GB2146534A

GB2146534A - Electrosurgical system

Info

Publication number: GB2146534A
Application number: GB08422992A
Authority: GB
Inventors: John Harold Blackett
Original assignee: Matburn Holdings Ltd
Current assignee: Matburn Holdings Ltd
Priority date: 1983-09-13
Filing date: 1984-09-12
Publication date: 1985-04-24
Also published as: GB8324442D0; GB8422992D0; GB2146534B

Abstract

A method of and circuit for detecting unacceptable levels of current flow from the plate circuit of an electrosurgical unit to earth. A voltage wave form is applied to the plate circuit by means of the electrosurgical generator or a dedicated source (2). A voltage related to that existing between the plate circuit and earth is derived at the input to circuit means (ICI, D1, C3, IC2 and 3) and operated on to provide a signal indicative of the nature of the impedance between the plate circuit and earth. This signal is supplied to switching means (IC3, IC6) which generate a signal for actuating an alarm and/or shutdown of the generator should this signal indicate that the resistive component of said impedance is below a predetermined value. <IMAGE>

Description

SPECIFICATION Electrosurgical system The present invention relates to a method and circuit for detecting unacceptably low values of impedance to earth of a patient's body connected to the plate circuit of an electrosurgical generator.

The most frequent cause of electrosurgical accidents results from current leakage from a patient's body via paths other than those of the patient's plate electrode and the patient's plate connector provided on the electrosurgical unit. Such alternative paths are not physically or electrically controllable and can, under certain circumstances, pass fairly high levels of electrosurgical current. Such current levels, over what can be a small contact area, can be more than sufficient to cause burns. Indeed, it has been shown that currents of the order of less than 100 milliamps can cause a temperature rise at a contact tissue interface sufficient to start dessication and a possible burn.

Recent designs of electrosurgical units have done a lot to overcome the problem of earth referenced or directly earthed types of plate circuit by providing fully floating or isolated patient plate circuits thereby eliminating the most common cause of alternative contact path burns. The currents between the plate and an alternative contact path are divided in the ratio of their impedances. Thus, fully floating circuits maintain the plate contact circuit at a low impedance, and due to the isolation provided by this type of circuit, provide a much higher impedance via any other patient contact. However, it is not possible to provide perfect or even near perfect isolation, the reason for this being that an electrosurgical unit must have for its operation an active lead connecting the active terminal of the generator to the surgeon's electrode.This active lead has, of necessity, a capacitance to earth or to earth related objects. This capacitance, of the order of 20 to 50 Pf, is sufficient to allow the passage of leakage current of sufficient-magnitude to cause at least minor burns.

Certain protective circuitry can be incorporated in the generator such as that described in US Patent No.

3,683,923 for return loop monitoring and that in British Patent No. 1,480,736 for plate voltage monitoring.

Specific safety requirements are laid down for the design of electrosurgical equipment by both British and IEC Standards. The British Standard, BS5724: Section 2.2:1983 IEC601-202:1982 UDC (615.47:621.313:614.8) : (616-72:621.3.023)) stipulates the maximum leakage current that may flow from either the plate or the active circuit of an electrosurgical unit to earth. This current is measured in a test rig designed to simulate the working conditions of an electrosurgical generator, i.e. employing the plate and active leads specified by the manufacturer. The current measuring instrument employed has an internal resistance of 200 ohms, this resistance being intended to simulate the actual impedance of a small contact area between a patient's body and an adjacent earthed conductive body.

Under these conditions, and employing the full output of the electrosurgical generator in its worst operating mode, the current measured must not exceed 150 milliamps. This value is clearly a compromise between absolute safety, where the value should approach zero, and a practical value achievable with an electrosurgical generator operating at normal frequencies and without requiring specific leads or balancing circuits.

Thus, any electrosurgical unit should clearly meet this 150 milliamp limit through a 200 ohm non-inductive resistive load and this imposes certain restrictions on the design of such units since current surgical techniques demand high open circuit voltages to produce effective coagulation. Such voltages are clearly not compatible with minimising leakage currents.

A method of providing effective coagulation without raising the output voltage is to employ a waveform with a higher energy per-cycle output than the conventional sinewave. To this end, certain recent electrosurgical units have employed a square wave or near square wave output. Indeed, the coagulation performance of such high harmonic content waveforms has been found to be much better than other waveforms in certain types of surgery. However, such high harmonic content can also contribute very substantially to the leakage current. Thus, in order that such advantageous waveforms may be employed it has become essential to produce a method of detecting dangerous levels of leakage current.

An object of the present invention is to provide a method for and circuit capable of descriminating between a distributed leakage path from a patient's body, such as is produced by the capacitance formed between the patient's body and earth through the operating table mattress, and a point contact path.

According to the present invention, there is provided a method of detecting unacceptably low values of impedance to earth, of a patients body connected to the plate circuit of an electrosurgical generator, comprising the steps of: deriving and operating on a voltage related to that existing between the plate circuit and earth to obtain a signal indicative of the nature of the impedance between the plate circuit and earth and generating a switching signal for actuating an alarm and/or shut-down of the generator should this signal indicate that the resistive component of said impedance is below a predetermined value.

In one embodiment of the method of the present invention, the electrosurgical voltage is applied to the plate circuit and the peak value of a voltage derived from that appearing between the plate circuit and earth is compared with its mean or RMS value to determine the nature of the impedance between the plate circuit and earth.

In an alternative method, a small voltage of intermediate frequency, for example 100 kilo hertz, is applied to the plate circuit and a voltage reflecting that between the plate and earth is determined and compared with a reference voltage.

According to a further aspect of the present invention, there is provided a circuit for detecting unacceptably low values of impedance to earth, of a patients body connected to the plate circuit of an electrosurgical generator, comprising circuit means coupled with the plate circuit so as to derive and for operating on a voltage related to that existing between the plate circuit and earth to obtain a signal indicative of the nature of the impedance between said plate circuit and earth, and switching means coupled to the output of said circuit means which are adapted to generate a switching signal for activating an alarm and/or shut-down of the generator should that signal indicate that the resistive component of said impedance is below a predetermined value.

In one such embodiment, a "square law" device and a rectifier circuit are provided for producing an RMS and a peak value signal, respectively, at a respective input of an analogue divider. The output of this analogue divider is connected to a Schmitt Trigger for activating an alarm and/or shut-down of the generator, should the ration of the peak to RMS voltage exceed a predetermined value.

In an alternative embodiment, an intermediate frequency oscillator having a high output impedance applies a voltage to the plate circuit via a transformer. A voltmeter which derives a voltage representative of that across the primary winding of the transformer, compares this with a reference voltage to determine the nature of the impedance between the plate circuit and earth. Actuation of an alarm and/or shut-down of the generator is again controlled by a Schmitt Trigger connected to the output of the voltmeter.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram of a first embodiment of the present invention, and Figure 2 shows a circuit diagram of a second embodiment of the present invention.

Figure 1 shows a safety circuit capacitively coupled via a capacitor C1 of small capacitance to the plate circuit of a radio frequency generator supplying a square wave output. The voltage between the plate circuit and earth is divided between this capacitor C1 and a capacitor C2 which has a large capacitance with respect to that of capacitor Cm, for example, of the order of ten times that of capacitor C1. The voltage across capacitor C2 which is connected in parallel with a resistor R1, is supplied to a series connected "square law" device ICI i.e. an RMS voltmeter, whose DC output is supplied to an input of an analogue divider IC2 constituted by an analogue multiplier used in the dividing mode.The voltage across capacitor C2 and resistor R1 is also supplied to a half-wave rectifier circuit comprising a diode D1 connected to a capacitor C3 which is connected in parallel with a resistor R2. The voltage across the smoothing capacitor C3 is supplied to a second input of the analogue divider IC2. The output of the analogue divider IC2 is connected to a Schmitt Trigger 1C3 which controls the operation of an alarm device and/or shut-down of the generator 1.

Thus, this circuit compares two voltages, one representing the peak value of the waveform across the capacitor C2 and resistor R1 and the other the RMS value of this waveform, by forming their ratio in the analogue divider IC2. Should this value exceed a predetermined value, the Schmitt Trigger IC3 is turned on and transmits a signal to operate an alarm condition and/or generator shut-down.

The present circuit therefore compares two components representative of the waveform that exists between the patient's body or the plate circuit and earth to determine the conductive nature of this path. The principle by which this operates is that when a capacitive path exists between the patient's body and earth, the voltage across the capacitor is an attenuated version of the square waveform of the electrosurgical output, and is thus also of a square nature.If, however, the nature of the path changes from a capacitive to a resistive path, as can happen if a patient is in contact with an earthed or nearly earthed metal object, such as part of the operating table or a drip stand, the capacitance which exists between the active output and earth, is placed in circuit with this resistance and the waveform obtained changes from the attenuated version of the square wave output to a differentiated form in which there is a large difference in the ratio of the RMS value to the peak value of the waveform.

Thus, the magnetude of the ratio formed by the analogue divider IC2 indicates whether a resistive or capacitive path exists between the patient's body and earth. If the path is capacitive it is safe for the equipment to operate and the ratio generated will be lower than the predetermined switch on voltage of the Schmitt Trigger whereas if the path is predominantly resistive the ratio produced will exceed this switch on voltage and the Schmitt Trigger will in turn activate an alarm condition and switch the electrosurgical generator off.

By way of example, specific values for components of this embodiment are listed below, Ci=33pF R1 = 100KQ C2=330pF R2 = 100 kQ C3~1.0 FF In the embodiment of the present invention shown in Figure 2, an intermediate frequency oscillator 2 operating at frequencies of approximately one fifth of the electrosurgical frequency produced by the radio frequency generator i.e. 100 kilo Hertz and which has a high output impedance is coupled to the plate circuit via a transformer T1, a capacitor C4, a radio frequency damping resistor R8, and a relay RLi. A voltmeter 3 is connected across the primary winding of the transformer T1 and its output is connected to a Schmitt Trigger IC6 for activation of an alarm condition.

This circuit has a low impedance between the plate circuit terminal and earth. For this reason, a relay RL1 is provided to disconnect the circuit from the plate circuit when the electrosurgical generator 1 is on. The relay RL1 has high voltage contacts designed to withstand the full output voltage of the electrosurgical generator and is controlled by logic circuitry (not shown) to open on the application of output power from the electrosurgical generator 1. Although the relay RL1 remains closed in the stand-by condition, i.e. when the plate is connected to the patient and the equipment is ready for use, it does not detract from the safety of this circuit as any supply frequency leakage currents are blocked by the presence of a capacitor C4 which provides patient isolation.

The oscillator 2 illustrated comprises a capacitor C6, one plate of which is earthed while the other is connected via a parallel arrangement of a Schmitt Trigger multivibrator circuit IC4 and a resistor R7 to the gate of a MOSFET TRI. The drain of the MOSFET is connected both to an inductor L1 and to the cathode of a zener diode D2 the anode of which is earthed. A Capacitor C7, across which the primary winding of the transformer T1 is coupled, is connected between the inductor L1 and a voltage supply rail +V.

In view of the presence of the inductor L1, such an oscillator has a very high output impedance and this ensures that the voltage seen across its output reflects the impedance of the load. Oscillators of this type are known and will therefore not be discussed further here.

The voltmeter 3 includes a half wave rectifier comprising a diode D3 connected to a capacitor C5 which is connected in parallel with a pair of resistors R5 and R6. The voltage across resistor R6 supplies one input of an operational amplifier IC5 protected against the entry of voltage spikes due to radio frequency pick up, by a decoupling capacitor C8. The other input of the amplifier Icy is connected to a bridge circuit connected to a source of constant potential +V and formed by two resistors R3 and R4, one of which is variable. This bridge circuit allows pre-selection of a desired reference voltage against which to compare that across resistor R6.

When the difference between these two inputs reaches a certain value, the signal eminating from the operational amplifier is sufficient to cause the Schmitt Trigger IC4 to activate the alarm condition. Thus, the setting of this bridge determines the minimum acceptable voltage across resistor R6 and hence across the primary winding of the transformer T1.

For reference, an example specification for the circuit shown in Figure 2 is given below: C4 = 0.027 FF IC4 = 40106 C5 = 0.22 ~lF IC5 = 3140 C6 = 220 pF T1 = 20:20 turns C7 = 0.01 ~lF TR1 = VN10 C8 = 0.1 ijF R3 = 10 KQ D2 = 47V R4 = 100 KQ R5 = 22 KQ V = +15V R6 = 12 KQ R7 = 27 KQ R8 = 10 Q L1 = 470 FH The entire circuit operates by injecting a small voltage into the plate circuit and measuring the voltage across the primary winding of the transformer T1 by means of the voltmeter 3. This voltage reflects the impedance that appears between the plate circuit and earth. Clearly a low resistance in this condition reduces that voltage. Once the voltage falls to the predetermined level, the signal eminating from the operational amplifier of circuit 3 causes the Schmitt Trigger IC6 to activate the alarm condition.

An advantage of this circuit is that it can be used to test patient isolation without the electrosurgical power being applied. Further, the circuit can be so constructed that if an alarm condition is indicated, switching on of the power output from the electrosurgical generator 1 is prevented. This is in contrast with the circuit shown in Figure 1 in which the electrosurgical generator 1 has to be on in order for the circuit to operate and, therefore, subjects the patient to greater hazard. This circuit also has certain other advantages in that it is easier to set up and can detect reliably a wider range of impedances both of a capacitive and of a resistive nature.

Thus, the present invention can improve the safety of an electrosurgical generator by being able to detect an alternative conductive path between the plate circuit, and, therefore, the patient's body, and earth. In practice, the value of the impedance of this conductive path can be much greater than that required by the regulations. Thus, such a circuit provides an electrosurgical system with a significant safety advantage over known electrosurgical units in which the leakage current is limited by circuit design to the regulatory limit.

Thus, the present invention provides without complex circuitry, a means for discriminating between a distributed leakage path from a patient's body and a point contact path. By this means, it is possible to meet the requirements of the British Standard which imposes a resistive path and to detect any direct alternative resistive path from the patient's body by eliminating the distributed self capacitance of his body to earth.

Claims

1. A method of detecting unacceptably low values of impedance to earth, of a patients body connected to the plate circuit of an electrosurgical generator, comprising the steps of: deriving and operating on a voltage related to that existing between the plate circuit and earth to obtain a signal indicative of the nature of the impedance between the plate circuit and earth and generating a switching signal for actuating an alarm and/or shut-down of the generator should this signal indicate that the resistive component of said impedance is below a predetermined value.

2. A method according to claim 1 wherein the peak value of a voltage derived from that appearing between the plate circuit and earth is compared with its mean or RMS value to determine the nature of the impedance between the plate circuit and earth.

3. A method according to claims 1 or 2 wherein the voltage appearing on the plate circuit is supplied by the electrosurgical generator.

4. A method according to claim 1 wherein a voltage waveform is applied to the plate circuit via a transformer, and the signal indicate of the impedance between the plate circuit and earth is obtained from the voltage across one winding of the transformer.

5. A method according to claim 1 or 4 wherein the voltage appearing on said plate circuit is supplied by an intermediate frequency oscillator.

6. A method of detecting unacceptable levels of current flow between the plate circuit of an electrosurgical generator and earth substantially as herein described with reference to either of Figures 1 and 2 of the accompanying drawings.

7. A circuit for detecting unacceptably low values of impedance to earth, of a patients body connected to the plate circuit of an electrosurgical generator, comprising circuit means coupled with the plate circuit so as to derive and for operating on a voltage related to that existing between the plate circuit and earth to obtain a signal indicative of the nature of the impedance between said plate circuit and earth, and switching means coupled to the output of said circuit means which are adapted to generate a switching signal for activating an alarm and/or shut-down of the generator should that signal indicate that the resistive component of said impedance is below a predetermined value.

8. A circuit according to claim 7 wherein said circuit means comprise a square law device and a rectifier circuit for producing an RMS and a peak value signal respectively, at a respective input of an analogue divider.

9. A circuit according to claim 8 wherein the imputs to said square law device and rectifier are coupled across a capacitor capacitively coupled to the plate circuit.

10. A circuit according to claim 8 or 9 wherein said switching means comprise a Schmitt Trigger connected to the analogue divider for generating said switching signal should the ratio of the RMS to peak voltage exceed a predetermined value.

11. A circuit according to claim 7 further comprising an alternating voltage supply coupled with the plate circuit for applying the voltage thereto.

12. A circuit according to claim 11 wherein said voltage source is an intermediate frequency oscillator having a high output impedance.

13. A circuit according to claims 11 or 12 wherein said voltage source is coupled with the plate circuit via a transformer.

14. A circuit according to claim 13 wherein a voltmeter connected across one winding of said transformer derives a voltage from that across that winding and compares this voltage with a reference voltage to determine the nature of the impedance between the plate circuit and earth.

15. A circuit according to claim 14 wherein said voltage derived from that across the transformer winding and said reference voltage are supplied to a Schmitt Trigger for generating said switching signal.

16. A circuit according to any of claims 11 to 15 further comprising a relay for connecting said voltage source with the plate circuit.

17. A circuit according to claim 16 further comprising a logic circuit coupled with the relay for opening said relay on application of output power to the plate circuit from the electrosurgical generator.

18. A circuit according to any of claims 11 to 17 wherein the voltage source is coupled to the plate circuit via a capacitor to provide patient isolation.

19. A circuit for detecting unacceptable levels of current flow from the plate circuit of an electrosurgical generator to earth substantially as herein described with reference to either of Figures 1 and 2 of the accompanying drawings.

20. An electrosurgical unit comprising an electrosurgical generator and a circuit according to any one of claims 7 to 19.