the exposed treatment portion and the fluid contact surface, and wherein the ratio of (i) the length of the shortest conduction path (P₁) through the fluid medium between the exposed fluid contact surface and that part of the exposed treatment portion which is furthest f from the exposed fluid contact surface, to (ii) the length of the shortest conduction path (P₂) through the fluid medium between the exposed fluid contact surface and the exposed treatment portion, is less than or equal to 2 to 1, but greater than or equal to 1.25 to 1, the method comprising the steps of:

(a) introducing the electrode assembly into a selected operation site;

(b) surrounding the electrode assembly with a first fluid constituting a conductive liquid; and

either

(c) applying sufficient radio frequency output power in a first voltage range to the electrode assembly to increase the temperature of the fluid adjacent the active electrode treatment portion to near boiling without creating a vapour pocket surrounding the treatment portion when the electrosurgical instrument is used for desiccation;

or

(d) applying sufficient radio frequency output power in a second voltage range to the electrode assembly to thermally evaporate the fluid adjacent the active electrode treatment portion to create a vapour pocket surrounding the treatment portion when the electrosurgical instrument is used for cutting or ablation.