Plasma surgical instrument and plasma generation method

Abstract

The instrument (10) according to the invention has an electrode (11) which is arranged in a gas-guiding lumen (16) and is held centered. The electrode (11) has an electrode body (22) which is made of a thermally stable material, such as hard metal, tungsten, steel, stainless steel, etc. The electrode (11) is equipped with a coating (23) which is made of a low-melting material, such as silver, a silver alloy or other low-melting metals. An adhesive layer (24), in particular a gold layer, can be provided between the coating (23) and the electrode body (22). This adhesive layer can contribute to the retraction of the coating (23) during the first commissioning and thus during the formation of the desired shape of the electrode (11) for operation.

Description

Field of the Invention

[0001] The present invention relates to an electrosurgical instrument, especially for plasma treatment of biological tissue, especially for plasma coagulation, and a plasma generation method. Background Art

[0002] A plasma coagulation instrument having an electrode configured as a flat plate and a plasma coagulation instrument having a linear electrode are known from DE 10030111 A1, and both are respectively arranged in the lumen of a hose-shaped fluid conductor. Due to their shape, the electrodes are respectively supported on the inner wall of the lumen, such that the tip of the corresponding electrode is centered in the lumen and remains fixed in position. The discharge emitted from the electrode forms a plasma jet with the gas flowing through the lumen.

[0003] A similar instrument having a linear or pen-shaped electrode is further known from WO 2005 / 046495 A1, and the linear or pen-shaped electrode is fixedly held in the center of the lumen of a hose-shaped fluid conductor. To fix the electrode, a metal plate that extends radially through the channel and is supported on the inner wall of the lumen with its long side is used, and the electrode is fixed to the metal plate. The electrode is formed of, for example, tungsten wire.

[0004] An electrosurgical electrode for contact coagulation is further known from EP 1743588 B1, and the electrosurgical electrode has an electrode body, and the core of the electrode body is made of molybdenum and plated with a silver alloy. The silver alloy is made of silver and 1.4% to 4% of germanium and 1% to 2% of indium. When cutting tissue, such an electrode is used to reduce the damage occurring on the tissue.

[0005] In the case of plasma coagulation instruments, a large amount of heat is generated on and around the electrode during operation. The electrode is usually arranged in a fluid conductor, and the fluid conductor may be damaged by the heat generated. The electrode itself may also be damaged. To minimize or avoid such damage, thermal protection for the fluid conductor has been provided in the past. Thus, DE 10030111 A1 mentioned above proposed to configure the electrode as a flat plate to improve its cooling. By the flat shape of the discharge part of the electrode, it is intended to avoid the probe from heating up. In contrast, according to WO 2005 / 046495 A1, a ceramic tube should be installed at the distal end of the fluid conductor, and the ceramic tube keeps the generated heat away from the plastic hose of the probe.

[0006] All the measures mentioned above limit the miniaturization of the instrument, especially when it comes to plasma coagulation instruments. Summary of the Invention

[0007] For this reason, the object of the present invention is to describe a design that can be used to further reduce the size of a plasma coagulation probe.

[0008] This object is solved by means of an electrosurgical instrument and also by means of a method:

[0009] The instrument has a fluid conductor which has at least one lumen, and an electrode is preferably arranged in a position-fixed and preferably centered manner in the lumen. The electrode consists of an electrode body which extends from its distal end in the proximal direction into the lumen, and a coating is arranged on the electrode body. The lumen can be connected to a gas source, in particular an argon gas source, such that the electrode is arranged in the fluid flow. The instrument can emit an axially or transversely directed plasma jet at the distal end during operation. The lumen can be realized as a channel of a hose or a pipe. The hose or the pipe can have one or more lumens.

[0010] The electrode is at least partially arranged in the lumen and is preferably axially adjustable or axially fixed in a centered manner for this purpose. For this purpose, the electrode can be fixed, for example, to a support that extends up to the interface of the lumen. Alternatively, the electrode itself can extend up to the interface in order to be supported on the interface.

[0011] In one embodiment of the present invention, the distal end of the electrode is placed inside the lumen. Thus, the plasma jet is formed in or on the distal part of the lumen. The end of the electrode does not protrude from the lumen and is thus protected against direct tissue contact.

[0012] However, the electrode can also be placed such that the distal end of the electrode protrudes from the lumen. The electrode can in particular carry an insulator at its distal end. The insulator is preferably made of an electrically insulating ceramic material.

[0013] The insulator can be configured as a sphere carried by the electrode. The insulator can also be configured as a hemisphere carried by the electrode. Here, the insulator can have its circular or its flat side facing the lumen. The insulator can also be configured as a cylinder with a rounded end side. The insulator can further be configured as a cone with a circular or flat base. Here, the insulator can have its base or its conical side facing the lumen. The insulator can also be configured as a disc. Preferably, the insulator is configured and arranged rotationally symmetrically with respect to the linear electrode.

[0014] A laterally directed opening can be left between the insulator and the distal end of the hose or the pipe, through which gas and / or plasma can escape. The opening can extend around the electrode by more than 360°. The opening can be configured, for example, as an annular groove or as an annular free area between the insulator and the end of the hose or the pipe. In this case, the insulator is only carried by the electrode. The insulator is not connected to the pipe or the hose or the elements arranged therein.

[0015] Laterally directed openings can also be divided into two or more sub-openings. For example, an insulator can connect one, two or more partitions to a hose or pipe, forming a lumen in the hose or pipe and arranging electrodes therein. One or more partitions can be connected to elements arranged in the lumen or elements arranged outside the pipe or hose, such as brackets. The bracket can be integrally formed with the partition and the insulator from ceramics.

[0016] In embodiments where the electrode extends out of the lumen, that part of the electrode where the discharge base point for forming plasma discharge is formed can be completely arranged outside the lumen. The electrode can extend out of the lumen by, for example, 1 mm to 3 mm.

[0017] Regardless of whether the electrode extends out of the lumen or is completely arranged in the lumen, the following applies:

[0018] A coating arranged on the electrode body coats the electrode body, and the cross-section of the coating at a distance of several millimeters from the distal tip accounts for at least 12% of the cross-sectional area of the entire electrode. Preferably, this applies to a relatively large part that extends at least several millimeters away from the distal end of the electrode in the proximal direction, regardless of the shape of the electrode.

[0019] According to the present invention, the coating at the distal end of the electrode is at least reduced or eliminated after a period of normal use and during this normal use, regardless of the geometry of the electrode. Alternatively, the distal end of the electrode may already be without a coating before the first use, or the coating may have a reduced thickness or be interrupted one or more times at the distal end. Within the framework of this specification, the distal end is understood to be an end portion of the electrode that can be 1 mm to 3 mm long. The distal end can be configured to be blunt, rounded, tapered or conical.

[0020] The electrode can be configured as a plate with a distal tip or as a pen-shaped or needle-shaped electrode (so-called wire electrode). The electrode is preferably arranged in a lumen of a fluid conductor in a position-fixed or also longitudinally movable manner by a suitable device, such as a bracket. The electrode is preferably centered.

[0021] Preferably, the coating of the electrode has a melting temperature lower than, preferably far lower than, the melting temperature of the electrode body. The electrode body is preferably made of a material with a melting point higher than 1000 °C. The electrode body can be made of steel, stainless steel, especially steel containing chromium and / or nickel, molybdenum, tungsten, hard metal or other preferably conductive materials. Preferably, the electrode body is made of a carbon-containing metal or a carbon-containing metal alloy. The carbon content is preferably greater than 0.02 weight percentage, preferably at least 0.05 weight percentage.

[0022] In contrast, the coating preferably has a low melting temperature of less than 1000 °C. Here, the melting temperature is preferably chosen to be low such that at least a part of the coating material melts at or near the distal end of the electrode under the operating conditions provided for the instrument. The operating conditions provided for the instrument are related to the gas flow and the electrical power provided at the patient during operation. Preferably, the coating material can be silver or a silver alloy. In particular, a material that reacts little or not at all with the gas flowing in the lumen is preferably used as the coating material.

[0023] By the measures mentioned, an electrode structure can be formed at the start of using the instrument, in which the electrode coating at the distal end of the electrode is absent or has a structure different from that of the remaining coating. Thereby, it can be achieved that the discharge is concentrated at the distal end of the electrode. Thereby, the heat input into the electrode is also concentrated at the distal end of the electrode. Thus, the electrode absorbs much less heat compared to known electrodes, in the case of which the discharge base point moves or jumps along the electrode.

[0024] At least after the initial commissioning, the electrode can have a rough surface at its distal end, where there are at least some islands without coating material. Preferably, the electrode substrate contains carbon, which forms accumulations in some places and is bare in some places. The carbon can be surrounded by the coating material or be located in an area without coating material. Carbon clusters can form the discharge base point.

[0025] The effects mentioned above occur especially in the case of an electrode with a diameter of less than 0.5 mm and a coating with a thickness exceeding the minimum size. This minimum size is reached if at least 10%, preferably 12%, of the cross-sectional area of the electrode consists of the coating material.

[0026] During operation, the coating can especially melt at the distal end and form an area that is wholly or partly liquid. The coating can retract slightly from the distal end of the electrode body and expose the electrode body wholly or partly. However, an increase in the service life of the electrode and a significant reduction in material erosion compared to an uncoated electrode are demonstrated.

[0027] It is shown that by concentrating the discharge at the outer distal end of the electrode, the input of thermal energy into the electrode is minimized. This effect is so strong that in at least some embodiments the fluid conductor can be formed entirely of plastic and the ceramic lining at the distal end is also redundant. Cooling of the electrode by means of a cooling body or other measures can also be dispensed with. Heat-insulating and / or electrically insulating materials such as ceramics or plastics can be used as the electrode support, without fear of damage to the instrument even in the case of long-term use.

[0028] This property is also facilitated if the electrode has, at its distal end, a portion, for example at least 2 mm long, preferably at least 2.5 mm long, which has a heat capacity of less than 4.5 mJ / K, preferably less than 4.17 mJ / K. The low heat capacity facilitates the positioning of the discharge and its fixation to the distal end of the electrode. The electrode quickly reaches its operating temperature at its distal end, at which the coating metal is at least partially liquid. The coating metal can be retracted from the hot electrode tip. The discharge base point is fixed to the hot electrode tip. The discharge base point does not move in the proximal direction and in particular does not cross the formed annular barrier made of molten coating metal, for example silver.

[0029] At the electrode, a very strong axial temperature gradient results, which has a strong drop between a high temperature at the distal end and a low temperature outside the approximately annular region in which the liquid coating material is present during operation.

[0030] The present invention also includes a design for constructing electrodes using one or more discharge base points anchored at the distal end of the electrode. A discharge base point is a point where electron escape is increased relative to the surrounding environment. These discharge base points are visible in the following locations, from which intense luminous streaks visible in the plasma are emitted.

[0031] The desired shaping of the electrode can also be achieved by shaping the electrode when it is first put into operation. Electrodes that have not yet been used can have a largely precisely defined geometry at their distal ends, in particular. The coating can extend with a thickness that is as constant as possible until the outermost distal end of the electrode. At the latest during the initial operation, one or more regions of the distal end of the electrode can be exposed or have a reduced coating thickness. Therefore, the electrode surface at the distal end is different from the electrode surface at the more proximal part of the electrode. The difference may be material and structural properties. The distal end of the electrode can have carbon particles at the surface, in particular. The difference between the surface of the distal end and the remaining electrode surfaces causes the discharge base point to be fixed to the outermost distal end of the electrode and therefore causes minimization of the heat input to the surrounding wall of the electrode and the fluid conductor. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Further details of advantageous embodiments of the invention are obtained from the drawings and the following description.

[0033] Figure 1 The apparatus and the equipment set up for feeding are shown schematically;

[0034] Figure 2 The distal portion of the instrument according to the invention is shown in an enlarged longitudinal section schematic diagram;

[0035] Figure 3 It shows that during operation Figure 2 The distal portion of the instrument;

[0036] Figure 4 shows a cross-section taken along line IV-IV through the electrode according to Figure 3 ;

[0037] Figure 5 shows the electrode in a longitudinal section in a longitudinal cut of its distal part before its first use;

[0038] Figure 6 shows the electrode according to Figure 5 in a molded state;

[0039] Figure 7 and 8 shows other embodiments of the electrode according to the invention before its first commissioning in a longitudinal section. Detailed description

[0040] Figure 1 Illustrates the instrument 10, which is designed as an endoscopic probe. The instrument is used for plasma coagulation, in particular argon plasma coagulation, that is to say, for treating human or animal tissue without direct physical contact between the electrode 11 of the instrument and the corresponding biological tissue. The instrument 10 is configured as a flexible probe. However, the principles set out below can also be implemented on rigid instruments suitable for laparoscopic use or for open surgical use.

[0041] The instrument 10 has a fluid conductor 12, for example in the form of a flexible hose 13, which extends from the distal end 14 to its proximal end 15. A lumen 16 runs through the length of the hose 13, which lumen comes in particular from Figure 2 . The lumen is flowed through by a gas, usually an inert gas, such as an argon stream, during operation. For this purpose, the instrument 10 is connected to a device 17, which contains a gas source 18 or provides a connection to such a gas source. During operation, the gas flows from the proximal end 15 of the hose 13 and thus the lumen 16 to the distal end 14 and out of the lumen from the open end of the hose 13.

[0042] An electrode 11 is arranged in the lumen 16. The distal end 19 of this electrode preferably does not protrude from the fluid conductor 12, but rather remains located in the lumen 16. However, in some embodiments, the electrode may also protrude partially from the fluid conductor 12 and / or the lumen 16. Preferably, the electrode 11 is a pen-shaped or needle-shaped electrode, which can be formed, for example, by a round or profiled wire or can also be formed by a tube or a sleeve. The electrode 11 can also be formed by the distal end of a wire extending through the lumen 16. The electrode 11 can have a substantially constant cross-section. Regardless of its specific shape, the electrode can preferably be held centered and fixed in position or axially movable in the lumen 16. The provided support 20 carries the electrode 11 and is internally supported on the fluid conductor 12 or the hose 13.

[0043] The electrode 11 is electrically connected to a generator 21, for example an HF generator, which outputs a high-frequency voltage to the electrode 11. The corresponding connecting conductor can extend from the electrode 11 through the entire lumen 16 up to the proximal end 15, at which the electrical connection provides contact with the generator 21.

[0044] The generator 21 is preferably set up to output a voltage that is high enough to generate a discharge at the tip of the electrode 11 and thereby at least partially ionize the air flow flowing along the electrode 11. A plasma jet for treating biological tissue is formed.

[0045] An important feature of the instrument 10 lies in the nature of the electrode 11. The electrode is, for example, configured as an elongated cylinder with a flat, round, conical or conical tip or is configured as an elongated cone as a whole. The electrode preferably has a diameter of less than 0.5 mm, preferably at most 0.3 mm, at least in the vicinity of its distal end 19. Therefore, the radius R visible from the Figure 4 is less than 0.25 mm, preferably less than 0.15 mm. Radii less than 0.1 mm are possible. However, the electrode 11 can also have a prismatic shape, for example by configuring the electrode as a profiled wire. The electrode can also be configured as a flat plate with a tip oriented in the distal direction.

[0046] Figure 4 Illustrates the cross-section of the electrode 11 at an axial distance of a few millimeters from the distal end 19 of the electrode 11. This distance is large enough that the structure of the electrode 11 remains unchanged during operation. As can be seen, the electrode 11 has an electrode body 22, which is equipped with a coating 23. The electrode body 22 and the coating 23 are made of different materials. Preferably, the melting temperature T of the electrode body 22 K is higher than 1000 °C. The electrode body 22 can also be made of other thermally stable conductive materials or can also be made of other thermally stable materials that are at least non-conductive in the cold state, such as ceramics.

[0047] The electrode body 22 can be made of a high melting point metal, such as steel, stainless steel, hard metal, molybdenum, tungsten, etc. In particular, alloys containing iron and / or chromium and / or nickel are suitable as the material of the electrode body 22. There may also be carbon and / or manganese and / or phosphorus and / or sulfur and / or silicon and / or nickel and / or nitrogen and / or molybdenum as additional alloying components. The stainless steel preferably used as the base material has the following composition:

[0048]

[0049] In contrast, the coating 23 is preferably at least partially made of a low melting point material with good electrical conductivity and a melting point preferably below 1000 °C. The coating 23 can be made of silver or a silver alloy, for example. The thickness D of the coating 23 is preferably at least large enough such that the cross-sectional area of the hatched part of the coating 23 in Figure 4 accounts for an area share of the total cross-sectional area of the electrode 11 greater than 10%, preferably greater than 12%. The total cross-sectional area is the cross-sectional area of a circle with radius R. This corresponds to the area of the cross-section of the electrode body 22 (hatched in the figure) and the area of the cross-section of the coating 23 (hatched in Figure 4 ) in Figure 4 . The stated relationship between the cross-sectional area of the coating 23 and the total cross-sectional area of the electrode 11 applies regardless of the specific cross-sectional shape of the coating and the electrode. Thus, the electrode 11 can have a cross-section delimited by a hollow cylinder or a polygon.

[0050] An intermediate layer 24 can be provided between the coating 23 and the electrode body 22. The intermediate layer can be made of a metal, preferably a noble metal, a noble metal alloy or an inert metal, such as nickel, a nickel alloy, gold or a gold alloy. The melting temperature T of the material of the intermediate layer 24 Z is preferably between the melting temperatures T of the materials of the electrode body 22 and the coating 23 K and T B (T K > T Z > T B ). The intermediate layer 24 can be used as an adhesion promoter and at the same time helps the molten coating 23 to retract from the electrode body 22 at the distal end 19.

[0051] Favorable thermal relationships are obtained while maintaining the mentioned parameters, that is, while maintaining the diameter of the electrode 11 to be less than 0.3 mm and the share of the cross-sectional area of the coating 23 in the total cross-sectional area of the electrode 11 to be greater than 10%, preferably greater than 12%. Thus, the instrument 11 can be designed to have delicate external dimensions. If necessary, the outer diameter of the fluid conductor 12 can be 1 mm or less.

[0052] The resulting miniaturization potential is based on low heat generation and heat radiation at and from the electrode 11. This is achieved by a combination of at least some of the above measures. In particular, it is thereby achieved that during operation, the discharge is concentrated at the distal end 19 of the electrode 11. The electrode has a preferably several millimeter long section 25 connected to the distal end 19. Here, the relationship between the cross-sectional areas of the electrode body 22 and the coating 23 described in connection with Figure 4 applies. Preferably, the section 25 terminates proximally in front of or at the support 20. However, the coating can also extend further proximally via the support 20 and continue. The section 25 terminates distally at the distal end 19 of the electrode 11. The distal end 190 begins in the region 26 visible from Figure 3 such that this region is so close to the formed discharge base point 27 that in this region 26, the material of the coating 23 is present or can be present in a liquid state during operation.

[0053] During operation, at least part of the electrode body 22 is exposed at the distal end 19. The exposed area forms the distal end 19. Starting from the end side of the electrode 11 in the proximal direction up to approximately 2 mm to 2.5 mm, a section 19a is formed, the heat capacity of which is preferably less than 4.5 mJ / K, more preferably less than 4.17 mJ / K. The section 19a can be formed by the distal end 19. The low heat capacity of the section 19a enables local melting of the coating 23. Even at low HF power, this section ensures that the end 19 continuously emits electrons immediately after plasma ignition. This promotes plasma discharge concentration at the distal end of the electrode and thus reduces the heat input into the electrode.

[0054] The instrument 10 described in this regard is used as follows and its electrode 11 is operated as follows:

[0055] During operation, first the lumen 16 is loaded with gas such that a gas flow in the distal direction is obtained. For example, argon can be used as the gas, and the argon flows along the electrode 11 in the downstream direction. The electrode 11 is electrically connected to the generator 21. The voltage applied at the distal end 19 causes a spark discharge to a corresponding electrode located nearby, which corresponding electrode can be, for example, biological tissue.

[0056] Just before or shortly after the process starts, the electrode 11 has the initial shape elucidated in Figure 5 which is geometrically specific. For example, the electrode body 22 is cylindrical, while the coating 23 has a substantially constant thickness everywhere. The coating 23 extends a few millimeters or centimeters in the proximal direction from the distal end 19 and can then terminate or continue. The coating 23 can extend beyond the end side of the electrode body 22 or can also leave the end side free, as can be seen from according to Figure 8can be seen from the examples. The coating 23 can also already be removed from the end region, for example from the distal end 19, during the production of the electrode, as Figure 7 shown. For this purpose, the distal end 19 of the electrode 11 can be configured to be tapered, configured as a frustum or can also be configured as a wedge. For example, the electrode 11 can be produced by cutting a sufficiently long section from a continuously supplied coated wire. If necessary, the distal end 19 can be machined by means of material erosion in order to remove the coating 23 at the end 19 completely or partially.

[0057] With the first commissioning, first the distal end 19 of the electrode 11 is heated so that the electrode is then adapted for continued operation. A region 26 is formed on the electrode 11 in which the material of the coating 23 is at least partially melted, as Figure 6 illustrated. In the region 26, the coating can be thicker than in the rest of the electrode 11. In contrast, the coating at the outer end 19 can have a smaller thickness, be interrupted or be completely absent.

[0058] The process described above in connection with the first commissioning can also be carried out within the framework of the manufacture of the instrument 10. For this purpose, the manufacturer can briefly commission the instrument 10 under controlled conditions. Here, the manufacturer can specify the gas type and gas flow as well as the voltage and current for operation at the patient. However, the manufacturer can also select a different gas type, gas flow or operating voltage or current.

[0059] During operation, the distal end 19 of the electrode 11 gets hot and is able to emit electrons, while the electrode 11 reaches a much lower temperature in the region 26 and especially closer to the proximal end of the part 25 and remains relatively cold there. The discharge base point 27 ( Figure 3 ) is fixed to the distal end 19 and does not move in the proximal direction. Thus, the electrode 11 dissipates very little heat and does not contribute significantly to the heating of the fluid conductor 12.

[0060] The instrument 10 according to the invention has an electrode 11 which is arranged in the gas-guiding lumen 16 and is held centered. The electrode 11 has an electrode body 22 which is made of a heat-stable material, such as hard metal, tungsten, steel, stainless steel and the like. The electrode 11 is equipped with a coating 23 which is made of a low-melting material, such as silver, silver alloy or other low-melting metals. An adhesive layer 24, especially a gold layer, can be provided between the coating 23 and the electrode body 22. This adhesive layer can contribute to the retraction of the coating 23 during the first commissioning and thus during the formation of the desired shape of the electrode 11 for operation (for example according to Figure 6 ).

[0061] Reference numerals:

[0062] 10 Instrument

[0063] 11 Electrode

[0064] 12 Fluid Conductor

[0065] 13 Hose

[0066] 14 Distal End of Fluid Conductor 12 / Hose 13

[0067] 15 Proximal End of Fluid Conductor 12 / Hose 13

[0068] 16 Lumen

[0069] 17 Device

[0070] 18 Gas Source

[0071] 19 Distal End of Electrode 11

[0072] 19a Portion of Electrode 11 (Length of 2 to 2.5 mm)

[0073] 20 Bracket

[0074] 21 Generator

[0075] 22 Electrode Body

[0076] 23 Coating

[0077] 24 Intermediate Layer

[0078] 25 Portion

[0079] 26 Region Where Coating Material Can Be Liquid

[0080] 27 Discharge Base Point.

Claims

1. An electrosurgical instrument (10), The instrument has a fluid conductor (12) having at least one lumen (16), The instrument has an electrode (11) at least partially disposed in the fluid conductor (12) and having an electrode body (22) that extends from its distal end (19) in a proximal direction into the fluid conductor (12), and a coating (23) is disposed on the electrode body, Characterized in that, The electrode body (22) is made of stainless steel and the coating (23) is made of silver or a silver alloy, Wherein the electrosurgical instrument (10) is configured for plasma treatment of biological tissue.

2. The instrument according to claim 1, characterized in that The instrument is used for plasma coagulation.

3. The instrument according to claim 1 or 2, characterized in that, The coating (23) has a melting temperature (T B ), and the electrode body (22) has a melting temperature (T K ), and the melting temperature (T B ) of the coating is lower than the melting temperature (T K ) of the electrode body (22).

4. The instrument according to claim 1 or 2, characterized in that, The electrode body (22) is made of a material with a melting temperature (T K ) higher than 1000 °C.

5. The instrument according to claim 1 or 2, characterized in that, The coating (23) is made of a material with a melting temperature (T B ) below 1000 °C.

6. The instrument according to claim 1 or 2, characterized in that, The coating (23) has a cross-section with a coated area and the electrode body (22) has a cross-section with an electrode area, wherein the coated area is at least 12% of the electrode area.

7. The instrument according to claim 1 or 2, characterized in that, The electrode (11) has a portion (19a) at its distal end that is at least 2.5 mm long and has a heat capacity of less than 4.5 mJ / K.

8. The instrument according to claim 7, characterized in that, The heat capacity of the portion is less than 4.17 mJ / K.

9. The instrument according to claim 1 or 2, characterized in that, The electrode (11) has a distal end (19) that is held in the lumen (16).

10. The instrument according to claim 1 or 2, characterized in that, To dispose the electrode (11) in the lumen (16), a bracket (20) is provided, the electrode (11) is held on the bracket and the electrode (11) is centered in the lumen (16) by the bracket.

11. The instrument according to claim 10, characterized in that, The bracket (20) is made of a material with poor thermal conductivity.

12. The instrument according to claim 10, characterized in that, The bracket (20) is made of an electrically insulating material.

13. The instrument according to claim 1 or 2, characterized in that, The coating (23) has a melting region (26) near the distal end (19) during operation.

14. The instrument according to claim 1 or 2, characterized in that, The electrode (11) has a portion (19a) at its distal end (19) during operation that is at least partially free of the material of the coating (23).

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