Surgery – Instruments – Electrical application
Reexamination Certificate
1999-06-30
2001-05-08
Dvorak, Linda C. M. (Department: 3739)
Surgery
Instruments
Electrical application
C606S037000, C606S041000
Reexamination Certificate
active
06228081
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electrosurgery system, an electrosurgical generator, and methods of operating the system and performing electrosurgery.
BACKGROUND OF THE INVENTION
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 ‘t
1
’. When the saline is vaporised, the active tip temperature rises more rapidly until, at time ‘t
2
’. active electrode destruction occurs at a temperature of 1600° C. (melting point of platinum). This destruction temperature is indicated by temperature ‘T
D
’ 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 T
D
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.
SUMMARY OF THE INVENTION
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 &mgr;s 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 the 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 T
D
shown in
FIG. 1
, despite the higher applied voltage. It follows that if, during treatment, the electrode is used in such a way as to prevent cooling by the quenching effect of the liquid, it is likely to be destroyed as a result of heat accumulation. Such a condition can arise when the electrode is buried in tissue. It is for this reason that it is beneficial to use electrode temperature sensing to limit t
Dvorak Linda C. M.
Gibson Roy
Gyrus Medical Limited
Oliff & Berridg,e PLC
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