MXPA99005760A - Electrosurgical generator and system for underwater operation - Google Patents

Abstract

A radio frequency generator for an electrosurgical system is provided, the system including an electrode assembly having two electrodes for use immersed in an electrically conductive fluid. The generator has control circuitry for rapidly reducing the delivered radio frequency output power by at least 50%within at most a few cycles of the peak radio frequency output voltage reaching a predetermined threshold limit. In this way, tissue coagulation can be performed in, for example, saline without significant steam generation. The same peak voltage limitation technique is used in a tissue vaporisation or cutting mode to limit the size of the steam pocket at the electrodes and to avoid electrode burning. The generator has a push-pull output stage with a series-resonant output circuit, the output stage being driven by a radio frequency oscillator at a frequency which, in general, differs from the resonant frequency of the resonant output circuit. Power control is achieved by varying the ON-time of switching transistors forming the push-pull output pair and by altering the frequency spacing between the excitation frequency and the resonant frequency of the series-resonant output circuit. In an alternative embodiment, a bridge configuration using two push-pull pairs is used, yielding a further power control variable:the relative phase of the driving signals to the respective transistor pairs.

Description

A GENERATOR ELECTROQUIRÚRGICO AND SYSTEM FOR OPERATION UNDER WATER This invention relates to an electrosurgical generator for sending electrosurgical energy specifically but not exclusively in so-called electrosurgery under water or in a humid environment. The invention also relates to an electrosurgical system comprising the combination of a generator and an assembly of electrodes. The term "electrosurgery underwater or in moist medium" is used in this specification to refer to electrosurgery that is performed using an electrosurgical instrument with an electrode or treatment electrodes submerged in liquid in the surgical field, generally liquid introduced to distend a body cavity that includes the surgical field or to draw blood from said field. Alternatively, surgery can be performed with the electrode (s) submerged in the fluids usually found within the body. The invention has specific application in the fields of urology, hysteroscopy and arthroscopy. However, it should be understood that the invention includes features that may also have application in electrosurgery that does not include electrode immersion. The antecedents of electrosurgery in a humid environment and intracavitary surgery, that is, surgery by which living tissue is treated by less invasive surgical access to a cavity of the body, is described in our European Patent Application, pending granting, No. 96304558.8 (0754437), whose content is incorporated into this specification by reference. Effective electrosurgical treatment of tissue that is totally immersed in liquid at the site of application is difficult to achieve because the heat generated by the flow of electric currents in both the tissue being treated and the surrounding fluid tends to to cause the liquid to go into a boiling state. The operating electrode is intermittently surrounded by water vapor rather than liquid, with consequent large variations in the electrical impedance of the charge presented to the generator that provides the electrosurgical energy to the electrode. While this variation is mitigated by the use of a non-conductive fluid, it can not be completely eliminated due to the release of bodily fluids in the surgical field that elevates the electrical conductance of the fluid. Changes in the type of tissue also alter the impedance of the load. These effects result in difficulty in controlling the electrosurgical output to produce uniform effects on the tissue being treated. As a result, high energies are commonly employed to overcome this variation of performance. According to a first aspect of this invention, an electrosurgical generator for supplying radiofrequency energy to an electrical instrument comprises a radio frequency output stage having at least one pair of electrical connections. electrosurgical output for the supply of radiofrequency energy to the instrument, a power source linked to the output stage to provide power to the output stage, and a control circuit system including detection means for deriving a representative detection signal of the radio frequency peak output voltage developed through the output connections, where the output stage comprises a resonant output circuit coupled to the output connections and a switching means connected to the resonant output circuit, and where the The circuitry is operable to vary the switching intervals of the switching device to reduce the radiofrequency energy sent in response to a predetermined condition of the detection signal. In a preferred embodiment of the invention, the series resonance output circuit comprises the series combination of an inductance and a capacitance, and it is attached to the switching means so that a radiofrequency output waveform is switched through the series combination, the generator output connections that are connected to the series resonance circuit to receive the voltage of radiofrequency developed through the inductance or the capacitance, preferably the inductance. The series combination can be joined between the switching means and a ground connection and one of a pair of power supply power supply rails, one of the output connections of the generator is coupled to a junction between the inductance and capacitance, and the other is preferably connected to said ground connection or one of said feed rails. With the capacitance connected to the switching means and the inductance connected to the ground connection or supply rail, the output connection is preferably connected to the junction of the inductance and the capacitance by a coupling capacitance whose value is less than the of the capacitance of the series resonant combination. Alternatively, the switching means may comprise semiconductor switches connected in a gate configuration, the serial combination is coupled between opposite phase nodes of the switching means. In order to achieve a rapid reduction of energy when, for example, the liquid in the region of electrodes connected to the generator is vaporized, the switching means and the circuit system are arranged in such a way that the closing interval can be reduced of the switching means to the extent of causing at least a 50% reduction in the output power delivered within 100 μs of having reached a predetermined radio frequency peak output voltage threshold. (The term "peak output voltage" in this context includes voltages measured on a peak-to-peak basis). In other words, the generator responds to a detection signal representing peak or peak-to-peak output voltage levels. The switching means may comprise a pair of electronic switches connected in a push-pull series arrangement between the power supply rails, the resonance output circuit in series coupled to the connection between the electronic switches. In this way, to reduce the output power, the closing interval of each switch, which is typically a MOS field effect transistor energy ("MOSFET"), is reduced during respective half cycles of radio frequency to cause the reduction of energy required. By causing a control overvoltage, since the output energy is reduced by an amount greater than the increase necessary to produce vaporization, the vapor bubbles are allowed to disappear. This allows the surgery to be carried out in a conductive fluid field, specifically in a saline solution. Large and rapid changes in the load impedance can occur substantially without causing undesired electrosurgical effects. For example, when it is desired to produce electrosurgical drying, any increase in impedance can be largely prevented due to vaporization of the surrounding saline solution in the region of an electrode of the instrument which could otherwise lead to arcing. unwanted at the energy level required for an effective desiccation. When electrosurgical tissue cutting or tissue vaporization is required, limiting the output voltage can be used to avoid electrode burn and / or excessive tissue vaporization. To avoid overloading the power devices of semiconductor used in the switching means, the switching means is preferably driven by an oscillator operating at a frequency different from the resonant frequency of the series resonance output circuit. By operating the oscillator at an excitation frequency greater than the resonant frequency of the series resonance output circuit, the available energy can be increased to comparatively high impedances associated with cutting or vaporization, while operating the oscillator way to excite the resonant circuit at a frequency lower than that of its resonant frequency is more suitable for electrosurgical drying involving comparatively low load impedances. The aforementioned control circuit system is preferably arranged in such a way that a 50% reduction in the output power occurs in a period of less than 20 μs, after the output voltage reaches the predetermined detection signal threshold by reducing the driving time of the electronic switches during individual cycles of the radio frequency output signal. Said alteration of the driving period is advantageously achieved independently of any variation in the supply voltage. In practice, the reduction in the output power is achieved by using a single control variable, that is, the peak output voltage or peak-to-peak output voltage, independently of the supply voltage and independently of the power of the output. supplied output which, of course, varies according to the load impedance and the supply voltage. In this way, the tripping of an energy reduction occurs at the same threshold of predetermined output energy with different output power and load impedance values, according to the circumstances. The technique for directly controlling the radio frequency output stage can be performed by repeatedly producing, first, a rapid reduction in the cycle-by-cycle driving period of the energy device from a peak level to a valley level when the threshold is reached output followed by, secondly, a progressive increase in the driving period until the driving period again reaches its peak level, the radio frequency output voltage is monitored during the progressive increase. The output stage preferably includes an output resonant circuit with a "Q" that is high enough to remove the switching noise from the switching device or devices of the stage without unduly reducing the response to the output voltage by reaching the threshold predetermined. Typically, the "Q" is at least 1 and is also sufficient to achieve a peak factor less than 1.5, the peak factor is the ratio between the peak and the r.m. (mean square value) of the output voltage waveform. Other aspects of the invention include a generator for electrosurgery in a humid environment with an output impedance in a range between 100 ohms and 250 ohms, and preferably between 130 and 190 ohms. Said generator has its radio frequency output stage operable to produce a continuous wave output, that is, with a working rate of 100% or without modulation of the on / off pulse width ("on / off"), at a frequency less than the frequency of radio frequency oscillation. In effect, the output stage can operate as an open circuit stage. According to a second aspect of the invention, an electrosurgical system is provided that includes a generator for generating radiofrequency energy and an electrosurgical instrument having at least one electrode for use submerged in a conductive liquid, wherein the generator comprises a step of output that includes at least one radiofrequency energy device, a series resonance output circuit and at least one pair of output connections arranged to receive radio frequency energy from the power device, one of the pairs of connections is connected to said electrode, and wherein the generator further comprises an operable control stage for reducing the driving time of the energy device during the radiofrequency cycles in response to a detection signal representative of the voltage presented to the generator through the connections of output that exceed a detection signal threshold value n default, by which the radiofrequency energy supplied to the electrode structure is reduced 11 feedback signal to control the radiofrequency energy sent. The invention will now be described by way of example in connection with the drawings in which: Figure 1 is a diagram showing an electrosurgical system according to the invention. Figure 2 is a fragmentary view of a first assembly of electrodes for tissue dewatering, shown in use and immersed in a conductive liquid. Figure 3 is a characteristic load graph illustrating the variation in load impedance produced by an electrode assembly such as that shown in Figure 2 when used in a conductive liquid, in accordance with the delivered power output. Figure 4 is a fragmentary view of a second electrode assembly for tissue vaporization, shown in use submerged in a liquid. Figure 5 is a block and combined circuit diagram of a generator according to the invention. Figure 6 is a waveform diagram. Figure 7 is a block diagram of part of the control circuitry of the generator of Figure 5; Figure 8 is a graph of charge impedance versus energy related to the generator when it is operating in a desiccation mode. quickly when the conductive liquid vaporizes. The electrode structure can include a distal treatment electrode and a liquid contact electrode spaced proximal to the distal electrode, both electrodes for use are surrounded by a conductive liquid and each is connected to a respective pair of output connections. , the control stage is operable to reduce the decrease time of the energy device when the conductive liquid at the distal electrode is vaporized. The electrosurgical instrument can provide a structure of electrodes having first and second juxtaposed electrodes to be immersed in the conductive liquid, the first and second electrodes respectively form a tissue contact electrode at a distal end of the instrument and a return electrode separated proximally from the tissue contact electrode. The system can be switched between at least one tissue drying mode and a tissue vaporization or cutting mode, using a mode selection control. In this case the control stage is automatically operated to adjust the radiofrequency energy supplied to the electrode structure to limit the peak output voltage of the generator to a first value when the desiccation mode is selected and at least a second value when the vaporization or cutting mode is selected, the second value or values are higher than the first value. The first and second values are advantageously in the range between 150V and 200V, and between 250V and 600V respectively, being These voltages peak voltages. The mode selection control may be coupled to the oscillator of the generator that drives the energy device so that, in the tissue desiccation mode, the oscillator oscillation frequency is less than the resonant frequency of the series resonance output circuit , although in the vaporization or tissue cutting mode said frequency is higher than the resonant frequency for an improved output energy at comparatively high and comparatively low impedances respectively, as mentioned above. According to a third aspect of the invention, an electrosurgical generator is provided to supply radiofrequency energy to an electrosurgical instrument, the generator comprises a radio frequency output stage having at least one pair of electrosurgical output connections for the power supply of radio frequency to the instrument, a radio frequency oscillator for feeding a radio frequency signal to the output stage, and a control circuit system including detection means for deriving a detection signal representative of the radio frequency signal sent from the radio frequency connections. output, where the output stage comprises a series resonance output circuit coupled to the output connections, the resonant frequency of the series resonance output circuit is different from the operating frequency of the oscillator, and where the circuit system of control provides a Figure 9 is a similar graph applicable to a vaporization or tissue cutting mode; and Figure 10 is a block diagram and combined circuit of an alternative generator according to the invention. The preferred embodiment of the present invention forms bipolar electrosurgery with electrodes submerged in a conductive liquid medium such as normal saline. The electrosurgery is performed using a system comprising a generator and an instrument, the instrument has a double electrode structure and the saline acts as a conductor between the tissue being treated and one of the electrodes that we will call, from now on, the "electrode return". The other electrode is applied directly to the tissue. This other electrode will henceforth be referred to as an "active electrode". In Figure 1, said system is shown. The generator 10 has a 10S output plug that provides a radio frequency (RF) output for an instrument in the form of an applicator 12 via the connecting cord 14. The generator activation can be performed from the applicator 12 via a power connection. control on the cord 14 or by means of a foot switch unit 16, as shown, connected separately to the back of the generator 10 by a connecting cord with foot switch 18. In the illustrated embodiment, the foot switch unit 16 has two foot switches 16A and 16B for select a desiccation mode and a vaporization mode of the generator respectively. The front panel of the generator has buttons buttons 20 and 22 to determine respectively the energy levels of drying and vaporization, which are indicated in the indicator 24. The push buttons 26 are an alternative means to select between the modes of desiccation and vaporization. The applicator 12 assembles a removable electrode assembly 28 with a double electrode structure, as shown in the fragmented view of Figure 2. Figure 2 is an enlarged view of the distal end of the electrode assembly 28. At the end of the distal end the assembly has an active electrode 30 which , in this embodiment, it is formed as a series of metallic filaments connected to a central conductor 32. The filaments can be made of stainless steel. Approximately the active electrode 30 and spaced from the latter by an insulator 34 extending longitudinally and radially is the return electrode 36. The return electrode 36 is disposed coaxially around the inner conductor 32 as a sleeve 38 which extends as a tubular shaft 40 to the proximal end of the assembly 28 where it is connected in the applicator 12 to the conductors in the connecting cord 14. Similarly, the inner conductor 32 extends to the applicator and is connected to a conductor in cord 14. The assembly Electrode 28 has an insulating jacket 42 that covers the shaft 40 and terminates near the insulator 34 to leave the distal end of the shaft 40 exposed as the return electrode 36. When functioning as a drying instrument, the assembly of electrodes 28 is applied as shown in Figure 2 to the tissue 44 to be treated, the surgical field is immersed in a normal saline solution (0.9% w / v), which is shown here as a drop 46 of surrounding fluid the distal end portion of the electrode assembly 28. The liquid immerses both the active electrode 30 and the return electrode 36. Referring again to Figure 2, the metallic filaments forming the active electrode 30 are all electrically connected together and to the inner conductor 32 of the electrode assembly to form a unitary active electrode. The insulator 34 is an insulator jacket, the distal end portion of which is exhibited proximate to the exposed portion of the active electrode 30. Typically, this jacket is made of a ceramic material to resist damage caused by arcing. The return electrode terminates at a point near the end of the insulator 36 such that it is spaced radially and axially from the active electrode, or tissue contact 30. The surface area of the return electrode is considerably greater than that of the active electrode. At the distal end of the electrode assembly, the diameter of the return electrode is commonly in the region comprising between 1 mm and 3 mm, with the longitudinal extent of the exposed part of the return electrode commonly being between 1 mm and 5 mm. with the longitudinal spacing from the active electrode between 1 mm and 5 mm. In effect, the electrode assembly is bipolar, and only one of the electrodes (30) actually extend to the distal end of the unit. This means that the return electrode, under normal circumstances, remains spaced from the tissue being treated and there is a current path between the two electrodes via the tissue and the conductive liquid which is in contact with the return electrode 36. The liquid Driver 46 can be considered, as regards the supply of bipolar electrosurgical energy, as a low extension of tissue impedance. The radiofrequency currents produced by the generator 10 flow between the active electrode 30 and the return electrode 36 via the tissue 44 and the conductive immersion liquid 46. The particular electrode assembly shown in Figure 2 is very suitable for the tissue drying. The axial as well as the radial separation between the electrodes avoids the small spacing of the conventional bipolar arrangement in which both electrodes are in contact with the tissue. As a result, there is less danger of an undesired formation of arcs through the insulation surface which comparatively allows a high energy dissipation for the drying treatment, and, in the case of cutting or vaporization of tissue, avoids the excessive formation of arcs which can cause damage in the inter-electrode insulation. The immersion saline solution can be provided from a conduit (not shown) that is part of the instrument 12. Thus, the invention can take the form of an electrosurgical system for the treatment of tissue immersed in a conductive fluid medium, comprising an electrosurgical instrument with an applicator and an instrument shaft, and at the end of the shaft, an assembly of electrodes, the assembly comprises a tissue contact electrode that is exposed at the end of the distal end of the instrument, and a return electrode that is electrically isolated from the tissue contact electrode and has a fluid contact surface spaced near the exposed portion of the tissue contact electrode, The system further comprises a radiofrequency generator attached to the electrode assembly of the instrument, a reservoir of electrically conductive fluid, such as normal saline, and a conduit, typically an integral part of an endoscope, to deliver the liquid from the reservoir to the region of the reservoir. assembly of electrodes. Pressure can be supplied to supply the liquid by means of a pump that is part of the apparatus. Because in this embodiment of the electrode assembly 28, the active electrode 30 is made of stainless steel filaments in the form of a brush, the electrode is flexible and provides a reproducible tissue effect that is comparatively independent of the application angle of the electrode. electrode to the tissue surface. The flexibility of the electrode 30 also results in a differential contact area of the active electrode which depends on the applied pressure, allowing variations in the drying width on the tissue surface, reducing the time of the procedure. The drying takes place by virtue of radiofrequency currents passing between the electrode 30 and the conductive liquid 46 via the outer layer of the tissue 44 immediately below and in an area surrounding the active electrode 30. The output impedance of the generator is set at a level commensurate with the load impedance of the electrode assembly when used as used as shown in Figure 2, with both electrodes in contact with the conductive liquid 46. In order to maintain this combined state for drying the In this case, the output energy of the generator is automatically controlled in the manner described below, so that the appearance of vapor bubbles of significant size in the active electrode 30 is substantially avoided, thus avoiding the consequent increase in the load impedance. In this way, the active electrode can be continuously moistened by the conductive liquid so that, while the water in the tissue is removed by thermal drying, the impedance reaches an upper limit that corresponds to the point at which the boiling of the liquid begins. driver. As a result, the system is capable of supplying high energy levels for desiccation without a vaporization of undesired conductive liquid that leads to unwanted tissue effects. The electrical behavior of the electrode assembly when the electrodes 30 and 36 are immersed in the conductive liquid 46 is now considered with reference to the graph of the Figure 3. When the energy is applied first, the generator is presented with a load impedance "r" that is governed by the geometry of the electrode and the electrical conductivity of the conductive liquid. The value of "r" changes when the active electrode touches the tissue. The higher the value of "r", the greater the propensity of the conductive liquid to vaporize. As the energy dissipates in the tissue and conductive liquid, the conductive liquid increases its temperature. In the case of normal saline solution the conductivity temperature coefficient is positive and the corresponding impedance coefficient is therefore negative, so that the impedance initially decreases. Thus, the curve in Figure 3 indicates a drop in the impedance of the load as the energy supplied increases, the impedance that falls through point A to the minimum at point B, at which point the saline solution in immediate contact with the electrode reaches the boiling point. Small bubbles of vapor form on the surface of the active electrode and the impedance begins to rise, as shown in the curve that rises from point B to point C. Thus, once the boiling point is reached, the assembly displays a dominant positive energy coefficient of impedance. As the vapor bubbles form, there is an increase in the energy density at the remaining active electrode to the saline interface (the exposed area of the active electrode not covered by vapor bubbles) that also accentuates the interface, producing more vapor bubbles, and thus a higher energy density. This is a decisive condition, with a point of equilibrium that only occurs once the electrode is completely wrapped in steam. Thus, for a certain assembly of variables, there is an energy threshold that corresponds to the point C in which this new equilibrium is reached. In light of the above, it will be appreciated that the region between points B and C of Figure 3 represent the upper limit of the drying energy that can be achieved. Before the formation of a vapor pocket that surrounds the electrode, the impedance rises to around 1 O, as shown in point D of Figure 3, the value of the real impedance depends on a number of system variables . The vapor is then held by discharges through the pocket between the active electrode and the vapor / saline interface. This state of the situation is illustrated by the diagram of Figure 4 showing an alternative electrode assembly 28A with a hemispherical or spherical electrode 30A instead of the brush electrode 30 of the embodiment of Figure 2. As in the above , the return electrode 36A is spaced close to the active electrode 30A by an intervening isolator 34A. The spherical electrode is preferred for tissue vaporization. Once in the vaporizing equilibrium state, the vapor pocket, which is shown in reference 50 of Figure 4, is it holds by discharges 52 through the vapor pocket between the active electrode 30A and the vapor to the saline interface. Most of the energy dissipation takes place inside this pocket with the consequent heating of the active electrode. The amount of energy dissipation in this conduction is a function of the energy supplied. It will be noted in Figure 3 that the vaporization mode, indicated by the contour dot lines, can be maintained at much lower energy levels than those required to generate the formation of the vapor pockets. The characteristic impedance / energy consequently displays the hysteresis. Once the vaporization mode has been established, it can be maintained over a comparatively wide range of energy levels, as shown by the inclined part of the feature that extends to both sides of point D. However, the increase of the output power supplied beyond that represented by point D causes a rapid increase in electrode temperature, potentially damaging the electrode. To dissolve the steam pocket and to return to the drying mode, a significant reduction of energy is required back to point A, so that the direct contact between the active electrode and the saline solution is restored and the impedance is extremely low. The density of the energy in an active electrode also decreases so that the temperature of the saline solution now drops below the boiling point and the electrode is then again in equilibrium of stable desiccation. The generator described below has the ability to maintain the two modes of desiccation and vaporization. While in general the electrode assemblies illustrated in Figures 2 and 4 can be used in any of the modes, the brush electrode of Figure 2 is the one preferred for desiccation due to its wide potential coverage area, and the spherical electrode of Figure 4 is preferred for vaporization due to its small percentage of surface area of active electrode / return electrode. As can be seen in Figure 4, tissue vaporization occurs when the vapor pocket 50 crosses the tissue surface, and the electrode assembly is preferably spaced above the tissue surface by a small distance (commonly 1 mm to 5 mm ). The decisive condition that occurs when the supplied power reaches the level shown in point C of Figure 3 is exacerbated if the generator has a significant output impedance., because the output voltage can then rise suddenly. With greater energy dissipation and without the presence of cooling liquid around the active electrode 30, the temperature of the electrode rises rapidly with consequent damage to the electrode. This also produces the uncontrollable breaking of the tissue instead of the required drying. For this reason, the preferred generator has an output source impedance that, at least, at least equalizes the load impedance of the electrode structure when it is wetted. The preferred generator to be described now allows electrosurgery by desiccation substantially without undesired cell disruption, and electrosurgical cutting or vaporization substantially without burning the electrode. Although primarily intended for operation in a conductive liquid distention medium, it has application in other electrosurgical procedures, for example in the presence of a gaseous distention medium, or wherever rapid charge impedance changes may occur. With respect to Figure 5, the generator comprises a radio frequency (RF) oscillator 60 that operates above or below 400 kHz, with any frequency from 300 kHz and above within the HF range being feasible. The oscillator 60 drives an energy output stage 62 comprising two MOS field effect transistors (MOSFET) of energy T1 t T2 coupled in a counter-phase arrangement between two supply rails Vs and 0V. The power supply rails are driven by a power supply stage 64. The transistors T ^ T2 are driven in a switching mode by an impulse duration controller 64 which in turn is driven by the oscillator 60. From this each transistor T ,, T2 receives an unlock pulse during each cycle of the RF oscillator 60, the pulses are synchronized so that T ,, it is switched during the half cycles of the oscillator of one polarity, while T2 is switched during the half cycles of the opposite polarity oscillator, and the duration of the unlocking pulses is controlled according to the required output energy. Coupled to the junction between the two electronic switches represented by the transistors T ,, T2 is a series resonance output circuit comprising a capacitor Cr and an inductor Lr. The series resonant frequency of these two components is around 400 kHz, but is generally different from the operating frequency of the oscillator 60. Coupled to the junction between the capacitor Cr and the inductor Lr, by means of a coupling capacitor Cc, there is one end of a primary winding of an output isolation transformer 65 which supplies the output terminals 66, 68 of the generator. The other end of the primary winding is coupled to one of the supply rails, in this case the ground connection OV. The coupling capacitor Cc is smaller than the capacitor Cr. The connection of the series resonant circuit Cr, Lr to the switches is direct; there is no second resonant circuit in series that intervenes, such as a parallel resonant circuit, and the load impedance presented to the switches (when its variation is considered in relation to the excitation frequency) demonstrates a predominant minimum in the resonant frequency of the resonant circuit in series. In parallel with the consumption and source connections of each transistor T- and T2 there is a respective power recovery diode D-i, D2. The switching of the transistors T ,, T2, effected in the manner cited, causes the application to the junction 65 between the transistors of an RF excitation voltage to have a waveform as shown in Figure 6. A sinusoidal waveform This occurs at the junction of the two series resonant components Cr, Lr, whose amplitude depends on the frequency difference between the frequency of the oscillator and the resonance frequency, and on the impedance of the load 70 connected through the terminals output 66 and 68. Coupled through the output connections 66 and 68 there is a voltage threshold detector 72 having an output 72A coupled to an "on" control circuit 74. When the generator is operated, power is applied to the power supply 64 when the surgeon requires the electrosurgical energy by operating an activation switch arrangement that can be supplied in an applicator or foot switch unit (see Fig. to 1). A constant output voltage threshold is determined independently of the supply voltage via the input 72B in accordance with the control settings on the front panel of the generator (See Figure 1). Typically, for drying or coagulation, the threshold is determined at a drying threshold value between 150 volts and 200 volts. When a cut or vaporization output is required, the threshold is determined by a value in the range of between 250 or 300 volts up to 600 volts. These voltage values are peak values. The fact that they are peak values means that for desiccation at least it is preferable to have an RF output waveform of low crest factor to give maximum energy before the voltage is set at the given values. Typically, a crest factor of 1.5 or less is achieved. When the generator is activated for the first time, the status of the control input 64I of the pulse duration controller 64 (which is connected to the "on" time control circuit 74) is "on", so that the transistors Ti, T2 forming the output stage 62 are each switched during a maximum conduction period during each oscillation cycle which can be a half full cycle of the output of the oscillator. If the supply voltage is high enough, the temperature of the liquid medium surrounding the electrodes of the electrosurgical instrument (or within a gaseous medium, the temperature of the liquids contained within the tissue) may rise to such an extent that the liquid medium evaporate, leading to a rapid increase in the load impedance and the consequent rapid increase in the output voltage applied across terminals 12. This is an undesirable state of affairs if a desiccation outlet is required. For this reason, the voltage threshold for a desiccation output is determined to cause a trip signal to be sent to the "on" time control circuit 74 when the threshold is reached. The "on" time control circuit 74 has the effect of virtually instantaneously reducing the duration of the unlocking pulses produced by the controller 64, thereby virtually instantly reducing the "on" time of the radio frequency switching device T ^ T2. The subsequent control of the "on" time control circuit of the mechanisms Ti, T2 in individual cycles of the oscillator 60 will be understood after considering the internal configuration of the "on" time control circuit 74 according to Figure 7. The circuit it comprises a sawtooth RF generator 80 (synchronized at the RF oscillation frequency via a synchronization signal derived from the oscillator and applied to a synchronization input 74I), as well as a ramp generator 82 which is preset through a reset pulse from the output 72B of the voltage threshold detector 72 (see Figure 5) that occurs when the set voltage threshold is reached. This reset pulse is the trigger signal indicated above. The "on" time control circuit 74 also comprises a comparator 84 for comparing the sawtooth ramp voltages produced by the ramp and sawtooth generators 80 and 82 to produce a square wave control signal for applied to the input 64I of the pulse width controller 64. As appears from the waveform diagrams of Figure 7, the nature of the ramp and sawtooth waveforms is such that the The ratio of the marks to the space of the square wave signal applied to the controller 64 increases progressively after each reset pulse. As a result, after a virtually instantaneous reduction in the "on" time by detecting that the voltage output reaches the set voltage threshold, the "on" time of the RF oscillator progressively increases back to the original maximum value. This cycle is repeated continuously until the temperature of the liquid surrounding the electrodes decreases to such a level that vaporization no longer occurs. The voltage output of the generator is important for the operation mode. In fact, the output modes are defined exclusively by the output voltage, specifically the peak output voltage. The absolute measurement of the output voltage is only necessary for the multiple duration control. Nevertheless, a simple single duration control (ie, the use of a control variable) can be used in this generator to restrict the output voltage to predetermined voltage limits. Thus, the power threshold detector 72 shown in Figure 5 resembles the peak output voltage of the RF with a preset threshold level DC, and has a response time sufficiently fast to produce a reset pulse for the control circuit of "on" time 74 within a half cycle of RF. When the series resonance output circuit is excited By connecting the transistors T1, T2 at a frequency close to the resonance, the power amplitude at the junction between the transistors T2, T2 can exceed the supply voltage. In this way, when both transistors are turned off, the diodes D ,, D2 recover energy from the resonance circuit to the supply. Intermediate levels of excitation are possible by using less than the average-wave switching of the push-pull devices, the "on" time excites, the "off" time recovers energy and provides decrease of the wave amplitude. Before continuing to deal with the generator operation, it is convenient to re-reference the energy / impedance characteristic of Figure 3. It will be noted that the most important threshold control is that applied during drying. Since the vapor bubbles that form in the active electrode are not conductive, the saline substance that remains in contact with the electrode has a higher energy density and consequently an even greater predisposition to form steam. This degree of instability produces a transition to a vaporization mode with the same energy level due to the decisive increase in energy density in the active electrode. Consequently, the local impedance for the active electrode increases. The maximum of the energy absorbed coincides with the condition of the existing electrode immediately prior to the formation of the vapor bubbles, given that it coincides with the distribution maximum energy and most of the wet area of the electrode. Therefore, it is advisable that the electrode remain in the wet state during the maximum drying energy. The use of voltage limit detection brings a reduction of energy that allows the dissolution of the vapor bubbles, which in turn increases the capacity of the electrode to absorb energy. For this reason, the generator described in this specification includes a control loop with a large overvoltage, since the return stimulus of the peak voltage that reaches the predefined threshold causes a significant instantaneous reduction of energy by causing a reduction in voltage peak output at a level significantly below the peak output voltage level set by the threshold detector 72. This overvoltage control ensures a return to the required wet level. In the generator described above with reference to the Figures 5, 6 and 7, the energy reduction occurs in response to the detection of the voltage threshold through an instantaneous reduction in the RF energy supplied to the resonant output circuit in earnest. In the preferred configuration, the instantaneous energy reduction is at least three quarters of the available energy (or at least half voltage) of the DC power supply and preferably higher. Accordingly, a high speed response is obtained at the RF stage itself. In a typical case of desiccation, the output voltage increases with higher load impedance to a point where the output voltage threshold is reached, from which the instantaneous reduction of the "on" time at the output level occurs. This causes a rapid decrease in the RF output voltage, followed by a progressive increase, again as indicated above. When the output voltage reaches the voltage threshold again, the "on" time of the oscillator is reduced again instantaneously and then progressively increases, so that the waveform of the voltage output repeats its previous model. However, the voltage threshold is reached, again the voltage output is instantly reduced, and again the "on" time can increase, and so on until the conditions in the surgical field are modified in such a way that it no longer forms steam. It will be noted, then, that the circuit control system 74, 64 (Figure 5) operates dynamically to control the output voltage sufficiently quickly and to a sufficient degree to maintain the voltage at a level in accordance with, in this case, the level required for desiccation without tissue breakage. to the formation of arcs. The same technique can be used with a different voltage threshold to restrict the output voltage and avoid electrode burn and / or excessive tissue vaporization. In the latter case, the voltage limit can be set at a level between 250 volts (preferably 300 volts) and 600 volts. Due to the high energy density in the active electrode during the vaporization mode, most of the energy emitted dissipates in the vicinity of the electrode. It is advisable that in the vaporization mode the minimum heating of the saline be produced, but that any tissue that passes the vapor limit of the active electrode vaporizes. In the vaporization mode, the vapor is held by arcs within the vapor pocket as described above in relation to Figure 4. The increase in the output voltage during vaporization causes an increase in tissue removal due to the increase in vaporization. size of the steam pocket. The dissolution of the vapor pocket during the vaporization of the tissue has more important consequences, due to the increase of necrosis as a consequence of the greater dissipation of energy in the surrounding saline substance. The dissolution of the vapor pocket can be avoided in the following way, firstly, by achieving that the impedance of the electrode in the vaporization mode is such that the instrument is in an incomparable condition with respect to the impedance, as a consequence of which the circuit of The resonant output Q is high and the output voltage does not change as quickly as with a lower load impedance and, second, the active electrode has a sufficient heating capacity that maintains the steam pocket for a considerable period. An undesirable increase in the size of the vapor pocket can be avoided by limiting the peak output voltage during the vaporization mode, which can be done by replacing a value of different threshold by the voltage threshold detector 72 (see Figure 5) during the vaporization mode. The circuitry of the voltage threshold detector 72, and the "on" time control circuit 74 (indicated in Figure 5) in the preferred generator according to the invention is as described and shown in our Application European Patent Pending No. 96304558.8. As described above, different threshold voltages for the desiccation and cutting or vaporization of tissue are applicable. Accordingly, the generator includes a mode selection control 86, as seen in Figure 5. In practice, this may be part of a microprocessor control system (not shown) that produces outputs that depend on the calibration for the apiicador or unit of foot switch. Thus, for a desiccation output, the mode selection control determines the threshold voltage of the threshold detector 76 to a first value through the input 72B, while for the cut or vaporization, a threshold value is determined different, higher Improved results can also be obtained by determining the frequency of the RF oscillator for different values depending on whether desiccation or cutting / vaporization is required. Thus, for desiccation, the mode selection control applies a control frequency signal to the RF oscillator through the control threshold 60I to determine the frequency of the oscillator below the resonance frequency of the oscillator. combination of series resonance Cr, Lr. Conversely, when tissue cutting or vaporization is required, the RF oscillator is determined at a frequency greater than the resonance frequency. The lower frequency causes the modification of the energy versus the load impedance characteristic to increase the available energy at low impedances, as demonstrated during the desiccation. The higher frequency of the oscillator causes the alignment of the energy versus the impedance load curve, favoring higher impedances, as observed when the submerged fluid vaporizes. These variations in the energy / load impedance are observed diagramatically in Figures 8 and 9, where fE and fr respectively represent the excitation frequency (ie, oscillator) and the resonance frequency. Combined modes can be used by alternating constantly between the drying and cutting states or altering the position of the thresholds. An alternative generator to that described above can be used in relation to Figure 5, with an output stage with switching instruments in gate configuration, as seen in Figure 10. In this case, the switching devices include four transistors of MOS field effect of energy placed in two pairs of two transistors, each pair arranged in a push-pull configuration. The first pair is indicated in Figure 10 as transistors T3 and T, and the second pair as transistors T5 and T6. The resonance output circuit in series covering the capacitor Cr and the conductor Lr is connected between the respective junctions of the two pairs in push-pull as transistors T3, T4 and T5, T6 so that when these pairs move in phase opposition by a pulse width and a circuit phase 64 controller, the radiofrequency energy signal is applied along the combination of series Cr, Lr. As in the case of the generator of Figure 5, the output to an electrosurgical instrument is taken through the inductor Lr, via a junction capacitor Cc and an isolation transformer 65, the radiofrequency output voltage being transformed into the terminals 66 , 68 for the connection of an electrosurgical charge 70. In other aspects, this alternative generator is similar to the generator described above in relation to Figure 5, and commonly used reference numbers for the same are used in Figures 10 and 5 respectively. common parts. The voltage waveform generated along the series resonance circuits Cr, Lr, typically has the same waveform as that developed by the generator of Figure 5, that is, as seen in Figure 6. However, in the case of this alternative generator, by incorporating a phase control function in the circuit 64 that moves the connection transistors T3 to T6, the phase difference between the motion signals that are applied to the respective pairs T3 , T4 and T5, T6 can vary from the maximum value of 180 ° downwards so that the output energy decreases. This constitutes an additional variable to vary the output energy Indeed, every time if required, the phase difference can be reduced in such a way that the output energy is zero, which means that, for example, the supply voltage Vs obtained from the power supply 64 can be kept constant at all times. to be supply voltage Vs obtained from the power supply 64 instead of being used as a secondary means of energy reduction. To summarize, then, the gate configuration allows for a rapid reduction of energy when the conductive liquid surrounding the electrodes of the instrument vaporizes not only by reducing the "on" time of the transistors, but by modifying the relative phase between the two pairs of transistors below 180 °. It is also possible to alter the excitation frequency of the RF oscillator 60 to be further away from the resonance frequency defined by the series combination of the capacitor Cr and the inductor Lr as a further way of reducing output energy. From a general point of view, a radio frequency generator for an electrosurgical system is provided, the system includes an electrode assembly with two electrodes for use submerged in an electrically conductive fluid. The generator has a control circuit system for rapidly reducing the radio frequency output energy emitted by at least 50% within at most some cycles of the radio frequency peak output voltage that reaches a predetermined threshold limit. From In this way, tissue coagulation can be performed in, for example, saline solution without significant steam generation. The same technique of peak voltage limitation can be used in a tissue vaporization as a cut-off to limit the size of the vapor pocket in the electrodes and to avoid burning of the electrode. The generator has a push-pull output stage with a series resonance output circuit, the output stage is moved by a radio frequency oscillator at a frequency which, in general, differs from the resonant frequency of the resonant output circuit. The energy control is obtained by varying the "on" time of the switching transistors that form the push-pull output and by changing the frequency separation that remains between the excitation frequency and the resonance frequency of the output circuit of resonance in series. In an alternative configuration, a gate configuration with two opposing pairs is used which produces an additional energy control variable: the relative phase of the movement signals to the respective pairs of transistors.

Claims (29)

1. An electrosurgical generator for supplying radiofrequency energy to an electrical instrument, wherein the generator comprises a radio frequency output stage having at least one pair of electrosurgical output connections for supplying radiofrequency energy to the instrument, a power source coupled to the output stage for supplying power to the output stage, and a control circuit system including detection means for deriving a detection signal representative of the radio frequency peak output voltage developed through the output connections, wherein the output stage comprises a series resonance output circuit coupled to the output connections and switching means coupled to the resonant output circuit, and where the control circuitry is operable to vary the switching intervals of the switching means to reduce the radiofrequency energy supplied and n response to a predetermined condition of the detection signal. A generator according to claim 1, wherein the series resonance output circuit comprises the series combination of an inductance and a capacitance, and is coupled to the switching means in such a way that a form of radio wave frequency output wave through the series combination, generator output connections are coupled to the series resonant circuit to receive the radio frequency voltage developed through the inductance or capacitance. 3. A generator according to claim 2, wherein the output connections are coupled to receive the radiofrequency voltage developed through the inductance. A generator according to claim 2, wherein the series combination is coupled between the switching means and a ground connection or one of a pair of supply rails of the power supply means, one of the output connections of the generator is coupled to a junction between the inductance and the capacitance. A generator according to claim 4, wherein the capacitance is connected between the switching means and the junction, and the inductance is connected between the junction and the ground connection on said supply rail. 6. A generator according to claim 2, wherein the switching means comprises semiconductor switching devices connected in a gate configuration, the serial combination is coupled between nodes in opposite phases of the switching means. 7. A generator according to claims 2 to 6, wherein it includes a coupling capacitance connected in series coupled in a signal step between the series resonance circuit and one of the output connections. 8. A generator according to claim 7, wherein the capacitance of. coupling is of a value lower than the capacitance of the series resonance combination. A generator according to any preceding claim, wherein the switching means comprises a pair of electronic switches connected in a push-pull series arrangement between a pair of supply rails of the power source, the series resonant output circuit is coupled to the connection between the electronic switches. A generator according to claim 9, wherein the switching means comprises two pairs of electronic switches in a gate configuration, each pair connected in a series arrangement in a counter-phase between the supply rails, the resonance output circuit In series is coupled between the connection between the switches of one pair and the connection between the switches of the other pair, the two pairs are arranged so as to be driven with respective opposite phases. A generator according to any one of the preceding claims, wherein the switching means is connected to switch power repeatedly through the resonant output circuit to a radio frequency, and where the control circuit system is arranged and coupled to such switching means in a manner so as to reduce the radiofrequency working rate of the switching means sufficiently quickly to cause at least a 50% reduction in the delivered output power within 100 μs after having reached a threshold of predetermined radiofrequency output peak voltage. A generator according to claims 9 and 11, or claims 10 and 11, wherein the switching means is connected to repeatedly switch current through the resonant output circuit to a radio frequency, and where the circuit system The control device is arranged and coupled in such a way to the switching means so as to reduce the working speed of the radio frequency of the switching means in a sufficiently fast manner to cause a reduction of at least 50% in the output power supplied, of 100 μs after having reached a predetermined radio frequency peak output voltage threshold and where the control circuit system is arranged to drive each of the electronic switches so as to effect a partial cyclic switching by which each has a variable closing interval during respective radiofrequency cycles, the working time of both inte The switches are controlled fast enough to effect said energy reduction of at least 50% within five radiofrequency cycles. 13. A generator according to claim 11 or 12, wherein the control circuit system includes an exciting stage including a ramp generator that operates to elicit a control signal that will be initially applied to the driving stage to reduce said working rate of radio frequency to cause reduction of at least 50% of the energy supplied by the output connections, and then progressively increase the operating rate at a slower speed until the detection signal indicates that the predetermined voltage threshold has been reached again. 14. A generator according to any preceding claim, wherein it also includes an oscillator for driving the switching means, the oscillator operates at a frequency that is different from the resonant frequency of the series resonant combination. A generator according to claim 14 and claim 7, wherein the values of the coupling capacitance and the components of the series resonant circuit are such that the difference between the oscillator frequency and the resonant frequency is between 1 / 4 (Ccfr / Cr) and (Ccfr / Cr), where Cc is the coupling capacitance, Cr is the capacitance element of the series resonant circuit and fr is the resonant frequency. 16. A generator according to any of the preceding claims, wherein the output stage includes energy recovery diodes arranged to recover the energy of the resonant circuit in series towards the power supply. 17. A generator according to claim 9 and claim 16, wherein the diodes are connected through switches. 18. A generator according to claim 14 or claim 15, which includes a mode selection means for adjusting the operating frequency of the oscillator to a frequency greater than the resonant frequency for cutting or vaporization and, alternatively, to a frequency smaller than said resonant frequency for desiccation. A generator according to any of claims 14 to 18, wherein the control circuitry includes a means for altering the operating frequency of the oscillator to be more from said resonant frequency as a means to reduce the energy of exit. 20. An electrosurgical system wherein it includes a generator for generating radiofrequency energy and an electrosurgical instrument having at least one electrode for use submerged in a conductive liquid, wherein the generator comprises an output stage that includes at least one device of radiofrequency energy, a series resonance output circuit, and at least one pair of output connections arranged to receive radio frequency energy from the power source, one of the pairs of connections is connected to said electrode, and wherein the generator further comprises an operable control stage for reducing the driving time of the power source during individual radio frequency cycles in response to a detection signal representative of the peak output voltage across the output connections that they exceed a predetermined detection signal threshold value, by which the radiofrequency energy supplied to the electrode structure is rapidly reduced when the conductive liquid is vaporized. A system according to claim 20, wherein the electrode structure includes a protruding treatment electrode and a liquid contact electrode separate from the treatment electrode, both electrodes for use are surrounded by the conductive liquid and each is connected to a respective one of the pair of output connections, the control stage works to reduce the conduction time of the energy source when the conductive liquid in the treatment electrode is vaporized by it to cause the disappearance of the steam bubbles in the treatment electrode and a decrease in the electric load impedance. 2

2. A system according to claim 21, wherein the electrosurgical instrument has an electrode structure having juxtaposed first and second electrodes to be immersed in a conductive liquid, the first and second electrodes respectively form a tissue contact electrode. at the end end of the instrument and a return electrode separated proximally from the tissue contact electrode. 2

3. A system according to claim 21 or 22, wherein the series resonant circuit coupled between the power source and the output connections has a resonant frequency different from the operating frequency of the generator. 2

4. A system according to any of claims 20 to 23 and operable in at least one tissue drying mode and a tissue vaporization or cutting mode, wherein the generator includes a mode selection control, and wherein the stage control is operable automatically to adjust the radio frequency energy supplied to the electrode structure to limit the peak output voltage of the generator to a first value when the desiccation mode is selected and at least one second value is selected when the vaporization or cutting mode, the second value or values are greater than the first value. 2

5. A system according to claim 23, wherein the first and second values are within the range of between 150V and 200V and between 250V and 600V respectively, the voltages being peak voltages. 2

6. A system according to any of claims 20 to 23, operable in at least one tissue desiccation mode and a tissue vaporization or cutting mode, wherein it has a radio frequency oscillator to drive the energy source, wherein the generator includes a mode selection control coupled to the oscillator to adjust the frequency of the oscillator so that it is greater than the resonant frequency of the resonant output circuit in series in the vaporization or cut mode and lower than said resonant frequency in the desiccation mode. 2

7. An electrosurgical generator for supplying radiofrequency energy to an electrosurgical instrument, wherein the generator comprises a radio frequency output stage having at least one pair of electrosurgical output connections for the supply of radiofrequency energy to the instrument, an oscillator of radio frequency to feed a radio frequency signal to the output stage, and a control circuit system that includes a detection means for deriving a detection signal representative of the radio frequency signal supplied from the output connections, where the Output stage comprises a series resonant output circuit coupled to the output connections, the resonant frequency of the series resonance output circuit is different from the operating frequency of the oscillator, and where the control circuitry provides a feedback signal to control the radiofrequency energy supplied. A generator according to claim 27, wherein the output stage is a symmetrical counter-phased output stage, and the control circuitry is operable to alter the closing interval of one or more semiconductor devices forming part of the exit stage regardless of the frequency of operation. 29. A generator according to claim 27 or 28, wherein the detection means is arranged to derive a detection signal representative of the radio frequency peak output voltage developed through the output connections. SUMMARY A radio frequency generator for an electrosurgical system is provided, the system includes an electrode assembly having two electrodes for submerged use in an electrically conductive fluid. The generator has control circuits for rapidly reducing the radio frequency output power delivered by at least 50% within the majority of the peak radio frequency output voltage cycles that reaches a predetermined threshold limit. In this way, tissue coagulation can be performed in, for example, saline solution without significant vapor generation. The same peak voltage limiting technique is used in a vaporizing or cutting mode of tissue to limit the size of the vapor pocket at the electrodes and prevent burning of the electrode. The generator has a push-pull output stage with a series resonant output circuit, the output stage being driven by a radio frequency oscillator at a frequency which, in general, differs from the resonant frequency of the resonant output circuit. Energy control is achieved by varying the ON time of the switching transistors that form the push-pull output pair and altering the frequency separation between the excitation frequency and the resonant frequency of the series resonant output circuit. In an alternative mode , a gate configuration is used that uses two pairs in push-pull, producing an additional energy control variable: the relative phase of the impulse signals for the respective pairs of transistors.