Electrosurgical generator, electrosurgical system, and method of operating an electrosurgical generator

Abstract

The present invention relates to an electrosurgical generator, an electrosurgical system and a method for operating an electrosurgical generator. The electrosurgical generator is configured to deliver high-frequency alternating current to an electrosurgical instrument for plasma cutting of body tissue during operation. The electrosurgical generator has an output terminal for connecting an electrosurgical instrument to provide high-frequency alternating current to the electrosurgical instrument connected to the output terminal and determine the impedance of a load connected to the output terminal. The electrosurgical generator has an impedance determination unit and a voltage detection unit and an output voltage control unit. The output voltage adjustment unit is designed to control the output alternating voltage according to an output voltage maximum value, which is set during operation according to an output value of the impedance determination unit and / or according to an output value of the voltage detection unit so that the output voltage maximum value in the vaporization phase specifies an output alternating voltage that is lower than the output alternating voltage in the ignition phase after the vaporization phase.

Description

Technical Field

[0001] The present invention relates to an electrosurgical generator configured to supply high frequency alternating current to an electrosurgical instrument for plasma cutting of body tissue, an electrosurgical system including the electrosurgical generator and the electrosurgical instrument, and a method of operating the electrosurgical generator. Background Art

[0002] With the aid of electrosurgery, biological tissue (ie body tissue) can be cut, coagulated (vascular occlusion) and / or vaporized. For electrosurgery, high-frequency alternating current with a frequency between 0.2 MHz and 3 MHz is usually used.

[0003] An electrosurgical system generally includes an electrosurgical generator for generating high-frequency alternating current. An electrosurgical generator generally has two output terminals to which an electrosurgical instrument can be connected and between which a high-frequency alternating voltage is provided during operation. To this end, an electrosurgical generator generally includes a high-voltage power supply part that generates direct current during operation and a high-frequency part that is connected to the high-voltage power supply part and generates high-frequency alternating current from direct current during operation.

[0004] Electrosurgical systems are used, for example, in urology or gynecology. In particular, electrosurgical generators are often used in combination with suitable electrosurgical instruments, such as resectoscopes, for plasma vaporization, for example within the TURIS range (transurethral resection in saline) for the treatment of benign prostatic hyperplasia. The active cutting electrode or vaporization electrode of such an electrosurgical instrument is located in an electrically conductive irrigation fluid, such as a saline solution (NaCl). With the aid of a high-frequency alternating voltage, an arc can be ignited at the active electrode of the electrosurgical instrument (also called plasma ignition).

[0005] Body tissue can be vaporized or removed by means of a plasma by generating an arc in the gas volume around the active electrode of an electrosurgical instrument, which plasma is generated in a vapor bubble (i.e., gas bubble) around the electrode in a conductive liquid compatible with the body, especially a saline solution. For this purpose, a high-frequency alternating voltage is applied to the electrode, the alternating current caused by which first causes the saline solution in the immediate vicinity of the active electrode to evaporate, thereby generating a vapor bubble around the electrode. Within the vapor bubble, an electric field is formed between the electrode and the liquid saline solution. When the electric field is strong enough, a gas discharge called an arc occurs due to the ionization of the gas in the vapor bubble, that is, a plasma (ionized gas) occurs.

[0006] Before the plasma and thus the arc can occur, the conductive liquid around the electrode must be evaporated in an initial phase, and subsequently the arc must be ignited in the vapor bubble that has thus emerged. This presupposes that the vapor bubble completely surrounds the electrode and that there is no longer any conductive liquid on the electrode, because otherwise the strong electric field required for ionization of the gas in the vapor bubble would not occur, because the direct current flow between the active electrode and the conductive liquid prevents the potential difference required for ionization.

[0007] After the initial ignition of the arc (i.e. after the initial formation of the plasma), a dynamically stable state occurs in which a strong electric field always appears at the point where the gas bubbles are about to collapse around the electrode due to the proximity of the conductive liquid to the electrode, which in turn causes a strong plasma formation and thus a more intense evaporation of the conductive liquid. In this way, the gas bubbles around the electrode become stable.

[0008] Thus, the initial phase, which is eventually followed by a phase of a stable plasma arc, typically comprises two sub-phases.

[0009] In a first sub-phase, also referred to herein as the evaporation phase, the conductive liquid around the active electrode is heated and vaporized by the flow of electric current, thereby first forming one or more vapor bubbles around the active electrode until the active electrode is completely surrounded by a gas layer, which ultimately electrically isolates the active electrode from the conductive liquid.

[0010] Once this is done, the arc ignition occurs in the second sub-phase of the initial phase. The second sub-phase is also referred to herein as the ignition phase. In the ignition phase, the high pressure drop across the gas layer between the active electrode and the conductive liquid causes the gas in the vapor bubble to ionize, resulting in a plasma that can be used for tissue cutting or gasification. The initial ignition of the plasma is difficult and depends to a large extent on the environmental conditions. In some cases, a stable plasma appears within a few milliseconds. In unfavorable cases, however, multiple starts are required before a stable plasma ignites.

[0011] The various phases and sub-phases before and during plasma ignition also differ in the amount of power provided by the electrosurgical generator.

[0012] As long as the active electrode is not or not completely surrounded by a gas layer, the impedance between the active electrode and the counter electrode is low impedance. The impedance is initially about 25 ohms to 50 ohms, depending on the electrode geometry, the temperature of the conductive liquid, etc. Therefore, the output voltage and output current output by the electrosurgical generator are in phase with the conductive conductor around the active electrode in the first heating phase and have no DC voltage component.

[0013] As soon as the conductive liquid starts to evaporate and the active electrode surface is partially surrounded by gas, the impedance between the output terminals of the electrosurgical generator increases because the surface resistance between the active electrode and the conductive liquid increases. However, the surface resistance accounts for only a small part of the total impedance between the electrodes (and thus between the output terminals of the electrosurgical generator). Therefore, the impedance increases only slightly during the evaporation phase until almost the entire active electrode is surrounded by gas.

[0014] As soon as the active electrode is completely surrounded by the gas, the evaporation phase ends and the ignition phase begins. In the ignition phase, ohmic current can no longer flow between the electrodes. The impedance is therefore much greater than in the evaporation phase and is theoretically purely capacitive. Since the impedance of the conductive liquid is very low, it forms an equipotential envelope around the gas layer. An electric field is formed across the gas layer between the electrode and the conductive liquid.

[0015] Since no current flow then occurs, the gas layer around the active electrode shrinks again, because part of the vapor that formed the bubble condenses again at the boundary layer between the gas and the conductive liquid. The thinning of the gas layer leads to an increase in the electric field strength through the thinner gas layer and can be so great that it causes ionization of the gas in the bubble and the appearance of the first arc. The duration of the ignition phase depends on the thickness of the gas layer after the evaporation phase. This duration can be very short, usually in the range of a few milliseconds, or it can be so short that it disappears.

[0016] Once the electric field through the gas layer is strong enough to cause gas ionization, plasma rupture occurs, i.e., an arc occurs and discharge occurs. Because the surface of the active electrode is smaller than the surface of the conductive liquid surrounding the bubble, the electric field strength on the active electrode side is greater than the electric field strength on the conductive liquid side. A DC voltage component of the output AC voltage appears. This DC voltage component (DC offset) is also called the "spark voltage" and is approximately 100 volts.

[0017] Due to electrical breakdown (plasma breakdown), the salt solution evaporates at the interface between the gas and the conductive liquid, and the thickness of the gas layer increases again at the location of plasma breakdown. Therefore, whenever the thickness of the gas layer decreases again due to condensation to increase the electric field strength to the extent that spark breakdown occurs, the thickness of the gas layer increases again due to the evaporation of the conductive liquid. In this way, the thickness of the gas layer around the active electrode is self-stable, and the plasma around the active electrode is in an equilibrium phase. In the equilibrium phase, an equilibrium state occurs, in which the thickness of the gas layer is so large that the increase in thickness caused by plasma penetration compensates for the decrease in thickness caused by condensation at the boundary layer between the gas and the salt solution. In the equilibrium phase, plasma breakdown occurs "automatically" where the gas layer around the active electrode is thinnest, because the electric field strength there is the highest. Accordingly, the new gas that appears due to evaporation appears exactly where the gas is consumed. The output voltage of the electrosurgical generator determines the thickness of the gas layer.

[0018] For plasma cutting, electrosurgical generators according to the prior art generally provide an output AC voltage of between 250 V and 350 V, for example between 280 V and 320 V.

[0019] In practice, problems occasionally occur when igniting a plasma to effect tissue cutting. Summary of the invention

[0020] It is an object of the present invention to improve an electrosurgical generator with respect to its suitability for plasma cutting of tissue.

[0021] According to the present invention, this object is achieved by an electrosurgical generator, which is configured to output high-frequency alternating current to an electrosurgical instrument in operation so as to perform plasma cutting on body tissue. The electrosurgical generator has an output terminal for connecting the electrosurgical instrument so as to supply high-frequency alternating current to the electrosurgical instrument connected to the output terminal.

[0022] The electrosurgical generator has an impedance determination unit for determining the impedance of a load connected to the output terminal. In addition, the electrosurgical generator has an output voltage adjustment unit and may have a voltage detection unit.

[0023] The output voltage regulating unit is configured to regulate the output AC voltage of the electrosurgical generator according to a predetermined output voltage maximum value.

[0024] The impedance determination unit is designed to determine the impedance of a load in the immediate vicinity of the output of the electrosurgical generator. For this purpose, the impedance determination unit can be designed to detect a voltage, a current and / or a phase shift. The impedance determination unit can determine the impedance, for example, from the detected current, the detected voltage and, if necessary, the detected phase. If the current detection unit and the voltage detection unit measure only the effective values ​​of the current and voltage, the measured phase is meaningful, since the impedance can be determined exactly from the effective values ​​of the current and voltage and the phase position, just as it can be determined from the instantaneous values ​​of the current and voltage.

[0025] The voltage detection unit is designed to detect a voltage applied to the output of the electrosurgical generator, in particular a DC voltage component of the voltage applied to the output.

[0026] The output voltage regulating unit is designed to control the output AC voltage, in particular its effective value, according to the output voltage maximum value. According to the invention, at least for a part of the initial phase, the predetermined output voltage maximum value is different from that in the subsequent balancing phase.

[0027] According to a first aspect of the invention, the predetermined maximum output voltage value in the evaporation phase (ie at the beginning of the initial phase) is smaller than the maximum output voltage value set for actual plasma cutting in the equilibrium phase.

[0028] According to a second aspect of the invention, the predetermined maximum output voltage value in the ignition phase (ie at the end of the initial phase) is greater than the maximum output voltage value set for actual plasma cutting in the equilibrium phase.

[0029] The two inventive aspects can be implemented independently of one another and are therefore separate inventions. However, the two aspects can also be combined with one another, in particular in such a way that the electrosurgical generator is configured to use a maximum output voltage for the vaporization phase that is less than the maximum output voltage set for the equilibrium phase and to use a maximum output voltage for the ignition phase that is greater than the maximum output voltage set for the equilibrium phase. Both aspects contribute, both independently and in combination, to reliable ignition of the arc at the start of plasma cutting.

[0030] In order to detect the end of the evaporation phase and the start of the ignition phase or the end of the ignition phase and the start of the equilibrium phase (or both), the output voltage control unit is preferably connected to the impedance determination unit or the voltage detection unit or to both.

[0031] In order to detect the end of the vaporization phase and the beginning of the ignition phase, the output voltage regulating unit may be configured to detect whether the impedance at the output of the electrosurgical generator increases by more than a predetermined degree. The predetermined degree may be derived from the impedance determined by the impedance determination unit at the beginning of the vaporization phase, for example a predetermined multiple of this impedance.

[0032] In order to detect the end of the ignition phase and the beginning of the balancing phase, the output voltage regulation unit can be configured to detect the DC voltage component in the output AC voltage and adopt the output AC voltage maximum value set for the balancing phase directly or after a predetermined time after detecting the DC voltage component.

[0033] Unlike the prior art, which provides a single maximum output AC voltage value for plasma cutting that is applicable to the balancing phase and is used in the initial phase, i.e., the evaporation phase and the ignition phase, at least one maximum output AC voltage value that is different from the maximum output AC voltage value applicable to the balancing phase is provided.

[0034] The invention includes the recognition that the maximum value of the output AC voltage applied during the equilibrium phase may lead to unstable conditions in the initial phase, which may prevent reliable ignition of the plasma.

[0035] Therefore, the output voltage regulating unit is designed to control the output AC voltage according to an output voltage maximum value, which is set during operation according to an output value of the impedance determination unit and / or according to an output value of the voltage detection unit (based on which the start and end of the evaporation phase or the ignition phase are detected), so that the output voltage maximum value in the evaporation phase predetermines a lower output AC voltage than in the ignition phase after the evaporation phase.

[0036] Therefore, the output voltage regulating unit is set to different output voltage maximum values ​​based on the initial phase and the equilibrium phase. Here, the initial phase can include an evaporation phase and / or an ignition phase, wherein in the evaporation phase there is direct conductive contact between the active electrode and the conductive liquid surrounding the active electrode, and the conductive liquid is evaporated by electrically heating the conductive liquid due to the flow of current through the conductive liquid. In the ignition phase, the active electrode is completely surrounded by bubbles generated by evaporation, and the impedance determined by the impedance determination unit is significantly higher than the impedance during the evaporation phase. Once the gas in the bubble is ionized and an arc occurs, a DC voltage component also appears in the AC voltage applied to the output of the electrosurgical generator. This DC voltage component is detected by the DC voltage detection unit, so that it can generate a corresponding output signal, which is a symbol of the (first) arc.

[0037] In the equilibrium phase, the plasma around the active electrode is stable. The output voltage maximum value according to which the output voltage of the electrosurgical generator is regulated by means of the output voltage regulating unit during the equilibrium phase is generally higher than the output voltage maximum value according to which the output voltage of the electrosurgical generator is regulated by means of the output voltage regulating unit during the evaporation phase.

[0038] According to a first aspect of the invention, the output voltage maximum value according to which the output voltage of the electrosurgical generator is regulated during the evaporation phase by means of the output voltage regulating unit is lower than the output voltage maximum value set for the output voltage regulation of the electrosurgical generator by means of the output voltage regulating unit during the balancing phase. Preferably, no fixed output voltage maximum value is set for the evaporation phase, but it is preferably set based on the impedance measured between the output terminals of the electrosurgical generator during the evaporation phase in order to obtain an approximately constant current density at the transition between the active electrode and the saline solution surrounding it.

[0039] According to a second aspect of the invention, the output voltage maximum value according to which the output voltage of the electrosurgical generator is regulated by the output voltage regulating unit during the ignition phase after the vaporization phase is higher than the output voltage maximum value set by the output voltage regulating unit for the output voltage regulation of the electrosurgical generator during the balancing phase. The increased output voltage maximum value for the ignition phase compared to the subsequent balancing phase results in an increase in the output AC voltage of the electrosurgical generator in the ignition phase and causes the plasma to ignite more quickly and reliably.

[0040] The present invention includes the recognition that by measuring the output voltage and output current of an electrosurgical generator, parameters such as impedance or DC voltage components can be derived, and thus different phases of the ignition process can be distinguished. By identifying different phases before and during the ignition of the plasma around the active electrode of the electrosurgical instrument, different maximum output voltage values ​​can be set independently and purposefully for each phase, rather than a fixed maximum output voltage value for all phases.

[0041] Preferably, the output voltage regulating unit is designed to control the output AC voltage according to a maximum output voltage value, which is adjusted during operation according to an output value of an impedance determination unit for detecting the start of an ignition phase and / or according to an output value of a voltage detection unit for detecting the end of the ignition phase, so that the maximum output voltage in the ignition phase is higher than the maximum output voltage in a balancing phase after the ignition phase.

[0042] The electrosurgical generator is preferably designed to detect the start of the vaporization phase and the ignition phase based on the measured impedance. The electrosurgical generator is preferably designed to use as a criterion for the start of the ignition phase a reduction in the current intensity of the output alternating current during the vaporization phase to a predetermined fraction, such as one third, of the current intensity of the output alternating current at the start of the vaporization phase. The electrosurgical generator is therefore preferably designed to detect the start of the ignition phase when the current intensity of the output alternating current during the vaporization phase has dropped to 1 / 3 or 1 / 4 of the current intensity of the output alternating current at the start of the vaporization phase.

[0043] According to another preferred variant embodiment, the electrosurgical generator is designed to detect the end of the ignition phase on the basis of a DC voltage component in the voltage applied to the output terminals of the electrosurgical generator.

[0044] The output voltage regulating unit is preferably designed to use an output voltage maximum value which is tracked during the evaporation phase as a function of the respective current output value of the impedance determination unit and / or the voltage detection unit. The tracking of the output voltage maximum value is preferably carried out in such a way that an at least approximately constant current density is present at the transition from the active electrode to the liquid in the immediate vicinity of the active electrode. Thus, when the active electrode is increasingly covered by gas bubbles during the evaporation phase and thus the area of ​​the conductive liquid contacting the active electrode decreases, the output voltage regulating unit correspondingly reduces the output AC voltage, so that the current density remains at least approximately constant.

[0045] Preferably, the electrosurgical generator is designed to determine the impedance between the two output terminals of the electrosurgical generator by means of the impedance determination unit before the start of the vaporization phase and to adjust the maximum output voltage value at the start of the vaporization phase so that the current intensity generated by the impedance and the output voltage is less than the maximum current intensity of the output alternating current that can be provided by the electrosurgical generator.

[0046] Alternatively, the output voltage regulating unit can be designed to use a fixed output voltage maximum value for the evaporation phase during the evaporation phase. The fixed output voltage maximum value is selected such that the maximum current that can be provided by the electrosurgical generator is not exceeded at the beginning of the evaporation phase even in the low impedance conditions that are then present, according to U max =Z gemessen *I max Since the impedance Z only increases further during the period until the end of the vaporization phase, the electrosurgical generator can maintain the predetermined maximum output voltage since the current is further reduced. Compared with the output voltage maximum value tracked according to the respective current output values ​​of the impedance determination unit and / or the voltage detection unit, this variant can be realized more easily and can also prevent excessive vapor generation at the end of the vaporization phase.

[0047] Preferably, the electrosurgical generator is sized to provide a maximum output AC voltage greater than 250 V and a maximum output AC current greater than 4 A, such as a maximum output AC voltage between 250 V and 400 V and a maximum output AC current between 4 A and 12 A.

[0048] According to the invention, an electrosurgical system is also proposed, which has an electrosurgical generator of the type described herein and an electrosurgical instrument, which is connected or can be connected to the output of the electrosurgical generator. The electrosurgical instrument has an active electrode and at least one counter electrode and at least one liquid line arranged relative to the active electrode, so that the active electrode can be flushed with a conductive liquid during operation.

[0049] The active electrode is preferably designed as a ring electrode or a button electrode.

[0050] The electrosurgical instrument is preferably a resectoscope.

[0051] Another aspect of the present invention relates to a method of operating an electrosurgical generator. A first variant of the method comprises the following steps:

[0052] - generating an output AC voltage and providing the output AC voltage to an output terminal of the electrosurgical generator, wherein an output AC voltage maximum value is set for the output AC voltage,

[0053] - determining the load applied to the output and comparing it with a limit value, the limit value being selected below which the presence of an evaporation phase is indicated,

[0054] - wherein, as long as the limit value is below, the maximum value of the output AC voltage is set to be preferably at least 30% lower than the maximum value of the output voltage set when the limit value is exceeded, and

[0055] - Adjusting the output AC voltage based on the respective set output AC voltage maximum value.

[0056] In this case, the output AC voltage maximum value is preferably tracked in the event of a load change as a function of the determined load, as long as the limit value is below.

[0057] The load can be determined, for example, from the impedance at the output of the electrosurgical generator. The limit value as a criterion for the identification of the vaporization phase is then, for example, an impedance value defining a predetermined level, which is preferably derived from the impedance measured by the impedance determination unit at the beginning of the vaporization phase. A vaporization phase is present if the limit value is below, i.e. the impedance is not above the predetermined level. The impedance determination unit determines the impedance, for example, from the measured current, the measured voltage and, if only the effective values ​​of the current and voltage are measured, also the phase.

[0058] Using this method, excessive vapor formation, especially at the end of the evaporation phase, can be prevented.

[0059] Additionally or alternatively, the method may further comprise the following steps:

[0060] - generating an output AC voltage and providing the output AC voltage to an output terminal of the electrosurgical generator, wherein an output AC voltage maximum value is set for the output AC voltage,

[0061] - determining the load applied to the output (18) and comparing it with a limit value, the limit value being selected such that the presence of an evaporation phase is indicated below the limit value,

[0062] - detecting the DC voltage component of the output AC voltage applied to the output terminal,

[0063] - wherein, once the limit value is exceeded and the DC voltage component in the output AC voltage loaded on the output terminal (18) is not detected, the maximum value of the output AC voltage is set to be greater than the maximum value of the output voltage set when the DC voltage component is detected,

[0064] -Regulating the output AC voltage based on the output AC voltage maximum value.

[0065] Instead of detecting the DC voltage component, a frequency analysis of the output AC voltage can also be performed to detect the occurrence of an arc. The frequency analysis can include a Fourier transform, in particular a fast Fourier transform (FFT).

[0066] With this second variant of the method, the arc can be stabilized after the initial ignition.

[0067] These two method variants can be combined with one another.

[0068] The method preferably further comprises the step of detecting the start of the ignition phase based on an impedance increase exceeding a limit value determined by an initial impedance value and a predetermined impedance value, or detecting the start of the ignition phase based on a decrease in the output AC current below a predetermined fraction of a predetermined initial output AC current value. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The present invention will now be described in detail based on embodiments and with reference to the accompanying drawings, in which:

[0070] Figure 1 An electrosurgical system having an electrosurgical generator and an electrosurgical instrument connected thereto is shown;

[0071] Figure 2 A diagram showing the use of a resectoscope as an electrosurgical instrument;

[0072] Figure 3 shows a schematic circuit diagram of an electrosurgical generator;

[0073] Figure 4 a) to f) show different stages in igniting a plasma around an active electrode of an electrosurgical instrument;

[0074] Figure 5 a) to c) show alternative embodiments of active electrodes for electrosurgical instruments;

[0075] Figure 6 A graph is shown illustrating voltage and impedance curves during the vaporization phase and the ignition phase when an arc occurs on an active electrode of an electrosurgical instrument.

[0076] Figure 7 shows a possible ideal course of the maximum value of the output voltage during the evaporation phase, which is controlled as a function of the impedance detected during the evaporation phase;

[0077] Figure 8 A possible simplified course of the maximum output voltage during the evaporation phase, the ignition phase and the equilibrium phase is shown.

[0078] Reference numerals list

[0079] 10 Electrosurgical System

[0080] 12 Electrosurgical generator

[0081] 14 Electrosurgical instruments

[0082] 16 Connect the cables

[0083] 18,18.1,18.2 Output of electrosurgical generator

[0084] 20 tubes

[0085] 22,22' Active electrodes

[0086] 24,26 Hose

[0087] 30 High voltage power supply section

[0088] 34 High frequency part

[0089] 36 Output voltage regulation unit

[0090] 38 Current detection unit

[0091] 40 Voltage detection unit

[0092] 42 Analysis Units

[0093] 44.1,44.2 Counter electrode

[0094] 46 Steam Bubble

[0095] 50 Plasma

[0096] 52 Organization

[0097] 60 Evaporation stage

[0098] 62 Ignition Stage

[0099] 64 Balance Phase

[0100] 66 Initial stage DETAILED DESCRIPTION

[0101] exist Figure 1 An electrosurgical system 10 is depicted in FIG. 1 . The electrosurgical system 10 comprises an electrosurgical generator 12 and an electrosurgical instrument 14 . The electrosurgical instrument 14 is connected to an electrical output 18 of the electrosurgical generator 12 via a connecting cable 16 .

[0102] In the illustrated embodiment, the electrosurgical instrument 14 is an electroresectoscope having a tube 20 through which a conductive liquid can be guided as an irrigation liquid to the distal end of the tube 20. For this purpose, a liquid line is provided in the tube 20. The conductive liquid flows out at the distal end of the tube 20 and surrounds and irrigates the active electrode 22 at the distal end of the tube 20. The conductive liquid circulates and is drained away again through the tube 20. Therefore, the tube 20 includes at least two lumens through which the conductive liquid can be guided to the distal end of the tube 20 so that it can flow out there, while the liquid is drained away again from the distal end of the tube 20 through another lumen. The lumens thus serve as liquid conduits. Figure 1 In FIG. 1 , a corresponding hose connection for an electrically conductive liquid is not shown on the electrosurgical instrument 14 .

[0103] like Figure 1 As shown, the active electrode 22 of the electrosurgical instrument 14 can be removed from the tube 20, or can be withdrawn into the tube 20. Typically, the electrosurgical instrument 14 has a spring that requires the active electrode 22 to be pushed out of the tube 22 by hand against the spring force, or conversely, the active electrode 22 to be pulled back into the tube 22 by hand against the spring force.

[0104] The active electrode 22 may have various shapes, such as a button electrode or a ring electrode. Figure 1 It is not shown in the figure, it can be formed, for example, by the tube 20, but can also be pushed out of the tube 20 or retracted into it together with the active electrode 22.

[0105] Figure 2 The electrosurgical instrument 14 is shown in use. In addition to the connecting cable 16, Figure 2Also shown are hoses 24 and 26 which are used to supply and drain the conductive liquid. At the distal end of the tube 20 of the electrosurgical instrument 14, an active electrode 22 projects from the tube 20.

[0106] When a high-frequency alternating voltage is applied between the active electrode 22 and the corresponding counter electrode, this first causes a current to flow through the conductive liquid around the active electrode 22. Due to the flow of current, the conductive liquid around the active electrode 22 becomes hot and thus evaporates. In this case, the current flows through the conductive liquid between the active electrode 22 and the corresponding counter electrode until the active electrode 22 is completely surrounded by a vapor bubble. Once this is the case, an alternating electric field is formed between the active electrode 22 and the conductive liquid surrounding the vapor bubble in the vapor bubble that appears in this way. If the field strength of the alternating electric field is large enough, the gas in the bubble around the active electrode 22 is ionized and plasma appears, which can be seen at the arc. Here, the first arc appears at the place where the vapor bubble around the conductive liquid surrounding the active electrode is closest to the active electrode 22, because the field strength of the alternating electric field there is the largest. The arc thus generated causes the other liquid of the conductive liquid to evaporate, so that the vapor bubble around the active electrode 22 does not collapse as a result, but forms a dynamically stable equilibrium state around the active electrode 22.

[0107] The electrosurgical generator 12 provides the AC voltage required for the generation and maintenance of the plasma around the active electrode 22, as well as the AC current required for vaporizing the conductive liquid. Figure 3 As can be seen in the figure, the electrosurgical generator 12 has a high-voltage power supply part 30 for this purpose, which can be connected to a common public power supply network, for example, and provides a high-voltage direct current at its output 32. This high-voltage direct current is supplied to a high-frequency part 34 of the electrosurgical generator 12. The high-frequency part 34 of the electrosurgical generator 12 acts as an inverter and generates a high-frequency alternating voltage, which is output to the output terminals 18.1 and 18.2 of the electrosurgical generator 12 via an output transformer (not shown) of the high-frequency part 34. Figure 1 As shown, the electrosurgical instrument 14 may be connected to outputs 18 . 1 and 18 . 2 of the electrosurgical generator 12 .

[0108] In order to control the output voltage of the electrosurgical generator 12, an output voltage regulating unit 36 ​​is provided, which controls the output voltage at the output terminals 18.1 and 18.2 based on the output voltage maximum value in such a way that the set output voltage maximum value is not exceeded during operation. The invention provides that the output voltage maximum values ​​to be used for different phases during plasma ignition should be set differently, wherein the plasma ignition phase should be identified based on the electrical output values ​​at the output terminals 18.1 and 18.2.

[0109] For this purpose, a current detection unit 38 and a voltage detection unit 40 are provided, which respectively detect the current output via the two output terminals 18.1 and 18.2 and the voltage dropped simultaneously via the two output terminals 18.1 and 18.2. The output values ​​of the current detection unit 38 and the voltage detection unit 40 are provided to an analysis unit 42, which is configured to determine the impedance of the load applied to the output terminals 18.1 and 18.2 on the one hand. In this sense, the analysis unit 42 is an impedance determination unit. The impedance determination unit can determine the impedance, for example, based on the measured current, the measured voltage and, if necessary, the measured phase. When the current detection unit and the voltage detection unit only measure the effective values ​​of the current and the voltage, it is meaningful to determine the phase, because the impedance can be determined based on the effective values ​​of the current and the voltage and the phase position, just as it is determined based on the instantaneous values ​​of the current and the voltage. On the other hand, the analysis unit 42 is also configured to measure the DC voltage component of the AC voltage dropped via the output terminals 18.1 and 18.2. In this sense, the analysis unit 42 also serves as a DC voltage detection unit. The evaluation unit 42 provides output values ​​dependent on the measured impedance and the measured DC voltage component to the output voltage regulation unit 36. The output voltage regulation unit 36 ​​is designed to determine respective output voltage maximum values ​​based on the output values ​​for the impedance and the DC voltage component provided by the evaluation unit 42, according to which the output voltage regulation is based.

[0110] The output voltage of the electrosurgical generator 12 is therefore controlled by forming a corresponding output voltage maximum value for the output voltage regulating unit 36 ​​.

[0111] exist Figure 4 Schematically illustrated in FIG. 1 are different stages in the ignition of a plasma around an active electrode, here a button electrode 22 ′.

[0112] Compared to the active electrode 22, the counter electrodes 44.1 and 44.2 are arranged around the supply line to the active electrode 22 with a larger area. During use, the active electrode 22' and the counter electrodes 44.1 and 44.2 are surrounded by a conductive cooling liquid, i.e., a saline solution. If a high-frequency AC voltage from the generator 12 is applied between the active electrode 22' and the counter electrodes 44.1 and 44.2, the AC current first flows through the conductive liquid surrounding the active electrode 22 and the counter electrodes 44.1 and 44.2. This Figure 4 As shown in a). Figure 4As shown in b), the conductive liquid becomes hot due to the flow of current, and then bubbles 46 appear. Once the active electrode 22' is completely surrounded by the vapor bubbles 46, no DC current will flow between the active electrode 22' and the conductive liquid. Instead, an alternating electric field is formed between the active electrode 22' on the one hand and the boundary layer 48 between the conductive liquid and the vapor bubbles 46 on the other hand. The thinner the vapor bubble 46, the greater the strength of the alternating electric field. The electric field strength is greatest where the conductive liquid or the boundary layer 48 is closest to the active electrode 22'. Figure 4 A fully formed vapor bubble 46 is shown in FIG.

[0113] When the electric field strength between the active electrode 22' and the conductive liquid surrounding the vapor bubble 46 exceeds a certain level, ionization of the gas in the vapor bubble 46 occurs, and a plasma 50 appears around the active electrode 22', which can be seen at the arc. Once the plasma is ignited, the active electrode 22 can be placed near the biological tissue 50 to be treated, so as to vaporize a portion of the biological tissue 52 with the aid of the plasma 50, thereby partially removing or cutting the biological tissue 52. This is Figure 4 It is shown in d) and e).

[0114] In order to cut biological tissue like a paring knife, a ring electrode 22' can also be provided as an active electrode. Figure 5 is shown in .

[0115] In e.g. Figure 4 During the evaporation phase shown in b) in FIG. 1 , an increasingly larger portion of the active electrode 22 is gradually surrounded by the vapor bubble 46, so that the contact area between the conductive liquid around the active electrode 22 and the active electrode 22 becomes increasingly smaller. Since the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 does not decrease to the same extent as the contact area between the conductive liquid and the active electrode 22 decreases, the current density of the conductive liquid increases where the active electrode 22 is in contact while the current density of the electrosurgical generator 12 remains constant. The reason for this is that the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 depends not only on the transition resistance between the active electrode 22 and the conductive liquid, but also on the conductive liquid resistance (or impedance) and the transition resistance to the counter electrode 44.1. For example, it can be assumed that during the evaporation phase, the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 is initially approximately 25Ω (see Figure 4a) in the figure). 25Ω is, for example, composed of a transition resistance of about 10Ω between the active electrode 22 and the conductive liquid and an impedance of about 15Ω of the conductive liquid (including the transition resistance between the conductive liquid and the counter electrodes 44.1 and 44.2). In this example, it can be assumed that the 15Ω impedance of the conductive liquid is more or less stable and does not change when the vapor bubble 46 around the active electrode 22 becomes larger. However, the increase in the transition resistance between the active electrode 22 and the conductive liquid is inversely proportional to the coverage of the active electrode 22 by the vapor bubble. A coverage of 0 means that the vapor bubble has not yet formed and the conductive liquid is in full contact with the active electrode 22. A coverage of 1 means that the active electrode 22 is completely surrounded by the vapor bubble 46. The result of this assumption is that the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 depends on the following formula:

[0116]

[0117] Wherein, Z is the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2, and sc is the coverage of the vapor bubble 46 covering the surface of the active electrode 22. As already explained, coverage sc=0 means that the conductive liquid contacts the surface of the active electrode 22 in its entirety, while coverage sc=1 means that the surface of the active electrode 22 is completely enclosed in the vapor bubble 46.

[0118] As a result, the impedance first increases only slowly in the initial phase of the vaporization phase and then rises sharply at the end of the vaporization phase, for example at a coverage of the order of 0.8 (80%). This impedance increase is accompanied by an increase in the output voltage, at least initially, since the electrosurgical generator 12 can only provide a limited maximum current, so that the electrosurgical generator 12 cannot reach its maximum current of, for example, 320 V at low load impedances. effektiv The maximum output voltage of Figure 6 Indicated by dotted line.

[0119] As a background, a typical electrosurgical generator 12 can only provide, for example, 4 to 5A. effektiv The maximum output current is limited, so it is not possible to obtain a 320V output current at low impedance values. effektiv or 350V effektiv In the case of an initial low impedance of approximately 25Ω between the active electrode 22 and the counter electrodes 44.1 and 44.2 and therefore also between the output terminals 18.1 and 18.2 of the electrosurgical generator 12, the limited maximum output current of the electrosurgical generator 12 acts as a limiting parameter, so that the electrosurgical generator 12 cannot output its maximum output voltage. If this is possible, the electrosurgical generator 12 will output 320V at an output voltage of 25Ω. effektiv Output 12.8A effektiv of current.

[0120] Therefore, as the coverage of the active electrode 22 increases, the output voltage of the electrosurgical generator 12 increases, while the current output by the electrosurgical generator 12 remains constant. Therefore, as the coverage sc increases, the output voltage, output power and current density at the transition between the active electrode 22 and the conductive liquid increase. Only when the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 is so large due to the enlarged vapor bubble that the electrosurgical generator 12 reaches its, for example, 320V effektiv The current intensity of the power output by the electrosurgical generator 12 is reduced only at the maximum output voltage of . However, this is only the case at the end of the vaporization phase, e.g. Figure 6 In the example shown, the coverage is approximately 0.9 (90%).

[0121] The increasing current density with increasing coverage causes the conductive liquid in contact with the active electrode 22 to heat up more and more rapidly, so that a sudden, almost explosive evaporation may occur, such as in Figure 5 As can be seen in c) in the figure.

[0122] This behavior is undesirable and can impair reliable and rapid ignition of the plasma. The invention therefore provides that during the evaporation phase the maximum output voltage of the electrosurgical generator 12 is reduced below a value which is set as the maximum output voltage for the subsequent balancing phase. This is done in such a way that the maximum output voltage for the evaporation phase is formed such that the maximum output voltage of the electrosurgical generator 12 is correspondingly low. Ideally, the output voltage maximum for the maximum output voltage of the electrosurgical generator 12 is tracked as a function of the coverage, so that the maximum output voltage is initially relatively high in the case of low coverage and then decreases as the coverage increases, so that the current intensity caused by the output voltage of the electrosurgical generator 12 decreases inversely proportional to the electrode coverage, resulting in an approximately constant current density. Since the coverage sc cannot actually be determined or measured directly, according to a preferred variant embodiment, the impedance predominantly present between the output terminals 18.1 and 18.2 is taken into account to form and track a suitable output voltage maximum during the evaporation phase, which impedance depends on the coverage sc as described above. Figure 7 It is shown how in the example shown here the maximum output voltage can depend on the measured impedance between the output terminals 18 . 1 and 18 . 2 in order to obtain a constant current density during the entire evaporation phase under the above assumptions (conductive liquid impedance of approximately 15Ω).

[0123] According to an alternative embodiment variant, the maximum output voltage during the evaporation phase is not tracked continuously as a function of the coverage or alternatively as a function of the determined impedance, but is fixed as a constant value for the evaporation phase, wherein the evaporation phase can still be identified as a function of the applied impedance value.

[0124] It should be pointed out that Figure 6 The ideal curve of the output AC voltage in an example case during the evaporation phase as a function of the coverage is shown, in particular, by Figure 6 As shown by the red line drawn in FIG. 1 . Thus, the effective value of the output AC voltage at the beginning of the vaporization phase is between 110 V and 120 V, and the effective value of the output AC voltage at the end of the vaporization phase is close to 50 V. Alternatively, a constant effective value can be set for the output AC voltage of the electrosurgical generator 12 during the vaporization phase, wherein the constant value can be, for example, between 50 V and 120 V, or even better, between 80 and 100 V. effektiv between.

[0125] As described in the introduction, once the plasma has ignited, the plasma around the active electrode 22 causes a DC voltage component (DC offset or also referred to as spark voltage) to appear in the output AC voltage of the electrosurgical generator 12. Accordingly, the successful ignition of the plasma can be identified by the appearance of a DC voltage component in the output AC voltage of the electrosurgical generator 12.

[0126] The end of the vaporization phase and thus the start of the ignition phase are preferably determined by an impedance rise exceeding a detection impedance value determined by a predetermined multiple of the impedance value determined at the start of the vaporization phase or by a decrease in the output AC current to a predetermined fraction below the output AC current value determined at the start of the vaporization phase. With the detection of the start of the ignition phase, the output AC voltage of the electrosurgical generator 12 is increased to a level as high as possible, which is higher than the output AC voltage set for the subsequent balancing phase. Reliable ignition of the arc can thus be ensured. As soon as the first arc occurs and the presence of a DC voltage component in the output AC voltage of the electrosurgical generator 12 is thus also detected, it is desirable to keep the output AC voltage of the electrosurgical generator 12 at a higher output AC voltage value for a certain time, which is greater than the output AC voltage set for the subsequent balancing phase. The determined time for which the output AC voltage of the electrosurgical generator 12 is now kept at the higher output AC voltage value can be, for example, 10 to 80 ms or 40 to 60 ms.

[0127] Accordingly, according to a preferred variant implementation, it is provided that the output voltage regulating unit increases the maximum output voltage to a voltage value higher than the voltage value set for the balancing stage. The output voltage value set for the balancing stage is, for example, 250V. effektiv to 320V effektiv Then, the increased output AC voltage value can be, for example, 300V effektiv Up to 350V effektiv. This increased output voltage of the electrosurgical generator 12 during the ignition phase leads to the arc being maintained as reliably as possible until the plasma in the vapor bubble around the active electrode 22 has stabilized. As soon as this is the case and the equilibrium phase mentioned in the introduction therefore begins, the output voltage of the electrosurgical generator 12 can be reduced again by correspondingly reducing the maximum output voltage for the output voltage regulating unit 36. This can be done gradually or suddenly. By reducing the output AC voltage of the electrosurgical generator 12 after the plasma has ignited, the plasma layer (vapor bubble) around the active electrode 22 becomes slightly thinner, but the plasma does not disappear, as long as the output AC voltage of the electrosurgical generator remains sufficiently high and is, for example, 280 V to 300 V.

[0128] In a simple embodiment, the electrosurgical generator 12 and its output voltage regulating unit 36 ​​can be designed so that the electrosurgical generator 12 initially outputs a maximum output AC voltage between 100V and 200V until the DC voltage detection unit detects a DC voltage component in the output AC voltage of the electrosurgical generator 12, which is, for example, greater than 50Ω. Once such a DC voltage component is detected in the output AC voltage of the electrosurgical generator 12, the output voltage regulating unit 36 ​​switches to another output voltage maximum value of, for example, 320V to maintain plasma stability during the ignition phase. The higher output voltage maximum value for the ignition phase can be reduced to a slightly lower value for the output AC voltage, for example, to 250 to 300V after a predetermined time of 10 to 50ms. effektiv The maximum output voltage between

[0129] With such an electrosurgical generator 12 , the plasma around the active electrode 22 can be ignited reliably and in a short but not too strongly fluctuating time and not too dependent on environmental parameters. This makes it much easier for the surgeon to use the corresponding electrosurgical system 10 .

[0130] Figure 8 A possible simplified curve of the maximum output voltage during the evaporation phase 60, the ignition phase 62 and the equilibrium phase 64 is shown. During the evaporation phase 60, the maximum output voltage is, for example, 100 V. effektiv or 120V effektiv Then, at the beginning of the ignition phase 62, the maximum output voltage is increased to, for example, 300 V. effektiv or 350V effektiv After the ignition phase 62 is completed, the maximum output voltage is reduced to, for example, 280 V set for the balancing phase 64. effektiv or 320V effektivThe evaporation phase 60 and the ignition phase 62 together form an initial phase 66 , in which the maximum output voltage is smaller or larger than the maximum output voltage in the subsequent balancing phase 64 .

Claims

1. An electrosurgical generator (12) configured to deliver high frequency alternating current to an electrosurgical instrument (14) for plasma cutting of body tissue (52) during operation, wherein: The electrosurgical generator (12) has an output terminal (18) for connecting to the electrosurgical instrument (14) to provide high-frequency alternating current to the electrosurgical instrument connected to the output terminal (18) during operation and determine the impedance of a load connected to the output terminal (18). The electrosurgical generator (12) further comprises an impedance determination unit and a voltage detection unit (40) and an output voltage adjustment unit (36), wherein: The output voltage adjustment unit (36) is configured to control or adjust the output AC voltage of the electrosurgical generator (12) according to a specified maximum output voltage value, and The impedance determination unit is designed to determine the impedance of a load applied to the output (18) of the electrosurgical generator (12) during operation, It is characterized in that The output voltage regulating unit (36) is designed to control the output AC voltage according to an output voltage maximum value, which is different from the output voltage maximum value in the subsequent balancing stage at least for a part of the initial stage, and the output voltage maximum value is set during operation according to the output value of the impedance determination unit and / or according to the output value of the voltage detection unit so that the output voltage maximum value in the evaporation stage is lower than the output voltage maximum value in the subsequent balancing stage.

2. The electrosurgical generator (12) according to claim 1, characterized in that The output voltage regulating unit (36) is designed to control the output AC voltage according to an output voltage maximum value, and the output voltage maximum value is set during operation according to an output value of the impedance determining unit and / or according to an output value of the voltage detecting unit (40) so that the output voltage maximum value in an ignition phase is higher than the output voltage maximum value in a balancing phase after the ignition phase.

3. The electrosurgical generator (12) according to claim 1, characterized in that The electrosurgical generator (12) is designed to detect the vaporization phase based on the detected impedance.

4. The electrosurgical generator (12) according to claim 3, characterized in that The electrosurgical generator (12) is designed to detect the end of the vaporization phase in dependence on an increase in impedance at the output of the electrosurgical generator exceeding a specified level, wherein the specified level is derived from the impedance detected by the impedance determination unit at the beginning of the vaporization phase.

5. The electrosurgical generator (12) according to claim 4, characterized in that The specified degree is a specified multiple of the impedance.

6. The electrosurgical generator (12) according to claim 2, characterized in that The electrosurgical generator is designed to detect the end of the ignition phase on the basis of a DC voltage component of a voltage applied to the output of the electrosurgical generator.

7. The electrosurgical generator (12) according to claim 1, characterized in that The output voltage regulating unit (36) is designed to use an output voltage maximum value tracked during the evaporation phase according to one of the current output values ​​of the impedance determining unit and / or the voltage detecting unit.

8. The electrosurgical generator (12) according to claim 1, characterized in that The output voltage regulation unit (36) is designed to use during the evaporation phase a maximum output voltage value which is fixedly specified for the evaporation phase.

9. The electrosurgical generator (12) according to claim 1, characterized in that The electrosurgical generator (12) is dimensioned such that it provides a maximum output AC voltage greater than 300V and a maximum output AC current greater than 4A.

10. An electrosurgical system (10), comprising an electrosurgical generator (12) according to any one of claims 1 to 9 and an electrosurgical instrument (14), the electrosurgical instrument (14) being connected to an output end (18) of the electrosurgical generator (12) or being connectable to the output end (18), and the electrosurgical instrument (14) having an active electrode (22) and at least one counter electrode (44.1, 44.2) and at least one liquid pipeline, the at least one liquid pipeline being arranged relative to the active electrode (22) so that the active electrode (22) can be flushed with a conductive liquid during operation.

11. The electrosurgical system (10) according to claim 10, characterized in that: The active electrode is designed as a ring electrode or a button electrode.

12. The electrosurgical system (10) according to claim 10 or 11, characterized in that: The electrosurgical instrument (14) is a resectoscope.

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