CA1200287A - Return electrode monitoring system for use during electrosurgical activation - Google Patents

Abstract

IMPROVED RETURN ELECTRODE MONITORING SYSTEM

Abstract A return electrode monitoring system for use with a patient return electrode adapted for contacting a patient, the return electrode having two, spaced apart conductors attached thereto for connecting the electrode to a genera-tor of electrosurgical current which passes through the electrode, the system comprising means for applying a monitoring current through the conductors to the elec-trode; detecting means responsive to the monitoring current for producing a signal which is a function of the impedance between the two conductors, the detecting means including means for substantially eliminating any effect the elec-trosurgical current might have on the production of the signal when the generator is operational and the patient is in contact with the electrode; means for establish-ing a desired range having at least an upper limit for the impedance; and determining means responsive to the signal for determining whether the impedance is within the desired range. The system also includes means for estab-lishing a desired range having an upper limit and a lower limit for the impedance when the patient is in contact with the electrode elements; determining means responsive to the signal for determining whether the impedance is within the desired range; and adjusting means for adjust-ing the upper limit to adapt the system to the particular impedance of the patient in response to the particular impedance occurring within the desired range.

Description

IMPROVED RETURN ELECTRODE MONITORING SYSTEM

BACKGROUND OF THE INVENTION
This invention is directed to electrosurgery and, in particular, to circuitry for monitoring patient re-turn electrodes employed in such surgery.
One risk involved in electrosurgery is a burn under the patient return electrode. The most co~non conditions which are thought to lead to such a burn are:
(1) Tentinq: Lifting of the return electrode from the patient due to patient movement or improper application. This situation may lead to a burn if the area of electrode-patient contact is significantly reduced.
(2~ Incorrect Application Site~ Application of a return electrode over a highly resistive body location (i.e. excessive adipose tissue, scar tissue, erythema or lesions, excessive hair) will lead to a greater, more rapid temperature increase. Or; if the `20 electrode is not applied to the patient (i.e. electrode hangs freely or is attached to ano~her surface~, the patient is in risk of being burned by contact at an alternate returll path such as the table or monitoring electrodesO
~ (3) Gel Dr~ing either due to premature opening of the electrode pouch or to use of an electrode which has exceeded the recommended shelf life.
Many monitor systems have been developed in the past, but most cannot directly guard ayainst all three of the above listed situations. In order to pro~ect against these potential hazard situations, the patient ;, , ~
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itself should be monitored in addition to the continuity of the patient return circuit.
Safety circuitry is known whereby split (or double) patient electrodes are employed and a DC
current ~see German Patent No. 1,139,927, published November 22, 1962) or an AC current (see U. S. Patent Nos~ 3,933,157 and 4,200,104) is passed between the split electrodes to sense the contact resistance or impedance between the patient and the electrodes.
U. S. Patent No. 3,913,583 d.iscloses circuitry for reducing the curren-t passing through the patient de-pending upon the area of contact of the patient with ~
solid, patient plate, there being employed a saturable reactor in the output circuit, the impedanc~ of which varies depending upon the sensed impedance of the contact area.
The above systems are subject to at least one or more of the following shortcomings: (a) lack of sensitivity or adaptiveness to different physiological ch~racteristics of patients and (b) susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation.
OBJECTS OF THE INVENTION
~ccordingly, it is a primary object of this invention to provide an improved return electrode monitoring system which has little, of any, susceptibi--lity to electrosurgi.cal current interference when monitoring is continued during electrosurgical activationO
It is a further object of this invention to provide an improved return electrode monitoring system where two conductors are connected to a co~non elec-trode.
It is a further object of this invention to provide an improved return electrode monitoring system where the type of monitoring depends on the type of ~g~ Z8 7 return electrode employed in the system.
In accordance with the present invention, there is provided a return electrode monitoring system for use with a patient return electrode adapted for contacting a patient, the electrode having two, spaced apart conductors attached thereto for connecting the electrode to a generator of electrosurgical current. The system comprises means responsive to the impedance between the two conductors for producing a signal which is a function of the impedance, means for establishing a desired upper limit for the impedance and determining means responsive to the signal for determining whether the impedance is below the desired upper limit.
Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram of an illustrative system in accordance with the invention.
Figure lA is a diagrammatic illustration of a common foll electrode and associated cable for use in the system of Figure 1.
Figure 2 is a diagrammatic illustration indicating physiological characteristics affecting the impedance between the elements oE a split patient electrode when the electrode is in contact with a patient's skin.
Figure 3 is a schematic diagram of the patient impedance detection circuitry of Figure 1.
Figure 4 is a graph illustrating the operation of the adaptive threshold circuitry of Figure 1.
Figures 5A and 5B are a flow chart of a program for implementing the operation illustrated by Figure 4.
Figure 6 is a flow chart of a program for implementing a non-adaptive threshold function.
Figure 7 is a schematic diagram of circuitry for implementing a non-adaptive function.

DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS OF THE INVENTION
Reference should be made to the drawing where like reference numerals refer to like parts.
Referring to Figure 1, there is shown a block diagram of the system of the present invention wherein electrosurgical generator 10 rnay include known circuitry such a radio frequency oscillator 12 and an output amplifier 14 which generate an electrosurgical current.
This current is applied to a patient (not shown) via an active electrode 16. The electrosurgical current is returned to the generator 10 via a split return electrode 18 comprising electrodes 20 and 22 and a two conductor patient cable 24 comprising leads 26 and 28, The split return electrode may be of the type shown in above-mentioned U. S. Patent No. 4,200,104. The electrosurgical current is then returned to amplifier 14 via a lead 30 connected between capacitors 32 and 34.
These cap~citors are connected across the secondar~
winding 36 of a transformer 38.
The primary winding 40 of the transformer is connected to patient impedance detecting circuitr~ 42, the purpose of which is to produce a voltage EREM which is a function of the impedance between electrodes 20 and 22. EREM is applied to adaptive threshold circuitry 4~ which determines whether the above impedance is within a desired range~ the range being preferably adaptable to the physiological characteristics of the patient. If it is not, an inhibit signal is applied over a line 46 to intexnally disable the generator.
A plug attached to the generator end of two conductor cable 24 is insertable into a patient connectox which is incorporated in generator 10. The plug/
connector arrangement is diagrammatically indicated at 47 and 49. A switch 51 in the connector is also pro-vided to indicate the mode of operation of the system.
That is, in a first mode, the system employs the split patient electrode 18 of Figure 1. Incorporated in the plug for the split patient return electrode cable is a pin which activates switch 51 to thereby indicate ovex lines 61 and 63 to adaptive threshold circuitry 44 the system is operating in its first mode that is 9 with a split patient electrode.
Diagrammatically illustrated in Figure lA is an electrode arrangement employed in a second mode of U~ 7 operation of the system, the electrode 53 comprising a common foil having connected thereto at spaced apart points the lea~s 5S and 57 of a two conductor cabl~ 59. ~ plug attached to the generator end of the cable is insertable in the connector disposed at the generator. However, it does not include a pin corresponding to that described above. Hence~ when the plug of the Figure lA arrangement is inserted in the connector, switch 51 is not activated. According-ly, an indication is provided over lines 61 and 63 the system is operating in its second mode of operation.
In Figure 2, patient impedance detection circuitry 42 is shown connected to electrodes 20 and 22 which in turn are in contact with the patient's skin.
Further, the physiological characteristics of the patient's skin, adipose and muscle layers are dia-grammatically indicated b~ resistances. As will be described in detail hereinafter, detection circuitry 42 applies a constant, physiologically benign, monitor current ~typically 140 kHz, 2mA) to conductor 26 which passes through electrode 20 and the patient and then returns to circuitry 42 via electrode 22 and conductor 28. Circuitry 42 processes the voltage appearing across conductors 26 and 28 to provide EREM which, as stated above, is a measurement of the impedance between electrodes 20 and 22.
Adaptive threshold circui-try 44 typlcally establishes a range, which may extend from 20 to 144 ohms, within which the impedance between the electrodes (or pads) 20 and 22 must fall. If not, the generator lO is disabled. Thus, the lower limit is fixed at the nominal value of 20 ohms whereby such hazards as applying the electrode to a surface other than the patient may be avoided~ The upper limit is set to avoid such problems as those mentioned d hereinbefore - that is, tenting, incorrect applica-tion site, gel drying, etc~
In accordance with an important aspect o~ the invention, the upper limit is ad~ustable from the absolute maximum (typicall~ 144 ohms) downward to as low as typically 20 ohms to thereby provide for automatic adaptiveness to the physiological character-istics of the patient. This provides the monitor of the present invention with significantly more con-trol over the integrity of the return electrode con-nection without limiting the range o~ patient types with which the system may be used or burdening the operator with additional concerns. Tha~ is, the physiological characteristics indicated in Figure 2 can vary significantly from patient to patient and from one location site for the return electrode to another. Thus, patients, of course, vary in their respective ~mounts of adipose tissueO Further, for a particular patient, one location site may be more fatty, hairy or scarred than anotherO All of the factors may affect the impedance measured between electrodes 20 and 22 and thus concern -the operator as to which site is optimal for a particular patient.
As stated above, such concerns are eliminated in the present invention by providing for automatic adaptability to the physiological characteristics of the patient.
Referring now to Figure 3, there is shown a circuit diagram of patient impedance detection cir-cuitry, which comprises an oscillator indicated at48. The output of the oscillator is connected to a flip-flop 50 which provides a s~mmetrical square wave of typically 140 kHz. The outputs of flip-flop 50 are applied to 52 and 54 which pro~ide fast edges for accurate multiplexer operation, as described below.

Z~3~7 Constant currents from 52 and 54 pass through resistors 56 and 58 and thence through the respective halves 60 and 62 of primary winding 40 of trans~ormex 38. The impedance reflected to the primary side of the transformer will vary as a function of the impe-danee between electrodes 20 and 22. Accordingly, i~
view of the constant currents flowing through resistors 56 and 58 the voltages appearing at terminals 64 and 66 will vary as the above impedance. It is these voltages which are processed to derive ER~M.
A synchronous detector 68 comprising analog transmission gates 70-76 rejeets electrosurgical cur-rent which may appear at terminals 64 and 66. Thus, in accordance with another important aspect o~ the invention, monitoring of the return electrode circuit may not only be effected prior to electrosurgical activation but may also be continued during such acti-vation. Since the 140 kHz gating signals applied over lines 78-84 to gates 70-76 are in phase with the 140 kHz sense currents flowing into terminals 64 and 66 from resistors 56 and 58, the sensing signals applied to the gates from these terminals via resistors 85 and 87 will be passed by the gates and additively applied to RC circuits 86 and 8B where RC circuit 86 comprises resistor 90 and capacitor 92 and RC circuit 88 com-prises resistor 94 and capacitor 96. However, the 750 kHz electrosurgieal current signal will be ortho-gonal to the 140 kHz gating signal and thus, over a period o time the electrosurgical signals applied to RC circuits 86 and 88 will subtract from one another to thereby provide a very high degree of rejection of ~he electrosurgical current signal and any other noise.
The signals appearing across RC circuits 86 and 88 are ~6~ 7 applied to a differential ampliFier circuit 98, the output of the circuit being EREM.
Reference should now be made to Figure ~ which is a graph illustrating ~he operation of adaptive threshold circuitry 44.
The return electrode monitor (REM hereinafter) impedance range (that is, the acceptahle range of the impedance detected between electrodes 20 and 22) is preset when the power is turned on to an upper limit of 120 ohms and a lower limit of 20 ohms as can be seen at time T = 0 seconds in Figure 4. If -the moni-tored impedance is outside of this range (T - A seconds~
for example, when the return ele~trode is not affixed to the patient, an RE~I alert will be asserted and the generator will be disabled over line 46, The REM
impedance at any instant is designated the REM Instant-aneous Value (RIV) in Figure 4. When the REM impedance enters the range (T - B seconds) bounded by the Upper Limit (UL), the Lower L,imit (LL), a timing sequence begins. If after five seconds the RIV is still within range (T = C seconds), the alert condition will cease and the REM impedance value is stored in memory. This is designated as 'RE~ Nominal Value (RNV). The upper limit is then reestablished as 120% of this amount.
~5 The 80 ohm RIV shown in Figure 4 causes the upper limit to be at 96 ohms. This feature o-E the invention is particularly important because it is at this time (T = C seconds) that adaptation is initially made to the phys:iological characteristics of the patient. Note if the RIV were to exceed 96 ohms at a time between T - C and T - F second (while the upper limit is 96 ohms), the alert will be asserted and the generator dis-abled. However, if the upper limit had not been adjusted to 96 ohms, the alert would not have been asserted until '35 after the RIV exceeded the initial 120 ohms upper limit, ~2~C~'7 thus possibly subjecting the patient to undue heating at the split re~urn e~ectrode. This situation is o~
course exacerbated if the patient's initial RIV within the preset 20 to 120 ohm range is 30 ohms, for ex~nple.
An initial RIV of 120 ohms within the preset range of 20 to 120 ohms sets an upper limit of 144 ohms.
In accordance with another aspect of the inven-tion, it has been observed the REM impedance de-creases over a relatively long period such as a number of hours. Since man~ surgical procedures can extend a number of hours, this effect is also taken into consideration in the present invention. Accord-ingly, RIV is continuousl~ monitored and any minim~
in REM impedance, i.e., a downward trend followed by a constant or upward trend in REM impedance, initiates a new five second tJming interval (T = E seconds) at the end of which the RNV is updated to the RIV
if the RIV is lower (T = F seconds). The REM upper limit of 120% of RNV is re-established at this time.
The five second interval causes any temporar~ negative change in REM impedance (T = D seconds~ to be dis-regarded. Operation will continue in this manner pro-viding RIV does not exceed the upper limit of 120%
RNV or drop below the lower limit of 20 ohms. Ex-ceeding the upper limit (T = G seconds) causes a ~EM
alert and the generator is disabled. It will remain in alert until the RIV drops to 115% o RNV or less (T = H seconds) or until the REM s~stem is reinitial-ized. RIV dropping to less than 20 ohms (T = I
seconds) causes a similar alert which continues until either the RIV exceeds 24 ohms (T = J seconds~ or ~he system is reinitialized. The h~steresis in the limits of the REM range ~that is, the chansing of the upper limit to 115% of RNV and the lower limit to 24 ohms in the previous examples~ prevents erratic alerting when RIV is marginal.

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It should be n3ted in the example of Figure 4 the alert actually does not turn off when RIV returns to a value greater than 24 ohms because the split return electrode is removed before 5 seconds after T = J seconds elapse. Thus, the alarm stays on due to the removal of the electrodes.
Removing the return electrode from the patient or unplugging the cable 24 from generator 10 (T =
K seconds) for more than one second causes the REM
system to be reinitialized to the original limits of 120 and 20 ohms. This permits a pad to be reloca-ted or replaced (T = L seconds) without switching the generator off. The RIV at the new location is 110 ohms and 120~ RNV is 132 ohms. Thus, as aescribed above, this is the one -time (whenever RIV enters the 20 to 120 ohms ranye (either as preset during power on or as reinitialized as at T = K seconds) for the first time3 that the upper limit can be raised during the normal REM cycle. Otherwise, it is continually decreased to adapt to the decreasing RIV impedance with the passage of time.
The preferred implementation of the foregoing Figure 4 operation o~ the adaptive threshold circuitry 44 is effected by a programmed microprocessor such as the INTEL 8048. Attached hereto as an Appendix is a program for the INTE~ 8048 for implementiny the Figure 4 operation.
Reference should now be made to Figures 5A and 5B, which are a flow chart of the above-mentioned programr As indicated at 100, the program is called by another program TIMINT (Tim.ing Interrupt) which samples EREM
approximately 50 times every second. First0 RIV is calculated at portions 102 of the program in accordance w.ith the following equation:
RIV = ERF~ (1) I - I

where Isense is the constant current flowing through resistors 56 and 58 of Figure 3 and I h t is shunt cur-rent which ~lows through shunt paths in transformer 38 and through resistors 85 and 87. Ideally Ishunt would no-t be present and EREM would only be a func-tion of the variable ~IV and the constant current Isense.
However, not all of ~ is employed to produce EREM
~ecause of the above-mentioned shunt paths. Ishunt may be determined from the parameters of the circuit of Figure 3 and thus RIV is readily calculated in accordance with equation (1~.
A determination is next made at step 104 as to which mode of operation the system is in. Assuming switch 51 has been activated, the system is in its first mode of operation and a split return electrode is being used. The program now moves to a portion generally indicated at 106 comprising steps 108-116 the purpose o which is to implement the function described at T = K seconds of ~igure 4 whereb~ removal of electrode 18 or unplugging of cable 24 for more than approximately one second causes reinitialization of the system. That is, as indicated at step 114, RNV is reset to 120 ohms, 115% ~NV to 138 and 120% RNV
to 144 ohms where RNV, 115% RNV and 120% RNV are preset to these values at the time power is initiall~
applied to the generator. Another parameter LSTRIV
(LAST RIV), which will be discussed below, is also preset to 120 ohms at the time o initial power appli-cation. At step 108, a determination is made as to `30 whether RIV is greater than 150 ohms Ithat is, whether electrode 18 has been removed or cable 24 unplugged).
If so, a one second counter is incremented at step 110. Fifty increments (corresponding to the 50 samples per second of EREM) will cause the counter to ovexflow to zero at one second. Thus, if the counter is set to zero, this indicates one second has elapsed since electrode 18 was removed or cable 24 was unplugged whe~eby the program will pass from step 112 to step 114 to effe~t the resetting of RNV, 115% RNV and 120% RNV as described above. I RIV
is less than 150 ohms, the one second counter is cleared at step 115.
The pxogram passes from portion 106 to step 116 where the upper limit U~ is set to 120~ RNV and the lower limit LL is set to 20 ohms.
The program next moves to portion 118 which includes steps 120-126. This portion provides the hysteresis in the limits of the REM range illustra~ed at T - G or I of Figure 4. Thus, as will be described lS below, when RIV drops below 20 ohms, a mode one lo (low) fault flag will be set. When EREM is sampled again approximately 1/50 second later, -the mode one lo fault flag will still be set as detected at step 120 and the lower limit ~L will be reset to 24 o~ns at step 122 as illustrated at T - I. In a similar manner, the upper limit UL will be reset to 115%
RNV at steps 124 and 126 as illustrated at T - G
assuming a previous mode one hi (high~ fault has occurred~
The program now passes to portion 128 which includes steps 130-136 where the actual determinations are made as to whether RIV has remained with the desired range extending between UL and LLo If RIV
is greater than U~ (T - G), this is determined at step 130 and indicates the presence of a fault. Ac-cor~ingly, at step 132, any previous mode two fault (-to be described hereinafter) is cleared and the mode one hi fault flag is set.
Appropriate alarms ma~ -then be activated at portion 137 o~ the program and the INHIBIT signal on line 46 of Figure 1 is generated to disable the generatox. Rather than generating the INHIBIT
signal directly from the Figure 6 program, it ma~
also be done (ana is done in the actual implementation of the invention) by communicating REM status informa-tion (such as the status of the mode one hi and lo faults) to a main program (which effects other operations as-sociated with the generatox 10 not forming a part of this invention) via specific registers. These regis-ters are continually checked and if an~ REM fault bits ar~ set, the generator is disabled.
Portion 137 includes steps 140~1~60 Step 140 turns on an REM alarm light. A sound alarm may also be activated to provide a pred~termined number of bongs.
If this alarm has not been activated, this will be determined at step 142 whereby at step 144, a bong flag will be set to indicate actuation of the sound alarm. The number of bongs produced b~ the alarm is determined at step 146 where, :in this example, the 2~ number is two. Even though the generator has been dis-abled and alerts have been turned on, the system will continue to monitor RIV~
In a manner similar to that described above, a test is made at step 134 to determine_if the lower limit LL is greater than RIV. If it is, any previous flag is set and a five second counter, which will be discussed below, is also cleared.
Assuming RIV is within the range established by the current value of UL and L~/ the program passes to step 149 where any previous fault (which may have been set at steps 13Z, 136 or 180) is cleared, REM alert li~hts (which may have been turned on at step 140) are turned off and the bong flag (which may have been set at step 144) is cleared.
The program then moves to portion 150 which in-cludes steps 152-168. At portion 150, a de-termina-1~
tion is made as to whether any new minimum in RIV, resuIting either from RIV entering the desired ranye for the first time as at T = B or ~ ox from a decrease in value thexeof as at T = D or E, should be dis~e~
regarded as being a ~ransient. If the minimum lasts more than five seconds, it i5 not disregarded and RNV
is updated to the RIV if ~IV is lower as indicated at T = F . Thus, at step 152, a de~ermination is made as to whether the current RIV is less than the last RIV (LSTRIV). If it is not (that is e~ual to or greater than) the current RIV is immediately moved at step 156 to a register for storing LSTRIV and thus becomes the last RIV for the next sample of EREM. I~
RIV is lncreasing in such a manner that it is moving out of the desired range, th.is will quickl~ be detected at step 130 as successive samples of EREM
are processed, at which time, portion 136 will be activated ~o disable the generator and turn on ap-propriate alarms.
If RIV is less than LSTRIV, this indicates the possible occurrence of a non-transient minimum and thus, a five second counter is started at step 154.
The operation of this counter is similar to the one second counter previously discussed and after 250 successive increments thereof~ approximately five seconds will have elapsed which is indicated b~ the counter overflowing to zero. After starting the counter, the new lower RIV is moved to LSTRIV at step 1560 Of course, if RIV ever becomes less than 20 o~ms, this will be detected at step 134.
A check is next made at step 158 ~s to whether the five second counter has been started. If it has, the program returns to TIMINT preparatory to processing the next sample. If it hasn't, the five second counter is incremented at step 160 and again7 at step 162, a ~2C3~ '7 check is made to see if five seconds have elapsed on the counter. If not, the program returns to ~I~IN~ it has, a check is made at step 164 to see if RIV is less -than RNV. I~ RIV is not less than RNV, this indica-tes the downwaxd trend initially detected in RIV was transien~ and is thus disregarded and -the program returns to TTMINT. However, if RIV
is less than RNV, a non-transient minimum has occurred whereby the current RIV becomes the new RNV as indica-ted at step 166. The new values of 115% RNV and 120%
RNV are also calculated and stored at step 168.
~ s stated above, the system is placed ln itssecond mode of operation, when single foil electrode 53 of Figure lA is emplo~ed. Portion 170 of the program is used to a~sure continuity o~ the cable/electrode of Figure lA and its connection to -the generator.
Only an upper resistance limit of typically 20 ohms is employed. The above continuit~ is verified when the measured resistance between the two connector prongs is less than 20 ohms. A resistance of greater than 20 ohms causes a REM alert, and the generator is inhibited over line 46. Causing the resistance to decrease to less than 16 ohms, typically by replacing the cord/return electrode, ~ill clear the REM fault condition.
Arcordingly, portion 170 of the program includes steps 172-182 whereby if, at step 104, it is deter-mined the system is in its second mode of operation, the upper limit is set to 20 ohms at step 172. If there has been a previous mode two fault, the upper limit is decreased to 16 ohms at step 176 in a manner similar to the decrease that occurs in the mode one upper limit at step 126. A check is then made at step 178 to determine whether RIV is less than or equal to the upper limit. If it is not, a fault has occurred.
Thus, at step 180, any previous mode one fault flags ~ a are cleared and the mode two fault flag is set.
The program then enters portion 137 at which time the generator is disabled and appropriate alerts are turned on, as described above. If RIV is less than or equal to UL, all fault flags are cleared, the REM
alert light i5 turned o~f and the bong flag is cleared prior to returning to TIMINT.
Refere~ce should now be made to Figure 6 which is a flow chart of a computer program which may be used ln in a non-adaptive system. In a non-adaptive system, the upper and lower limits are fixed typically at 120 and 20 ohms. Of course, the advantages o the adaptive system as described hereinbefore are not available.
Howe~er, the protection afforded by such a system is adequate in many applications.
As can be seen in Figure 6, the program for a non-adaptive system is a simplified version of the Figure S adaptive program. Hence, in Figure 6, there is no portion 106 to reinitialize the upper limit since the upper limit is not changedO The same applies to portion 150 of the Figure 5 program where the upper limit is downwardl~ adjusted with the passage of time. Accordingly~ portions 106 and 150 are not included in the non-adaptive program of Figure 6. The r~m~;n;~g portions of the Figure 6 program are the same as the corresponding portions of the Figure 5 program with the following exceptions. In portion 118, the upper limit is set to 114 ohms if there has been a previous mode one hi fault at step 190. Further, there is no need to clear a five second counter as is done at step 136 of the Figure 5 program. With these exceptions, the operation of the Figure 5 program corresponds to that described above for the Figure 5 program. Hence, the operation of the Figure 6 program will not be repeated hexe.

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The software embodiment of Figure 6 is preferred for implementing a non-adaptive s~stem when a processor such as the INTEL 8~48 is employed for effecting other functions of the generator. However, when such a processor is not employed, a preferred implementation would be the threshold circuitry shown in Figure 7.
This circuitry includes comparators 220 and 222 which are respectively set via voltage dividers 2~1 and 223 to provide the high and low limits of 120 and 20 ohms.
Input terminals 224 and 226 preferably are connected to output terminal 228 of the synchronous detector 68. Thus, a double-ended output is presented so that the detector will be s~mmetrically loaded; however, only the Outpllt occurring at terminal 228 is used by the comparator circuits. If they are connected to terminal 228, the operational amplifier circuitr~ 98 of Figure 3 may be eliminated. Alternatively, the EREM output of Figure 3 may be applied to terminals 224 and 226 of Figure 7. ~ysteresis is respectively provided via elements 225 and 227 on comparators 220 and 222 to provide stable switching.
Exclusive OR gate 228 is keyed by the signal occurring on lines 61 and 63 of Figure 1 to thereby establish the mode of operation o~ the threshold cir-cuitry. Thus, if a common foil electrode is employed (mode two), the low resistance value of comparator 222 is employed as the upper limit. If the input slgnal at terminal 226 exceeds this upper limit signal established at the other input to comparator 222, an inhibit is

3~ applied to terminal ~30 (connected to line 46 of Figure 1) via gates 228 and 232 and inverter 234 to thereby disable the generator.
I~ a split patient electrode is employed (mode one1~ the low resistance value of comparator 222 is employed as the lower limit and the high resistance t7 value of comparator 220 is employed as the upper limit.
If either the input signal at terminal 224 exceeds the upper limit established at comparator 220 or the input signal at terminal 226 is less than the lowex ~imit established at comparator 222, an inhibit signal is applied to terminal 230. ~ppropriate visual and sound alarms may also be provided as needed upon occurrence of the inhibit signal.
It is to be understood that the above detailed description of the various embodiments of the invention is provided by way of example onl~ Various details of design and construction may be modified without departing rom the true spirit and scope of the invention as set forth in the appended claims.

Claims (25)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A return electrode monitoring system for use with a patient return electrode adapted for contacting a patient, said return electrode having two, spaced apart conductors attached thereto for connecting the electrode to a generator of electrosurgical current which passes through the electrode, said system comprising means for applying a monitoring current through said conductors to said electrode;
detecting means responsive to said monitoring current for producing a signal which is a function of the impedance between said two conductors, said detecting means including means for substantially eliminating any effect the electrosurgical current might have on the production of said signal when the generator is operational and the patient is in contact with the electrode;
means for establishing a desired range having at least an upper limit for said impedance; and determining means responsive to said signal for determining whether said impedance is within said desired range.

2. A system as in Claim 1 where said means for establishing a desired upper limit includes means for generating a reference signal corresponding to the upper limit and where said determining means includes comparator means for comparing the signal which is a function of said impedance with the reference signal.

3. A system as in Claim 1 where said desired range includes a lower limit for said impedance and where said means for generating a desired range includes means for generating a further reference signal corresponding to the lower limit and where said determining means includes further comparator means for comparing the signal which is a function of said impedance with the further reference signal.

4. A system as in Claim 1 where said detecting means is a synchronous detector.

5. A system as in Claim 1 where said means for generating the monitoring current includes an oscil-lator which also generates a gating signal for the synchronous detector.

6. A system as in Claim 5 where said means for generating the monitoring signal also includes a transformer for coupling the two conductors to the oscillator, the secondary winding of the transformer being connected to the two conductors and the primary winding thereof being in circuit with the output of the oscillator where the output of the oscillator is a constant current so that the im-pedance reflected from the secondary circuit including the electrode and the two conductors to the primary winding causes the voltage across the primary winding to follow any variations in the impedance between said two conductors.

7. A system as in Claim 1 where said synchro-nous detector is connected across said primary winding.

8. A system as in Claim 1 where the frequency of said electrosurgical current is sub-stantially different from that of said monitoring current.

9. A system as in Claim 8 where the frequency of the electrosurgical current is 750 kHz and that of the monitoring current is 140 kHz.

10. A system as in Claims 1 or 8 where said return electrode is a single foil electrode with the two conductors attached at spaced apart points on the electrode.

11. A system as in Claims 1 or 8 where said return electrode is a split patient electrode having two, electrically isolated electrode elements with the two conductors respectively attached to the elements.

12. A return electrode monitoring system for use with a return electrode adapted for contacting to a patient r said return electrode being either (a) a split patient electrode having two, electrically isolated electrode elements with two conductors respectively attached to the elements or (b) a single foil electrode with two conductors attached at spaced apart points on the common foil electrode, said system comprising electrode selecting means for detecting whether the return electrode is of the split patient type or the single foil type;
means responsive to the impedance between the two conductors for producing a signal which is a function of said impedance regardless of which type of return electrode is employed;
means for establishing a first desired range having an upper and a lower limit for said impedance when (a) the split patient electrode is employed and (b) the patient is in contact with the electrode element;
means for establishing a second desired range having at least an upper limit for said im-pedance when the single foil electrode is employed;
first determining means responsive to said electrode detecting means detecting the employment of a split patient electrode for determining whether said impedance is within said first desired range;
and second determining means responsive to said electrode detecting means detecting the employment of a single foil electrode.

. 13. A system as in Claim 12 where said lower limit of the first desired range equals the upper limit of the second desired range.

14. A return electrode monitoring system for use with a single foil, patient return electrode adapted for contacting a patient, said electrode having two, spaced apart conductors attached thereto for connecting the electrode to a generator of electrosurgical current, said system comprising means responsive to the impedance between said two conductors for producing a signal which is a function of said impedance;
means for establishing a desired upper limit for said impedance; and determining means responsive to said signal for determining whether said impedance is below said desired upper limit.

15. A system as in Claim 14 where said means fox establishing a desired upper limit includes means for generating a reference signal corresponding to the upper limit and where said determining means includes comparator means for comparing the signal which is a function of said impedance with the reference signal.

16. A return electrode as in Claim 14 where said desired upper limit is 20 ohms.

17. A system as in Claim 14 including means for detecting a transition of said impedance from a value less than said upper limit to a value greater than said upper limit and means for disabling the system in response to the occurrence of the transition.

18. A system as in Claim 14 including means for detecting a transition of said impedance from a value less than said upper limit to a value greater than said upper limit and means for generating an alarm signal in response to the occurrence of the transition.

19. A system as in Claim 14 including transition detecting means for detecting a transition of said impedance from a value less than said upper value to a value greater than said upper limit and means for decreasing the initial value of the upper limit to a lower value in response to said transition.

20. A system as in Claim 19 including means for disabling the system in response to the occurrence of the transition.

21. A system as in Claim 19 including means for generating an alarm signal in response to the occurrence of a transition.

22. A system as in Claim 4 where the frequency of said electrosurgical current is substanstially different from that of said monitoring current.

23. A system as in Claim 22 where the frequency of the electrosurgical current is 750 kHz and that of the monitoring current is 140 kHz.

24. A system as in Claim 22 where said return electrode is a single foil electrode with the two conductors attached at spaced apart points on the electrode.

25. A system as in Claim 22 where said return electrode is a split patient electrode having two, electrically isolated electrode elements with the two conductors respectively attached to the elements.