|Publication number||USRE39358 E1|
|Application number||US 10/323,004|
|Publication date||Oct 17, 2006|
|Filing date||Dec 19, 2002|
|Priority date||May 21, 1999|
|Also published as||CA2308881A1, CA2308881C, EP1053720A1, US6228081, USRE41921|
|Publication number||10323004, 323004, US RE39358 E1, US RE39358E1, US-E1-RE39358, USRE39358 E1, USRE39358E1|
|Inventors||Colin C. O. Goble|
|Original Assignee||Gyrus Medical Limited|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Referenced by (161), Classifications (7), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,228,081. The other reissue application is Reissue Application No. 11/436,186 filed May 18, 2006. The other reissue application is a continuation reissue application of Reissue Application No. 10/323,004 filed Dec. 19, 2002.
This invention relates to an electrosurgery system, an electrosurgical generator, and methods of operating the system and performing electrosurgery.
The cutting or removal of tissue electrosurgically using an instrument having a tip with one or more active electrodes supplied with a radio frequency (r.f.) voltage usually involves cell rupture as a result of arcs between the active electrode and the tissue being treated or, in the case of underwater electrosurgery, between the active electrode or electrodes and a conductive liquid such as saline overlying the tissue to be treated. As described in EP-A-0754437, electrode destruction can occur if sufficient radio frequency power is supplied to an electrode to cause burning or melting of the electrode material, and this can be avoided by sensing peak electrode voltage and applying feedback to reduce the applied power so as to set a maximum peak voltage. It will be understood that for a given power setting, the temperature of the electrode depends on the rate at which heat can be dissipated which, in turn, depends on such variables as the degree of tissue engagement, electrode structure, and fluid flow around the electrode. Consequently, to avoid electrode destruction the peak voltage limit must be set at a sufficiently low level to prevent damage in the worst case dissipation situations, i.e. when there is an absence of cooling fluid and/or the electrode is surrounded by tissue.
In the absence of such control, the temperature of the electrode follows an asymptotic curve as shown in FIG. 1. The saline absorbs power until the point of vaporisation is reached at time ‘t1’. When the saline is vaporised, the active tip temperature rises more rapidly until, at time ‘t2’, active electrode destruction occurs at a temperature of 1600° C. (melting point of platinum). This destruction temperature is indicated by temperature ‘TD’ in FIG. 1. The time taken to reach this temperature after vaporisation occurs is dependent on both thermal capacity and thermal dissipation mechanisms. A low mass electrode heats up faster. The principal dissipation mechanism is infra-red emission and is, therefore, dependent on surface area.
Limitation of peak voltage is used, as described above, to control the applied r.f. power so as to prevent the electrode temperature reaching TD under all normal operating conditions. It will be appreciated that this limits the rate at which tissue can be removed.
It is an object of the present invention to provide a means of increasing the rate of tissue removal.
According to a first aspect of the present invention, an electrosurgical generator comprises a source of radio frequency (r.f.) energy, an active output terminal, a return output terminal, a d.c. isolation capacitance between the source and the active output terminal, and a pulsing circuit for the source, wherein the source and the pulsing circuit are arranged to generate a pulsed r.f. output signal at the output terminals, which signal has a peak-to-peak voltage of at least 1250V, a pulse mark-to-space ratio 1:1 or less, and a pulse length of 100 μs or less. The pulse repetition rate is preferably between 5 Hz and 15 kHz or, more preferably, below 2 kHz. Advantageously, the mark-to-space ratio of the modulation is dynamically variable in response to a temperature signal from a temperature sensing arrangement, the signal being representative of the temperature of an electrode when coupled to the active output terminal.
The preferred generator includes a pulse modulator arranged to modulate the r.f. energy so as to produce a pulsed signal having alternate ‘off’ and ‘on’ periods during which the peak-to-peak output voltage of the generator is substantially zero and at least 1250V respectively, the duration of the ‘on’ periods being controlled in response to the temperature signal reaching a predetermined threshold value. When the load impedance drops to 50 ohms the peak current is at least 3 A.
It is possible to control the mark-to-space ratio on a pulse-by-pulse basis by using a temperature sensing arrangement having a response time which is less than the modulation period. Such an arrangement is one which is responsive to thermionic emission from the electrode, detected by monitoring the d.c. offset voltage on the output terminal coupled to the treatment electrode resulting from the thermionic effect.
According to a second aspect of the invention, an electrosurgical generator comprises a source of r.f. energy, a pair of output terminals coupled to the source, and a pulsing circuit for the source, wherein the pulsing circuit and the source are arranged, in a pulsed mode of operation, to deliver to the output terminals a peak current of at least 3 A into a 50 ohm load and a peak-to-peak voltage of at least 1250V into a 1 kilohm load.
According to a third aspect of the invention, an electrosurgery system comprises a generator having a source of radio frequency (r.f.) energy and, coupled to the generator, an bipolar electrosurgical instrument having an electrode assembly with at least a pair of electrodes for operating in a wet field, wherein the generator is adapted to deliver r.f. energy to the electrode assembly as a pulse modulated r.f. signal which, in use with the pair of electrodes immersed in liquid, has a peak current of at least 3 A and a peak-to-peak voltage of at least 1250V.
According to a fourth aspect of the invention, there is provided an electrosurgery system comprising a generator including a source of radio frequency (r.f.) energy and, coupled to the generator, an electrosurgical instrument having a treatment electrode, wherein the system includes an electrode temperature sensing arrangement and the generator is adapted to supply the r.f. energy to thee electrode as a pulse modulated r.f. signal, the mark-to-space ratio of the modulation being dynamically variable in response to a temperature signal from the temperature sensing arrangement representative of the electrode temperature.
The generator and system disclosed in this specification make of the property that the tissue removal rate increases disproportionally with the applied peak voltage. Accordingly, by pulsing the output signal and increasing the peak voltage beyond that which would normally create destructive conditions for the electrode, it is possible to increase the tissue removal rate without a corresponding increase in the applied power. The way in which the tissue removal rate varies is best understood by considering some examples. For instance, an electrode using a peak-to-peak voltage of 1250V yields approximately twice the tissue removal rate of an electrode operating at 1000V. Thus if an electrode is driven at a voltage of 1250V peak-to-peak with a 50% duty cycle, the removal rate is approximately equivalent to that achieved with continuous application of a voltage of 1000V peak-to-peak. However, it is possible to use higher voltages still. An electrode normally limited to 1000V peak-to-peak can be operated at up to 1500V peak-to-peak and the removal rate can be doubled again. Thus, an electrode powered at a 50% duty cycle at a voltage of 1500V peak-to-peak will have approximately twice the removal rate of an electrode operating continuously with 1000V peak-to-peak.
Higher-than-normal peak voltages cause higher temperatures when used in a continuous mode of operation. However, in the presence of liquid, the “off” period of a pulsed signal, allows quenching and cooling of the electrode by the liquid, which causes the electrode temperature to remain below the electrode destructive value TD shown in
In this way, it is possible to perform electrosurgical removal of tissue at a higher rate than previously, not only due to being able to operate at higher temperature in other than worst case dissipation conditions, but also due to the high removal rate associated with high instantaneous voltage.
It is possible, within the scope of the invention, to drive a treatment electrode at much lower pulse mark-to-space ratios, depending on the applied voltage, the average power delivered, the electrode configuration and the rate at which heat is dissipated from the electrode due to, for instance the rate of flow of fluid adjacent the electrode. Accordingly, advantageous tissue removal rates can be achieved with a duty cycle as low as 5% and peak-to-peak voltages in the region of 3 kV or 4 kV. Indeed, it is possible to achieve rapid tissue removal with instantaneous power levels of up to 10 kW peak currents 20 A (i.e. both within ‘on’ bursts) and a pulse repetition rate of 2 kHz or higher. The pulse length, i.e. the duration of the ‘on’ bursts may be as short as 5 ms or even 1 ms. Such pulse lengths may be shorter than the thermal response time constant of the treatment electrode. Particular benefits can be achieved with high instantaneous power and short pulses when high liquid pumping rates are used since with high voltages vaporisation and tissue removal tends to occur very quickly, so that less of the incident energy is lost due to the flow of heated liquid away from the electrode.
Typically, the control circuitry of the generator and the detector are operable to limit the d.c. offset to a predetermined d.c. voltage level in the region of from 50V to 100V. In practice, the actual voltage level depends on electrode configuration and electrode material. Thus, if a platinum electrode is used, the voltage limit is set to that which occurs when the electrode voltage approaches 1600° C., the melting point of platinum.
In a preferred embodiment of the invention, the generator has an output terminal connectible to the treatment electrode and isolated from the r.f. source at d.c., and the detector has (i) a detection input which is connected to the output terminal and (ii) an isolation device connecting the detector to the control circuit. The detector may be powered from the generator r.f. output energy by having a power supply circuit coupled to the generator output terminal and including a rectifier for rectifying the r.f. electrosurgery signal applied to the output terminal. This is permissible since the thermionic effect does not occur until the r.f. output voltage reaches a level consistent with arcing. The fact that the detector does not function at lower voltages is, as a result, no disadvantage. Typically, to achieve isolation at the output of the detector, it comprises an oscillator for generating an alternating measurement signal representative of the d.c. offset, and the isolation device comprises an opto-isolator coupled to receive the alternating measurement signal and to feed it to the control circuit. The preferred detector also includes a reverse polarity d.c. offset detector as a fault condition indicator which can be used to disable the r.f. source when, for instance, in use of a bipolar electrode assembly in a conductive fluid field, a lack of fluid causes d.c. polarity reversal.
According to a further aspect of the invention, there is provided a method of operating an electrosurgery system including an electrosurgical r.f. generator and an electrode assembly having a treatment electrode coupled to the generator, wherein the method comprises applying to the electrode a pulse-modulated r.f. signal produced by the generator, generating a temperature signal indicative of the temperature of the electrode, and dynamically varying at least the mark-to-space ration of the pulse modulation of the r.f. signal in order to control the temperature of the electrode.
According to yet a further aspect of the invention, a method of performing electrosurgical tissue cutting or ablation comprises applying r.f. energy to an electrosurgical instrument so as to promote arcing at a treatment electrode of the instrument, wherein the energy is applied as a pulsed r.f. signal with a peak-to-peak voltage of at least 1250V and a pulse mark-to-space ratio of 1:1 or less. The r.f. energy may be regulated by regulating the mark-to-space ratio dynamically to maximise the temperature of the electrode without substantial electrode damage, the d.c. voltage being limited to a threshold value of less than 100V.
The invention will be described below by way of example with reference to the drawings.
in the drawings:
The present invention is applicable primarily but not exclusively to wet field electrosurgery. Referring to
Handpiece 12 mounts a detechable electrode assembly 28 having a dual electrode structure, as shown in the fragmentary view of FIG. 3.
In operation as an instrument for cutting or removing tissue in a conductive fluid field, the electrode assembly 28 is applied as shown in
The electrode assembly is effectively bipolar, with only one of the electrodes (active electrode 30) axially extending to the distal end of the unit. This means that the return electrode, in normal circumstances in a wet field, remains spaced from the tissue being treated and a current path exists between the tissue and the return electrode via the conductive liquid in contact with the return electrode. The conductive liquid 46 may be regarded, as far as the delivery of bipolar electrosurgical energy is concerned, as a low impedance extension of the tissue.
When sufficient r.f. voltage is applied between the electrodes 30, 36, power dissipation in the conductive liquid 46 causes the liquid to vaporize, initially forming small vapour bubbles on the surface of the active electrode 30, which ultimately coalesce until the electrode is completely enveloped in a pocket of vapour 50. Vapour pocket 50 is sustained by discharges 52 across the vapour pocket between the active electrode 30 and the vapour-to-saline interface. The majority of power dissipation now occurs within this pocket with consequent heating of the active electrode, the amount of energy dissipated being a function of the delivered power. By holding the active electrode 30 adjacent the surface of the tissue 44, as shown in
This mode of operation can be maintained over a comparatively wide range of power levels, but increasing the delivered power beyond this range causes a rapid rise in electrode temperature as described above with reference to
In describing above the system with reference to
The pulse modulator 61 is actuated by a processor 70 which, in turn, receives mode signals from the front panel of the generator or the foot switches (see FIG. 1). Accordingly, the generator may have a vaporisation mode in which the r.f. power stage 60 is modulated by the pulse modulator 61 with a mark-to-space ratio of 1:1 or less (i.e. successive “on” times representing a 50% duty cycle or less). The frequency of the modulation is typically 300 Hz. The processor 70 also controls the peak voltage of the r.f. output stage 60 according to mode. In addition, the processor has a temperature signal input 74 allowing control of the pulse modulator 61 in response to electrode temperature, as will be described in detail below.
A representation of the variation of the electrode temperature with time when r.f. power is applied at a relatively high peak-to-peak voltage with 100% pulse modulation depth is shown in FIG. 5. The mark-to-space ratio is 1:1. In other words, power is only applied for 50% of the time. This yields two potential benefits. Compared with continuous 1000V peak-to-peak operation, application of pulsed power at 1000V peak-to-peak results in a reduction in the averaged delivered power by as much as 25%. Since the peak delivered power is higher (i.e. during the r.f. burst when the pulse modulation is a logic level 1, the electrode is less susceptible to quenching effects caused by high flow rates of saline passed the electrode. This is explained by considering the saline at the surface of the active electrode. The ability to vaporise this saline is defined by the power it absorbs before leaving the electrode surface. When convection due to fluid flow is high, the silane refresh rate is high and, therefore, the power absorbed by per unit volume of saline at the electrode surface is smaller. If the waveform crest factor is increased by the use of modulation, as described above, but with similar average power levels, then the power absorbed per unit volume of saline during each power burst is higher.
The above described advantages are achieved because, during the “off” period of the modulation, the electrode is quenched and cooled. It is for this reason that the electrode temperature never reaches the steady state destructive value tD. If the electrode is used in such a manner that cooling by quenching is interrupted, there is a damage that the electrode will be destroyed by heat accumulation. This condition can arise when the electrode is burried in tissue. Accordingly, in accordance with the invention, the pulsing of the electrosurgical power is performed in conjunction with temperature monitoring, as provided for by the temperature signal input 74 to the process 70 in FIG. 4. The temperature signal applied to the input 74 is produced by an electrode temperature sensing arrangement, which may take a number of forms, for instance, a circuit for measuring a d.c. offset voltage across terminals 74 and 66 due to the thermionic effect occurring when the active electrode becomes very hot.
Processor 70 acts in such a way as to modify the mark-to-space ratio of the pulse modulation generated by pulse modulator 61 according to the level of the electrode temperature signal applies on input 54. Specifically, in this embodiment, a characteristic of the electrode temperature signal applied to the input 74 is compared with a threshold value which is a function of the maximum allowed temperature, so that the pulse modulator applied on “on” signal to the r.f. output stage 60 until the temperature signal reaches the predetermined threshold value, whereupon the r.f. output stage is switched off for a predetermined period.
This manner of operating is illustrated in
The modulation rate is primarily dependent upon the time taken for the vapour pocket around the active electrode to collapse, so that the electrode can be cooled. Ideally, power is reapplied as soon as the quenching occurs, in order that the resulting saline is not lost by either convection or flow. The burst length is preferably sufficiently long that re-establishing the vapour pocket occurs at least within the first half of the “on” burst. Modulation rates of 5 Hz to 2 kHz are appropriate.
As mentioned above, temperature sensing is done indirectly by monitoring the thermionic effect, as will now be described with reference to
The temperature-dependent positive potential (the d.c. offset voltage) is monitored using a detector connected as a shunt input across the generator output, on the output terminal side of the isolation capacitance. The detector has an input circuit with a series r.f. choke 78 coupled to the output terminal 64, and a smoothing capacitor 80 coupled to the common rail 81 which is connected to the return terminal 66. Therefore, d.c. component of the voltage at the active output terminal 64 accumulates at the junction of the choke 78 and the smoothing capacitor 80 where it is applied to a potential divider 82, 84 which present an input resistance of at least 2 MΩ, and typically between 50 and 100 MΩ, the output of the potential divider 82, 84 is applied to a high temperature buffer 86 the output of which provides a driving signal to a voltage controlled oscillator (VCO) 88. Providing an input impedance in the region of 50 to 100 MΩ, yields a detection current in the region of 1 μA for d.c. offsets in the region of 50 to 100V. Maintaining a low detection current has the advantage that never stimulation due to a direct current between the target tissue and the return electrode is avoided.
Conversion of the d.c. offset voltage to an alternating signal in the VCO 88 allows the signal to be transmitted to an isolated control circuit (not shown in
When the bipolar electrode assembly shown in
In this embodiment, power for the buffer 86, VCO 88, comparator 94, and OR-gate 96 is derived from the r.f. voltage itself delivered to the output terminals 64 and 66 of the generator, avoiding the need for a further isolation barrier. A suitable power supply for this purpose is illustrated in
Use of the invention is not restricted to wet field (underwater) electrosurgery. Arcing also occurs with monopolar or bipolar electrosurgery instruments in dry field surgery and power can be controlled using the thermionic effect in the same way as described above.
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|U.S. Classification||606/34, 606/41, 606/37|
|International Classification||A61B18/12, A61B18/04|
|Oct 9, 2008||FPAY||Fee payment|
Year of fee payment: 8
|Oct 1, 2012||FPAY||Fee payment|
Year of fee payment: 12