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

Description

[0001] The invention relates to an electrosurgical generator configured to supply an electrosurgical instrument for plasma cutting of body tissue with high-frequency alternating current. The invention also relates to an electrosurgical system comprising an electrosurgical generator and an electrosurgical instrument, as well as a method for operating an electrosurgical generator.

[0002] Electrosurgery can be used to cut, coagulate (ablate), and / or vaporize biological tissue—that is, body tissue. Electrosurgery typically uses high-frequency alternating currents with a frequency between 0.2 MHz and 3 MHz.

[0003] An electrosurgical system typically includes an electrosurgical generator to produce the high-frequency alternating current. The generator usually has two outputs to which an electrosurgical instrument can be connected, and between which a high-frequency alternating voltage is supplied during operation. An electrosurgical generator also typically includes a high-voltage power supply that generates a direct current during operation, and a high-frequency section connected to the power supply that converts the direct current into a high-frequency alternating current during operation.

[0004] Electrosurgical systems are used, for example, in urology and gynecology. In particular, for plasma vaporization, such as during TURI (transurethral resection in saline) to treat benign prostatic hyperplasia (BPH), an electrosurgical generator is typically used in conjunction with a suitable electrosurgical instrument, such as a resectoscope. The active cutting or vaporization electrode of such an electrosurgical instrument is immersed in an electrically conductive irrigation fluid, such as saline solution (NaCl). A high-frequency alternating current is used to ignite an electric arc at the active electrode of the electrosurgical instrument (also called plasma ignition).

[0005] Body tissue can be vaporized or cut using plasma generated in a vapor bubble—that is, a gas bubble—around an electrode in an electrically conductive, biocompatible liquid, particularly a saline solution. This is achieved by generating an electric arc within a gas volume around the active electrode of the electrosurgical instrument. A high-frequency alternating voltage is applied to the electrode, causing alternating currents. These currents initially vaporize the saline solution in the immediate vicinity of the active electrode, forming a vapor bubble around it. An electric field develops within the vapor bubble between the electrode and the liquid saline solution. If the electric field is sufficiently strong, a gas discharge, also known as an arc, occurs as a result of the ionization of the gas in the vapor bubble, thus creating a plasma (ionized gas).

[0006] Before the plasma, and thus an electric arc, can be generated, an electrically conductive liquid surrounding the electrode must be vaporized in an initial phase. Subsequently, an electric arc must be ignited within the resulting vapor bubble. This requires that the vapor bubble completely surrounds the electrode and that no electrically conductive liquid remains in contact with the electrode. Otherwise, the strong electric field necessary for ionizing the gas in the vapor bubble cannot be generated, as the direct current flow between the active electrode and the electrically conductive liquid prevents the potential difference required for ionization.

[0007] After the initial ignition of the arc – that is, after the initial formation of plasma – a dynamically stable state is established. Wherever the gas bubble around the electrode threatens to collapse, a particularly strong electric field arises due to the approach of the electrically conductive liquid to the electrode. This, in turn, results in particularly strong plasma formation and thus greater vaporization of the electrically conductive liquid. In this way, the gas bubble around the electrode stabilizes.

[0008] The initial phase, which is eventually followed by the phase of a stable plasma arc, thus typically comprises two sub-phases: In a first sub-phase, which is also referred to here as the evaporation phase, the electrically conductive liquid around the active electrode is heated and evaporated by current flow, so that initially one or more gas bubbles of vapor form around the active electrode, until the active electrode is completely surrounded by a layer of gas, which finally electrically insulates the active electrode from the electrically conductive liquid.

[0009] Once this is achieved, a second sub-phase of the initial phase ignites an electric arc. This second sub-phase is also referred to as the ignition phase. During the ignition phase, the high voltage drop across the gas layer between the active electrode and the electrically conductive liquid ionizes the gas in the vapor bubble, creating a plasma that can be used to cut or vaporize tissue. Initial plasma ignition is challenging and highly dependent on environmental conditions. In some cases, a stable plasma is generated within a few milliseconds. However, under unfavorable conditions, several attempts are required before a stable plasma is ignited.

[0010] The various phases and sub-phases before and during plasma ignition also differ with regard to the electrical quantities supplied by the electrosurgery generator.

[0011] As long as the active electrode is not completely surrounded by a gas layer, the impedance between the active electrode and a counter electrode is low. Initially, the impedance is approximately 25 to 50 ohms, depending on the electrode geometry, the temperature of the electrically conductive fluid, and other factors. Therefore, during this initial heating phase, the output voltage and current from the electrosurgical generator are in phase with the electrically conductive fluid surrounding the active electrode and contain no DC component.

[0012] As soon as the electrically conductive liquid begins to evaporate and the surface of the active electrode is partially surrounded by gas, the impedance between the outputs of the electrosurgical generator increases because the surface resistance between the active electrode and the electrically conductive liquid increases. However, the surface resistance contributes only a relatively small portion to the total impedance between the electrodes—and thus the outputs of the electrosurgical generator. Therefore, the impedance increases only slightly during the evaporation phase until almost the entire active electrode is surrounded by gas.

[0013] Once the active electrode is completely surrounded by gas, the vaporization phase ends and the ignition phase begins. During the ignition phase, no ohmic current can flow between the electrodes. Therefore, the impedance is much higher than during the vaporization phase and—theoretically—purely capacitive. Because the impedance of the electrically conductive liquid is very low, the liquid forms an equipotential shell around the gas layer. An electric field is generated across the gas layer between the electrode and the conductive liquid.

[0014] As a result of the cessation of current flow, the gas layer around the active electrode thins again because some of the vapor forming the gas bubble condenses at the interface between the gas and the electrically conductive liquid. This thinning of the gas layer causes the electric field strength across it to increase, eventually becoming strong enough to ionize the gas within the bubble and generate an initial arc. The duration of this ignition phase depends on the thickness of the gas layer after the vaporization phase. It can be very short—typically on the order of a few milliseconds—or even negligible.

[0015] As soon as the electric field across the gas layer is strong enough to ionize the gas, plasma breakdowns occur—that is, arcs form—and an electrical discharge takes place. Because the surface area of ​​the active electrode is smaller than the surface area of ​​the electrically conductive liquid surrounding the gas bubble, the electric field strength is greater on the side of the active electrode than on the side of the electrically conductive liquid. This results in a DC component to the output AC voltage. This DC component (DC offset) is also known as the "spark voltage" and is approximately 100 volts.

[0016] As a result of electrical breakdown (plasma breakdown), salt solution evaporates at the interface between the gas and the electrically conductive liquid, and the thickness of the gas layer increases again at the site of plasma breakdown. Therefore, whenever the thickness of the gas layer decreases again due to condensation to such an extent that the electric field strength increases enough to cause a spark, the thickness of the gas layer increases again due to the evaporation of electrically conductive liquid. In this way, the thickness of the gas layer around the active electrode is self-stabilizing, and the plasma around the active electrode is in an equilibrium phase. In this equilibrium phase, a state of equilibrium is established in which the thickness of the gas layer is precisely such that the increase in thickness due to plasma breakdowns compensates for the decrease in thickness due to condensation at the interface between the gas and the salt solution.In the equilibrium phase, plasma breakthroughs therefore occur "automatically" where the gas layer around the active electrode is thinnest, because the electric field strength is highest there. Accordingly, new gas is generated by vaporization precisely where it is needed. The output voltage of the electrosurgery generator determines the thickness of the gas layer.

[0017] For plasma cutting, electrosurgical generators typically have an output AC voltage between 250V and 350V, for example 280V or 320V, according to the state of the art.

[0018] US2014236142A1 discloses a state-of-the-art electrosurgery generator.

[0019] In practice, problems occasionally arise when igniting the plasma for tissue cutting.

[0020] The aim of the invention is to improve an electrosurgical generator with regard to its suitability for plasma cutting of tissue.

[0021] The invention is defined by independent claims 1, 3 and 5, further embodiments are described in the dependent claims.

[0022] According to the invention, this objective is achieved with an electrosurgical generator configured to deliver a high-frequency alternating current to an electrosurgical instrument for plasma cutting of body tissue during operation. The electrosurgical generator has outputs for connecting an electrosurgical instrument in order to supply such an instrument with a high-frequency alternating current.

[0023] The electrosurgical generator features an impedance determination unit to determine the impedance of a load connected to its outputs. It also includes an output voltage control unit and may feature a voltage sensing unit.

[0024] The output voltage control unit is configured to regulate an AC output voltage of the electrosurgery generator according to a predetermined maximum output voltage value.

[0025] The impedance measurement unit is designed to determine the impedance of a load connected to the outputs of the electrosurgical generator. For this purpose, the impedance measurement unit can be configured to detect voltage, current, and / or phase shift. The impedance measurement unit can determine the impedance, for example, from the detected current, voltage, and, if applicable, phase. The detected phase is important if a current and voltage measurement unit only measure the RMS values ​​of current and voltage, since the impedance can be determined from the RMS values ​​of current and voltage and the phase shift just as easily as from the instantaneous values ​​of current and voltage.

[0026] The voltage detection unit is designed to detect the voltage applied to the outputs of the electrosurgery generator, in particular a DC voltage component of a voltage applied to the outputs.

[0027] The output voltage control unit is designed to control the output AC voltage – in particular its RMS value – as a function of a maximum output voltage value. According to the invention, the predetermined maximum output voltage value is different, at least for part of the initial phase, than in the subsequent equilibrium phase.

[0028] According to a first aspect of the invention, the predetermined maximum output voltage value in the evaporation phase - i.e. at the beginning of the initial phase - is smaller than the maximum output voltage value provided for the actual plasma cutting in the equilibrium phase.

[0029] According to a second aspect of the invention, the specified maximum output voltage value in the ignition phase - i.e. towards the end of the initial phase - is greater than the maximum output voltage value provided for the actual plasma cutting in the equilibrium phase.

[0030] Both aspects of the invention can be implemented independently and thus each constitutes a separate invention. However, both aspects can also be combined, in particular by configuring the electrosurgical generator to apply a maximum output voltage for the vaporization phase that is lower than the maximum output voltage for the equilibrium phase, and to apply a maximum output voltage for the ignition phase that is higher than the maximum output voltage for the equilibrium phase. Both aspects, individually and in combination, contribute to more reliable arc ignition at the start of plasma cutting.

[0031] To detect the end of the evaporation phase and the beginning of the ignition phase, or the end of the ignition phase and the beginning of the equilibrium phase – or both – the output voltage control unit is preferably connected to the impedance determination unit or the voltage sensing unit, or both.

[0032] To detect the end of the vaporization phase and the beginning of the ignition phase, the output voltage control unit can be configured to detect an increase in impedance at the outputs of the electrosurgical generator above a predefined level. This predefined level can be derived from an impedance determined by the impedance measuring unit at the beginning of the vaporization phase, for example, a predefined multiple of that impedance.

[0033] To detect the end of the ignition phase and the beginning of the equilibrium phase, the output voltage control unit can be configured to detect a DC component in the output AC voltage and to apply the maximum output AC voltage value specified for the equilibrium phase immediately or after a specified time period following the detection of a DC component.

[0034] Unlike the state of the art, plasma cutting does not specify a single maximum output AC voltage value suitable for the equilibrium phase and already applied in the initial phase - i.e. the vaporization phase and the ignition phase - but at least one maximum output AC voltage value that differs from the maximum output AC voltage value suitable for the equilibrium phase.

[0035] The invention includes the finding that a suitable maximum output AC voltage value for the equilibrium phase can lead to unstable states in the initial phase, which can impair reliable plasma ignition.

[0036] The output voltage control unit is thus designed to control the output AC voltage depending on a maximum output voltage value, which is specified during operation depending on an output value of the impedance determination unit and / or depending on an output value of the voltage detection unit (based on which the beginning and end of the evaporation or ignition phase are detected) such that the maximum output voltage value in an evaporation phase specifies a lower output AC voltage than in an ignition phase following the evaporation phase.

[0037] The output voltage control unit is therefore configured such that it is based on different maximum output voltage values ​​for the initial phase and the equilibrium phase. The initial phase can include the evaporation phase and / or the ignition phase. In the evaporation phase, there is direct electrical contact between the active electrode and the electrically conductive liquid surrounding the active electrode, and the electrically conductive liquid is evaporated by electrical heating due to the current flowing through it. In the ignition phase, the active electrode is completely enclosed by the gas bubble formed as a result of evaporation, and the impedance detected by the impedance measuring unit is significantly higher than during the evaporation phase.As soon as the gas in the gas bubble is ionized and an arc is formed, a DC component is also introduced into the AC voltage applied to the outputs of the electrosurgical generator. This DC component is detected by the DC voltage detection unit, which can then generate a corresponding output signal that indicates a (first) arc.

[0038] In the equilibrium phase, the plasma around the active electrode is stable. The maximum output voltage value, on which the output voltage regulation of the electrosurgical generator by the output voltage control unit is based during the equilibrium phase, is typically higher than the maximum output voltage value on which the output voltage regulation of the electrosurgical generator by the output voltage control unit is based during the vaporization phase.

[0039] According to the first aspect of the invention, the maximum output voltage value on which the output voltage regulation of the electrosurgical generator by the output voltage control unit is based during the evaporation phase is lower than the maximum output voltage value specified for the output voltage regulation of the electrosurgical generator by the output voltage control unit during the equilibrium phase. Preferably, no fixed maximum output voltage value is specified for the evaporation phase; instead, it is preferably adjusted based on the impedance detected between the outputs of the electrosurgical generator during the evaporation phase such that an approximately constant current density is maintained at the interface between the active electrode and the surrounding saline solution.

[0040] According to the second aspect of the invention, the maximum output voltage value on which the output voltage regulation of the electrosurgical generator by the output voltage control unit is based during the ignition phase following the vaporization phase is higher than the maximum output voltage value specified for the output voltage regulation of the electrosurgical generator by the output voltage control unit during the equilibrium phase. A higher maximum output voltage value for the ignition phase compared to the subsequent equilibrium phase leads to a higher AC output voltage of the electrosurgical generator in the ignition phase and results in faster and more reliable plasma ignition.

[0041] The invention incorporates the insight that by measuring the output voltage and output current of the electrosurgical generator, it is possible to derive quantities such as impedance or a DC component, and thus to differentiate between the various phases of the ignition process. By recognizing the different phases before and during the ignition of a plasma around the active electrode of an electrosurgical instrument, it is possible to individually set different maximum output voltage values ​​for each phase, instead of a fixed maximum output voltage value for all phases.

[0042] Preferably, the output voltage control unit is designed to control the output AC voltage as a function of a maximum output voltage value, which in operation is set as a function of an output value of the impedance determination unit for detecting the beginning of the ignition phase and / or as a function of an output value of the voltage detection unit for detecting the end of the ignition phase, such that the maximum output voltage value in the ignition phase is higher than in an equilibrium phase following the ignition phase.

[0043] The electrosurgical generator is preferably configured to detect the vaporization phase and the beginning of the ignition phase based on a measured impedance. Preferably, the electrosurgical generator is configured to use as a criterion for the beginning of the ignition phase a drop in the output alternating current during the vaporization phase to a predetermined fraction – for example, one-third – of the output alternating current at the beginning of the vaporization phase. The electrosurgical generator is thus preferably configured to detect the beginning of the ignition phase when the output alternating current has dropped to one-third or one-quarter of the output alternating current at the beginning of the vaporization phase.

[0044] According to another preferred embodiment, the electrosurgery generator is designed to detect the end of the ignition phase by means of a DC voltage component in a voltage applied to the outputs of the electrosurgery generator.

[0045] The output voltage control unit is preferably configured to apply a maximum output voltage value during the evaporation phase, which is adjusted based on the current output value of the impedance determination unit and / or the voltage sensing unit. The adjustment of the maximum output voltage value preferably occurs such that an at least approximately constant current density is maintained at the interface between the active electrode and the liquid in contact with it. Thus, as the active electrode becomes increasingly covered by a vapor bubble during the evaporation phase, and the area in contact with the conductive liquid decreases, the output voltage control unit reduces the AC output voltage accordingly, so that the current density remains at least approximately constant.

[0046] Preferably, the electrosurgical generator is designed to determine the impedance between the two outputs of the electrosurgical generator before the start of the evaporation phase using the impedance determination unit and to adjust the maximum output voltage value at the beginning of the evaporation phase such that the current resulting from impedance and output voltage is smaller than a maximum current of the output alternating current that can be supplied by the electrosurgical generator.

[0047] Alternatively, the output voltage control unit can be configured to apply a fixed maximum output voltage value during the evaporation phase. This fixed maximum output voltage value is chosen so that the maximum current that the electrosurgical generator can deliver is not exceeded, even at the beginning of the evaporation phase when the impedance is low, as measured according to Umax = Z * Imax. Because the impedance Z only increases until the end of the evaporation phase, the electrosurgical generator can maintain the fixed maximum output voltage, as the current only decreases.This variant is easier to implement than a maximum output voltage value that is tracked depending on the current output value of the impedance determination unit and / or the voltage detection unit, and can also prevent excessive vapor development towards the end of the evaporation phase.

[0048] Preferably, the electrosurgical generator is dimensioned such that it can supply a maximum AC output voltage of more than 250 V and a maximum AC output current of more than 4 A, for example a maximum AC output voltage between 250 V and 400 V and a maximum AC output current between 4 A and 12 A.

[0049] According to the invention, an electrosurgical system is also proposed, comprising an electrosurgical generator of the type described herein and an electrosurgical instrument which is connected, or can be connected, to outputs of the electrosurgical generator. The electrosurgical instrument has an active electrode and at least one counter electrode, as well as at least one fluid line which is arranged relative to the active electrode such that the active electrode can be surrounded by an electrically conductive fluid during operation.

[0050] The active electrode is preferably designed as a loop electrode or as a button electrode.

[0051] Preferably, the electrosurgical instrument is a resectoscope.

[0052] Another aspect of the invention is a method for operating an electrosurgical generator. A first embodiment of the method comprises the following steps: Generating an output AC voltage and supplying the output AC voltage to outputs of the electrosurgery generator, wherein a maximum output AC voltage value is specified for the output AC voltage, determining the load applied to the outputs and comparing the load with a limit value that is chosen such that falling below this limit value indicates the existence of an evaporation phase, wherein as long as the limit value is not exceeded, a maximum output AC voltage value is specified, which is preferably at least 30% smaller than a maximum output voltage value that is specified when the limit value is exceeded, and regulating the output AC voltage based on the respective specified maximum output AC voltage value.

[0053] Preferably, the maximum output AC voltage value is adjusted as a function of the specific load when the load changes, as long as the limit value is not exceeded.

[0054] The load can be determined, for example, by measuring the impedance at the outputs of the electrosurgical generator. The threshold value, which serves as a criterion for detecting a vaporization phase, is then, for example, an impedance value that defines a predetermined level, preferably derived from an impedance measured by the impedance detection unit at the beginning of the vaporization phase. The vaporization phase is present when the threshold value is undershot, i.e., when the impedance has not exceeded the predetermined level. The impedance detection unit determines the impedance, for example, from the measured current, the measured voltage, and—if only the RMS values ​​of the current and voltage are measured—the phase.

[0055] Such a method can prevent excessive steam formation, especially towards the end of the evaporation phase.

[0056] Additionally or alternatively, the procedure may include the following steps: Generating an AC output voltage and supplying the AC output voltage to outputs of the electrosurgery generator, wherein a maximum AC output voltage value is specified, determining the load applied to the outputs (18) and comparing the load with a limit value chosen such that falling below this limit value indicates the existence of an evaporation phase, detecting a DC component in the AC output voltage applied to the outputs, wherein as soon as the limit value is exceeded and no DC component is detected in the AC output voltage applied to the outputs (18), a maximum AC output voltage value is specified which is higher than a maximum output voltage value specified when a DC component is detected, and regulating the AC output voltage based on the maximum AC output voltage value.

[0057] Instead of detecting a 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).

[0058] With such a second variant of the process, an electric arc can be stabilized after initial ignition.

[0059] The two process variants can be combined.

[0060] Preferably, the method also includes a step of detecting the start of an ignition phase based on an increase in impedance above a limit value determined from an initial impedance value and a predetermined value, or based on a drop in the output AC current below a predetermined fraction of an initial output AC current value.

[0061] The invention will now be explained in more detail with reference to exemplary embodiments and the figures. The figures show: Figure 1: An electrosurgical system with an electrosurgical generator and an electrosurgical instrument connected to it; Figure 2: An illustration of the use of a resectoscope as an electrosurgical instrument; Figure 3: A schematic circuit diagram of an electrosurgical generator; Figures 4a-f: Different phases during the ignition of a plasma around an active electrode of an electrosurgical instrument; Figures 5a-c: An alternative embodiment of an active electrode for an electrosurgical instrument; Figure 6: A diagram illustrating the voltage and impedance profiles during the vaporization and ignition phases when an arc is generated at the active electrode of the electrosurgical instrument; Figure 7: A possible ideal profile of a maximum output voltage value during the vaporization phase, controlled as a function of the impedance detected during the vaporization phase;and Figure 8: a possible simplified curve of a maximum output voltage value during the vaporization phase, the ignition phase and the equilibrium phase.

[0062] In Figure 1 An electrosurgical system 10 is shown. The electrosurgical system 10 comprises an electrosurgical generator 12 and an electrosurgical instrument 14. The electrosurgical instrument 14 is connected to electrical outputs 18 of the electrosurgical generator 12 via a connecting cable 16.

[0063] In the illustrated embodiment, the electrosurgical instrument 14 is a resectoscope with a tube 20 through which an electrically conductive fluid can be guided as irrigation fluid to a distal end of the tube 20. A fluid channel is provided in the tube 20 for this purpose. The electrically conductive fluid exits at the distal end of the tube 20 and flows around an active electrode 22 at the distal end of the tube 20. The electrically conductive fluid circulates and is also drained away through the tube 20. The tube 20 thus encloses at least two lumens through which electrically conductive fluid can be guided to the distal end of the tube 20, where it can exit, while simultaneously fluid is drained away from the distal end of the tube 20 through another lumen. The lumens therefore serve as fluid channels.Appropriate hose connections for the electrically conductive fluid are located on the electrosurgical instrument 14 in . Figure 1 not shown.

[0064] The active electrode 22 of the electrosurgical instrument 14 can be pushed out of the tube 20, as shown in Figure 1 The active electrode 22 is indicated or may be retracted into the tube 20. Typically, the electrosurgical instrument 14 has a spring that either causes the active electrode 22 to be pushed out of the tube 22 by hand against spring force, or conversely, allows it to be retracted into the tube 22 by hand against spring force.

[0065] The active electrode 22 can have various shapes, for example a button electrode or a loop electrode. In Figure 1Not shown is a counter electrode, which may be formed, for example, by the tube 20, but may also be extended out of or retracted into the tube 20 together with the active electrode 22.

[0066] Figure 2 shows an electrosurgical instrument 14 in use. Figure 2 In addition to the connecting cable 16, hoses 24 and 26, which serve to supply and drain the electrically conductive fluid, can also be removed. An active electrode 22 is extended from the distal end of the tube 20 of the electrosurgical instrument 14.

[0067] When a high-frequency alternating voltage is applied between the active electrode 22 and a corresponding counter electrode, this initially causes a current to flow through the electrically conductive liquid surrounding the active electrode 22. As a result of the current flow, the electrically conductive liquid around the active electrode 22 heats up and consequently evaporates. Current continues to flow between the active electrode 22 in the corresponding counter electrode and through the electrically conductive liquid until the active electrode 22 is completely surrounded by a vapor bubble. Once this occurs, an alternating electric field forms within the resulting vapor bubble between the active electrode 22 and the electrically conductive liquid surrounding the vapor bubble.If the field strength of the alternating electric field is sufficiently high, the gas in the vapor bubble surrounding the active electrode 22 is ionized, and a plasma – recognizable by an electric arc – is formed. The first arc forms where the vapor bubble around the electrically conductive liquid surrounding the active electrode 22 is closest to it, as the field strength of the alternating electric field is greatest there. The resulting arc causes further liquid in the electrically conductive liquid to evaporate, so that the vapor bubble around the active electrode 22 does not collapse, but rather a dynamically stable equilibrium state is established around the active electrode 22.

[0068] The alternating voltage required to generate and maintain a plasma around the active electrode 22 – and likewise the alternating current required to vaporize the electrically conductive liquid – are provided by the electrosurgical generator 12. How Figure 3As can be seen, the electrosurgical generator 12 has a high-voltage power supply 30, which can be connected to the standard public power grid and provides a high-voltage direct current at its output 32. This high-voltage direct current is supplied to a high-frequency section 34 of the electrosurgical generator 12. The high-frequency section 34 of the electrosurgical generator 12 acts as an inverter and generates a high-frequency alternating voltage, which is delivered via an output transformer (not shown) of the high-frequency section 34 to outputs 18.1 and 18.2 of the electrosurgical generator 12. The electrosurgical instrument 14, as shown in Figure 1 is shown.

[0069] To control the output voltage of the electrosurgery generator 12, an output voltage control unit 36 ​​is provided, which controls the output voltage at outputs 18.1 and 18.2 based on a maximum output voltage value such that a set maximum output voltage value is not exceeded during operation. According to the invention, the maximum output voltage value to be applied is predefined differently for the various phases during plasma ignition, with the plasma ignition phases being recognized based on the electrical output values ​​at outputs 18.1 and 18.2.

[0070] For this purpose, a current sensing unit 38 and a voltage sensing unit 40 are provided, each of which detects the current delivered via the two outputs 18.1 and 18.2 and the voltage simultaneously dropping across the two outputs 18.1 and 18.2. The output values ​​of the current sensing unit 38 and the voltage sensing unit 40 are fed to an evaluation unit 42, which is configured to determine the impedance of a load connected to outputs 18.1 and 18.2. In this sense, the evaluation unit 42 is an impedance determination unit. The impedance determination unit can determine the impedance, for example, from the detected current, the detected voltage, and, if applicable, the detected phase.The detected phase is important when a current sensing unit and a voltage sensing unit measure only the RMS values ​​of current and voltage, since the impedance can be determined from the RMS values ​​of current and voltage and the phase angle just as easily as from the instantaneous values ​​of current and voltage. Furthermore, the evaluation unit 42 is also configured to detect a DC component in the AC voltage drop across outputs 18.1 and 18.2. In this sense, the evaluation unit 42 also serves as a DC voltage detection unit. The evaluation unit 42 provides output values ​​to the output voltage control unit 36 ​​that depend on the detected impedance and the detected DC component.The output voltage control unit 36 ​​is designed to determine a respective maximum output voltage value underlying the output voltage control based on the output values ​​for the impedance and the DC voltage component supplied by the evaluation unit 42.

[0071] The output voltage of the electrosurgery generator 12 is thus controlled by forming corresponding maximum output voltage values ​​for the output voltage control unit 36.

[0072] In Figure 4 The different phases of igniting a plasma around an active electrode - in this case a button electrode 22' - are shown schematically.

[0073] Larger-area counter electrodes 44.1 and 44.2 are provided around the leads 42 to the active electrode 22' compared to the active electrode 22' itself. In this application, both the active electrode 22' and the counter electrodes 44.1 and 44.2 are surrounded by a conductive coolant, namely a saline solution. When a high-frequency alternating voltage from the generator 12 is applied between the active electrode 22' and the counter electrodes 44.1 and 44.2, an alternating current initially flows through the conductive fluid surrounding the active electrode 22 and the counter electrodes 44.1 and 44.2. This is indicated in Figure 4a. The current flow heats the conductive fluid, and bubbles 46 are formed, as shown in Figure 4b. Once the active electrode 22' is completely surrounded by the vapor bubble 46, there is no direct current flow between the active electrode 22' and the conductive liquid.Rather, an alternating electric field forms between the active electrode 22' on one side and the boundary layer 48 between the conducting liquid and the vapor bubble 46 on the other. The field strength of this alternating electric field is greater the thinner the vapor bubble 46 is. The electric field strength is greatest where the conducting liquid or the boundary layer 48 is closest to the active electrode 22'. The fully formed vapor bubble 46 is shown in Figure 4c.

[0074] When the electric field strength between the active electrode 22' and the electrically conductive liquid surrounding the vapor bubble 46 exceeds a certain threshold, the gas in the vapor bubble 46 is ionized, and a plasma 50—recognizable by an electric arc—is generated around the active electrode 22'. Once the plasma is ignited, the active electrode 22 can be brought close to the biological tissue 50 to be treated in order to vaporize a portion of the biological tissue 52 using the plasma 50, thereby partially removing or cutting the biological tissue 52. This is illustrated in Figures 4d and 4e.

[0075] To cut biological tissue as with a paring knife, a 22" loop electrode can also be used as the active electrode. This is in Figure 5 depicted.

[0076] During the evaporation phase, as indicated, for example, in Figure 4b, an increasingly larger portion of the active electrode 22 is successively surrounded by a vapor bubble 46, so that the contact area between the conductive liquid surrounding the active electrode 22 and the active electrode 22 becomes progressively 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 electrically conductive liquid and the active electrode 22 decreases, the current density in the electrically conductive liquid increases where it contacts the active electrode 22, even with a constant output voltage of the electrosurgical generator 12. This is because the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 decreases.2 is not solely determined by the contact resistance between the active electrode 22 and the electrically conductive liquid, but also by the resistance (or impedance) of the electrically conductive liquid and the contact resistance to the counter electrodes 44.1 and 44.2. For example, it can be assumed that the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 is initially about 25 Ω during the evaporation phase (see Figure 4a). This 25 Ω is composed, for example, of about 10 Ω contact resistance between the active electrode 22 and the electrically conductive liquid, and about 15 Ω impedance of the electrically conductive liquid (including the contact resistance between the electrically conductive liquid and the counter electrodes 44.1 and 44.2).In this example, the 15 Ω impedance of the electrically conductive liquid is assumed to be more or less constant and does not change as the vapor bubble 46 around the active electrode 22 grows. The contact resistance between the active electrode 22 and the electrically conductive liquid, however, increases—inversely proportional to the degree of coverage with which the vapor bubble covers the active electrode 22. A coverage of 0 means that no vapor bubble has yet formed and the electrically 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 a vapor bubble 46. Under this assumption, the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 depends on the coverage according to the following formula: . Z = 15 Ω + 10 Ω 1 − sc where Z is the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2, and sc is the degree of coverage with which the vapor bubble 46 covers the surface of the active electrode 22. As already stated, a degree of coverage sc = 0 means that the electrically conductive liquid is in full contact with the surface of the active electrode 22, while a degree of coverage sc = 1 means that the surface of the active electrode 22 is completely enveloped by a vapor bubble 46.

[0077] The result is that the impedance initially rises only slowly in the initial phase of the evaporation phase and then increases sharply towards the end of the evaporation phase, for example, at a coverage of approximately 0.8 (80%). Such an increase in impedance is accompanied, at least initially, by an increase in the output voltage, because the electrosurgical generator 12 can only deliver a limited maximum current, meaning that the electrosurgical generator 12 cannot effectively reach its maximum output voltage of, for example, 320 V at low load impedance. This is in Figure 6 indicated by a dotted line.

[0078] The reason for this is that a typical electrosurgical generator 12 can only deliver a limited maximum output current of, for example, 4 to 5 A RMS, so that a specified maximum output voltage of, for example, 320 V RMS or 350 V RMS cannot be achieved at low impedance values. With an initially low impedance of approximately 25 Ω between the active electrode 22 and the counter electrodes 44.1 and 44.2 – and thus also between the outputs 18.1 and 18.2 of the electrosurgical generator 12 – the limited maximum output current of the electrosurgical generator 12 acts as a limiting factor, preventing the electrosurgical generator 12 from delivering its maximum output voltage. If this were possible, the electrosurgical generator 12 would deliver a current of 12.8 A RMS at an output voltage of 320 V RMS when the load is 25 Ω.

[0079] As the coverage of the active electrode 22 increases, the output voltage of the electrosurgical generator 12 rises, while the current delivered by the electrosurgical generator 12 remains constant. Thus, with increasing coverage sc, the output voltage, output conductance, and current density at the interface between the active electrode 22 and the electrically conductive liquid all increase. Only when the impedance between the active electrode 22 and the counter electrodes 44.1 and 44.2 becomes so large due to the growing vapor bubble that the electrosurgical generator 12 reaches its maximum output voltage of, for example, 320 V effective, does the current delivered by the electrosurgical generator 12 decrease. However, this only occurs at the very end of the evaporation phase – when the Figure 6 The illustrated example shows a coverage level of approximately 0.9 (90%).

[0080] The increasing current density with increasing coverage means that the electrically conductive liquid in contact with the active electrode 22 heats up ever faster, so that sudden, almost explosive vaporizations can occur, as is the case in Figure 5c can be seen.

[0081] This behavior is undesirable and can impair reliable and rapid plasma ignition. Therefore, according to the invention, the maximum output voltage of the electrosurgery generator 12 is reduced during the vaporization phase below a value that is intended as the maximum output voltage for the subsequent equilibrium phase. This is achieved by setting a maximum output voltage for the vaporization phase such that the maximum output voltage of the electrosurgery generator 12 is correspondingly low.Ideally, the maximum output voltage of the electrosurgical generator 12 is adjusted according to the coverage ratio, so that the maximum output voltage is initially higher at low coverage ratios and then decreases with increasing coverage ratio. This results in the current caused by the output voltage of the electrosurgical generator 12 decreasing inversely proportional to the coverage ratio of the electrode, ultimately achieving an approximately constant current density. Since the coverage ratio sc cannot practically be determined or measured directly, a preferred embodiment uses the impedance between outputs 18.1 and 18.2 – which, as previously explained, depends on the coverage ratio sc – to establish and adjust a suitable maximum output voltage during the evaporation phase. Figure 7shows how, in the example presented here, the maximum output voltage value can depend on the measured impedance between outputs 18.1 and 18.2, so that, under the aforementioned assumptions (impedance of the conducting liquid of approximately 15 Ω), a constant current density is achieved during the entire evaporation phase.

[0082] According to an alternative design variant, the maximum output voltage value during the evaporation phase is not continuously adjusted depending on the coverage or, alternatively, depending on the detected impedance, but is set to a constant value for the evaporation phase, whereby the evaporation phase is still detected based on the applied impedance value.

[0083] It should be noted that Figure 6An ideal output AC voltage profile during the evaporation phase as a function of the coverage degree is illustrated in the example case, namely by the in Figure 6 The red line indicates the effective value of the output AC voltage. Accordingly, the RMS value of the output AC voltage at the beginning of the evaporation phase is between 110 and 120 V and at the end of the evaporation phase is just under 50 V. Alternatively, a constant RMS value for the output AC voltage of the electrosurgical generator 12 can be specified during the evaporation phase, whereby this constant value can be, for example, between 50 and 120 V or, even better, between 80 and 100 V RMS.

[0084] As also explained at the beginning, the plasma around the active electrode 22 leads to a DC voltage component ( DC offset or spark voltage(referred to as DC) in the output AC voltage of the electrosurgery generator 12, as soon as the plasma has been ignited. Accordingly, the successful ignition of the plasma can be recognized by the appearance of a DC voltage component in the output AC voltage of the electrosurgery generator 12.

[0085] The end of the vaporization phase, and thus the beginning of the ignition phase, is preferably detected by an increase in impedance above a detection impedance value determined from an impedance value determined at the beginning of the vaporization phase and a predetermined factor, or by a drop in the output AC current below a predetermined fraction of an output AC current value determined at the beginning of the vaporization phase. Upon detection of the beginning of the ignition phase, the output AC voltage of the electrosurgical generator 12 is increased to the highest possible level, which is greater than the output AC voltage intended for the subsequent equilibrium phase. This ensures reliable ignition of the arc.As soon as an initial arc is generated—and thus a DC component is detected in the AC output voltage of the electrosurgical generator 12—it is desirable to maintain the AC output voltage of the electrosurgical generator 12 at the elevated AC output voltage value for a specific period of time. This elevated AC output voltage is higher than the AC output voltage intended for the subsequent equilibrium phase. The specific period during which the AC output voltage of the electrosurgical generator 12 is maintained at the elevated AC output voltage value can be, for example, 10 to 80 ms or 40 to 60 ms.

[0086] Accordingly, in a preferred embodiment, the output voltage control unit increases the maximum output voltage to a value exceeding that specified for the equilibrium phase. The output voltage specified for the equilibrium phase is, for example, between 250 V RMS and 320 V RMS. The increased output AC voltage can then be, for example, between 300 V RMS and 350 V RMS. Such an increased output voltage of the electrosurgical generator 12 during the ignition phase ensures that the arc is maintained as reliably as possible until the plasma in the vapor bubble around the active electrode 22 has stabilized.Once this is the case—and thus the aforementioned equilibrium phase begins—the output voltage of the electrosurgical generator 12 can be reduced again by correspondingly lowering the maximum output voltage value for the output voltage control unit 36. This can be done gradually or abruptly. By lowering the output AC voltage of the electrosurgical generator 12 after plasma ignition, the plasma layer (vapor bubble) around the active electrode 22 becomes somewhat thinner, but without the plasma extinguishing, as long as the output AC voltage of the electrosurgical generator remains sufficiently high and is, for example, 280 to 300 V.

[0087] In a simple embodiment, the electrosurgical generator 12 and its output voltage control unit 36 ​​could be configured such that the electrosurgical generator 12 initially outputs a maximum AC output voltage between 100 and 200 V until a DC voltage detection unit detects a DC component in the AC output voltage of the electrosurgical generator 12 that is greater than, for example, 50 Ω. As soon as such a DC component is detected in the AC output voltage of the electrosurgical generator 12, the output voltage control unit 36 ​​switches to a different maximum output voltage value of, for example, 320 V in order to maintain a stable plasma during the ignition phase.The increased maximum output voltage value for the ignition phase can, for example, be reduced again to a slightly lower value for the output AC voltage after a predetermined time of 10 to 50 ms, for example to a maximum output voltage value that is between 250 and 300 V RMS.

[0088] With such an electrosurgical generator 12, it is possible to reliably ignite a plasma around an active electrode 22 within a short, relatively stable time, with less dependence on environmental parameters. This greatly facilitates the use of a corresponding electrosurgical system 10 by a surgeon.

[0089] Figure 8Figure 1 illustrates a possible simplified curve of a maximum output voltage value during the evaporation phase 60, the ignition phase 62, and the equilibrium phase 64. During the evaporation phase 60, the maximum output voltage value is, for example, 100 V RMS or 120 V RMS. At the beginning of the ignition phase 62, the maximum output voltage value is then increased to, for example, 300 V RMS or 350 V RMS. After the end of the ignition phase 62, the maximum output voltage value is then reduced to the maximum output voltage value intended for the equilibrium phase 64, for example, 280 V RMS or 320 V RMS. The evaporation phase 60 and the ignition phase 62 together form the initial phase 66, during which the maximum output voltage value is either lower or higher than during the subsequent equilibrium phase 64. Reference sign

[0090] 10 Electrosurgical system 12 Electrosurgical generator 14 Electrosurgical instrument 16 Connecting cable 18, 18.1, 18.2 Outputs of the electrosurgical generator 20 Tube 22, 22' Active electrode 24, 26 Tubing 30 High-voltage power supply 34 High-frequency unit 36 ​​Output voltage control unit 38 Current sensing unit 40 Voltage sensing unit 42 Evaluation unit 44.1, 44.2 Counter electrodes 46 Vapor bubble 50 Plasma 52 Tissue 60 Vaporization phase 62 Ignition phase 64 Equilibrium phase 66 Initial phase

Claims

1. The method of operating an electrosurgical generator (12), wherein the method comprises the following steps: - generating an AC output voltage and supplying the AC output voltage to outputs (18) of the electrosurgical generator (12), wherein a maximum AC output voltage value is predefined for the AC output voltage. - determining the impedance of a load present at the outputs (18) and comparing the impedance with a threshold value, which is defined such that an impedance falling below this threshold value indicates the occurrence of a vaporization phase. - wherein, as long as the impedance remains below the threshold value, a maximum AC output voltage value is preset that is lower than a maximum output voltage value that is preset when the threshold value is exceeded. - controlling the AC output voltage based on the respectively preset maximum AC output voltage value.

2. The method of operating an electrosurgical generator (12) according to claim 1, characterized in that the maximum AC output voltage value is updated depending on the determined impedance in a changing impedance, as long as the determined impedance remains below the threshold value.

3. The method of operating an electrosurgical generator (12), wherein the method comprises the following steps: - generating an AC output voltage and supplying the AC output voltage to outputs (18) of the electrosurgical generator (12), wherein a maximum AC output voltage value is predefined for the AC output voltage. - determining the impedance present at the outputs (18) and comparing the impedance with a threshold value, which is defined such that an impedance falling below this threshold value indicates the occurrence of a vaporization phase. - determining a DC offset in the AC output voltage present at the outputs (18), - wherein, as soon as the threshold value is exceeded and a DC offset is not detected in the AC output voltage present at the outputs (18), a maximum AC output voltage value is preset that is higher than a maximum output voltage value that is preset if a DC offset is detected, and - controlling the AC output voltage based on the maximum AC output voltage value.

4. The method of operating an electrosurgical generator (12), wherein the method comprises the steps according to claims 1 and 3.

5. Electrosurgical generator (12) that is configured to implement a method according to claim 1 and / or claim 3, and to supply, during operation, a high-frequency alternating current to an electrosurgical instrument (14) for plasma cutting of body tissue (52), wherein the electrosurgical generator (12) has outputs (18) for connecting an electrosurgical instrument (14) to supply an electrosurgical instrument connected to the outputs (18) during operation with a high-frequency alternating current, and for determining the impedance of a load connected to the outputs (18); wherein the electrosurgical generator (12) further features an impedance measuring unit and a voltage measuring unit (40) as well as an output voltage control unit (36), of which the output voltage control unit (36) is configured to control or regulate an AC output voltage of the electrosurgical generator (12) in accordance with a predefined maximum output voltage value, and the impedance measuring unit is designed to determine an impedance of a load present at the outputs (18) of the electrosurgical generator (12) during operation; characterized in that the output voltage control unit (36) is designed to control the AC output voltage depending on a maximum output voltage value that is set depending on an output value of the impedance measuring unit and / or depending on an output value of the voltage measuring unit (40), such that the maximum output voltage value during the vaporization phase being part of the initial phase is lower than the maximum output voltage value during a later equilibrium phase.

6. Electrosurgical generator according to claim 5, characterized in that the output voltage control unit (36) is designed to control the AC output voltage depending on a maximum output voltage value that is set during operation depending on an output value of the impedance measuring unit and / or depending on an output value of the voltage measuring unit (40), such that the maximum output voltage value is higher during an ignition phase than the maximum output voltage value during an equilibrium phase occurring subsequently to the ignition phase.

7. Electrosurgical generator (12) according to at least one of claims 5 or 6, characterized in that the electrosurgical generator (12) is designed to detect the vaporization phase based on a measured impedance.

8. Electrosurgical generator (12) according to claim 7, characterized in that the electrosurgical generator (12) is designed to detect the end of the vapor phase based on an impedance increase at the outputs of the electrosurgical generator beyond a predefined value, wherein the predefined value is preferably derived from the impedance measured by the impedance measuring unit at the start of the vaporization phase, and is preferably a predefined multiple of this impedance.

9. Electrosurgical generator (12) according to at least one of claims 7 to 8, characterized in that the electrosurgical generator is designed to detect the end of the ignition phase based on a DC offset in the voltage present at the outputs of the electrosurgical generator.

10. Electrosurgical generator (12) according to at least one of claims 5 to 9, characterized in that the output voltage control unit (36) is designed to apply, during the vaporization phase, a maximum output voltage value updated depending on a respectively current output value of the impedance measuring unit and / or the voltage measuring unit.

11. Electrosurgical generator (12) according to at least one of claims 5 to 9, characterized in that the output voltage control unit (36) is designed to apply, during the vaporization phase, a maximum output voltage value that is predefined for the vaporization.

12. Electrosurgical generator (12) according to at least one of claims 5 to 11, characterized in that the electrosurgical generator (12) is dimensioned in such a way that it can supply a maximum AC output voltage of more than 300 V and a maximum AC output current of more than 4 A.

13. Electrosurgical system (10) with an electrosurgical generator (12) according to at least one of claims 5 to 12, and with an electrosurgical instrument (14) that is or can be connected to outputs (18) of the electrosurgical generator (12), and that has an active electrode (22) and at least one return electrode (44.1, 44.2) as well as at least one fluid line that is arranged relative to the active electrode (22) in such a way that the active electrode (22) can be surrounded by an electroconductive fluid during operation.

14. Electrosurgical system (10) according to claim 13, characterized in that the active electrode is designed as a loop electrode or a button electrode and / or wherein the electrosurgical instrument (14) is a resectoscope.