CA1200286A

CA1200286A - Adaptive, return electrode monitoring system

Info

Publication number: CA1200286A
Application number: CA000413745A
Authority: CA
Inventors: David W. Newton; William G. Paterson
Original assignee: Valleylab Inc
Current assignee: Covidien AG
Priority date: 1981-10-26
Filing date: 1982-10-19
Publication date: 1986-02-04
Also published as: US4416276A; JPS58103445A; JPH0349577B2

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

BAC~GRO~ND 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 common conditions which are thought to lead to such a burn are:
(l) Tentinq: Lifting of the return electrode from the patient due to patient movement or improper application. This si-tuation 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. eYcessive adipose tissue, scar tissue, erythema or lesions, excessive hair~ will lead to a greater, more rapid temperature increase. Or, if the electrode is not applied to the patient li.e. electrode hangs freely or is attached to another surface), the patient is in risk oE being burned by contact at an alternate return path such as the table or monitoring electrodes.

(3) Gel Drying 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 mos-t cannot directly guard against all three of the above listed situations. In order to protect against these potential hazard situations, the patient ~''.'' ~,y ,~

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itself should be monitore~ in addition to the continuity o~ the patien-t return circuit.
Safety circuitry is known whereby split (or double) patient electrodes are emplo~ed and a DC
current (see German Patent NoO 1,139,927, published Novelnber 22, 1962) or an AC current (see U. S. Patent Nos. 3,933,157 and ~,200,104) is passed between the split electrodes to sense the contact resistance or impedance between the pa-tient and the electrodes.
U. S. Patent No. 3,913,583 discloses circuitry for reducing the current passing through the patient de-pending upon the area of contact of the patient with a solid, patient plate, there ~eing emplo~ed a saturable reactor in the output circuit, the impedance oE which varies depending upon the sensed impedance of the contact area.
The ahove systems are subject to at least one or more of the followins shortcomings: (a) lack of sensitivity or adaptiveness to different physiological characteristics of patients and (b) susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation.
OBJECTS OF THE INVENTION
Accordingly, it is a primary object of this invention to provide an improved return electrode monitoring system which has little, of any, susceptibi-lity to electrosurgical current interference when monitoring is continued during electrosurgical activation.
It is a further object of this invention to provide an improved return electrode monitoring system where two conductors are connected to a common elec-trode.
It is a further object of this invention to provide an improved return electrode monitoring sys-tem where the type of monitoring depends on -the type of ~ ~3~

return electrode employed in the system.
In accordance with one aspect of the inven~ion, there is provided a return electrode monitoring system for use with a split patient electrode having two, electrically isolated electrode elements adapted for contactin~ a patient. The system cpmprises means responsive to the impedance between the two electrode elements for producing a signal which is a function of the impedance, means for establishing 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 adjusting the upper limit to adapt the system to the particular impedance of the patient in response to the particular impedance occuring within the desired range.
In accordance with a further aspect of the present invention, there is provided a return electrode monitoring system for use with a common foil, pa-tient 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. ~he 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 ob~ects 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 foil electrode and associated cable ~or use in the system of Figure 1.
Figure 2 is a diagrammatic illustration indicating physiological charac~eristics affecting the impedance between the elements of 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 circultry of Figure 1.
Figure 4 is a graph illustrating the operation of the adaptive threshold circuitry of Figure 1.
Figures ~ and 5B are a flow chart of a program for implementing the operation illustrated by Figure 4O
Figure 6 is a flow chart of a program Eor implementing a non-adaptive threshold functionO
Figure 7 is a schematic diagram of circuitry for implementing a non-adaptive function.

DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS OF THE INVENTION
Referenee 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 may inelude known circuitry such a radio frequency oscillator 12 and an output amplifier 1~ which generate an electrosurgical cur~ent.
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 elect~ode may be o -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 capacitors are connected across the secondary winding 36 of a transEormer 38.
~he 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 o~ 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 ran~e being preferably adaptable to the physiological characteristics of the patient. If it is not~ an inhibit signal is applied over a line ~6 to internally disable the generator.
A plu~ attached to the generator end of two conductor cable 24 is insertable into a patient connector 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 o:E the system.
That is, in a first mode, the syste~l 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 thereb~ indicate over lines 61 and 63 to adaptive threshold circuitry 44 the system is operating in its first mode - that is, with a split patient electrode.
Diagrammatically illustrated in Figure lA is an electrode arrangement employed in a second mode o~

6''~

operation of the system, the electrode 53 comprising a common foil having connected thereto at spaced apart points the leads 55 and 57 of a two conductor cable 59. A plug attached to the generator end of the cable is inserta~le 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 1~ arrangement is inserted in the connector, switch 51 is not activated. According-1~, 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 eleetrodes 20 and 22 which in turn 2re in contact with the patient's skin.
Further, the physiological characteristics of the patient's skin, adipose and muscle layers are dia-grammatically indicated by resistances. As will be described in detail hereinafter, detection circuitry 42 applies a constant, physiologicall~ 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 circuitry 44 typically 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 notr the generator 10 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 pakient ma~ be avoided. The upper limit is set to avoid such problems as those mentioned hereinbefore - that is, tenting, incorreet applica-tion site, gel drying, etc.
In accordance with an important aspect of the invention, the upper limit is adjustable from the absolute maxlmum (typically 144 ohms) downward to as low as typically 20 ohms to thereby provide for automatic adaptiveness to the physioloyical character-istics of the patient. This provides the monitor of the present invention witn significantly more con-trol over the integrity of -the return eleetrode con-nection without limiting the ran~e of patient types with which the system may be used or burdening the operator with additional concerns. That is, the physiological characteristies indicated in Figure 2 can vary si~nificantly from patient to patient and from one location site for the return electrode to another. Thusl patients, of course, vary in their respective amounts of adipose tissue. Further, for a particular patient, one location site may be more fatty, hairy or scarred than another. All of the ~aetors may affect the impedance measured between electrodes 20 and 22 and thus eoncern the operator as to which site is optimal for a particular patient.
As stated above, sueh concerns are eliminated in the present invention by providing for automa-tic adaptability to the physiologieal 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 at43. The output of the oscillator is connected to a flip-flop 5~ which provides a s~mmetrical s~uare wave of typically 140 kHz. The outputs of flip-~lop 50 are applied to 52 and 54 which provide fast edges for accurate multiplexer operation, as described bel~w.

Constant currents ~rom 52 and 54 pass through resistors 56 and 58 and thence through the respective halves 60 and 62 of primary winding 40 of transformer 38. The impe~ance reflected to the primary side of the transformer will vary as a function of the impe-danee between electrodes 20 and 22. Accordingly, in 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 EREMn 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 of 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 addi-tively applied to RC circuits 86 and 88 where RC circuit 86 comprises resistor 90 and capacitor 92 and RC circuit 88 com-prises resistor 94 and eapacitor 96. However, the 750 kHz electrosurgical current signal will be ortho-gonal to the 140 kHz gating signal and thus, over a period of 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 an~ other noise.
The signals appearing across RC circuits 86 and 88 are applied to a differential amplifier circuit 98, the output of the circuit being EREM.
Reference should now be made to Figure 4 which is a graph illustrating the operation of adaptive threshold circuitry 44.
The return electrode monitor (RE~I hereinafter) impedance range (that is, the acceptable 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 o~ns 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 electrode is not affixed to the patient, an ~EM 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 IRIV) in Figure 4. When the REM impedance enters the range (T = B seconds~ bounded b~ the Upper Limit (UL), the Lower Limit (LL)I 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 memor~. This is designated as REM Nominal Value (RNV). The upper limit is then reestablished as 120% of this amount.
The 80 ohm RIV shown in Figure 4 causes the upper limit to be at 96 ohms. This feature of the invention is particularly im~ortant because it is at this time (T = C seconds) that adaptation is initially made to the physiological characteristics of the patient. Note `30 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 asser-ted until after the RIV exceeded the initial 120 ohms upper limit, thus possibly subjecting the pa~ient to undue heating at the split return electrode. This si-tuation is of course exacerbated if the pa-tient's initial RIV within the preset 20 to ]20 ohm range is 30 ohms, for example.
An initial RlV 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 ~he inven-tion, it has been observed the REM impedance de-creases over a relatively long period such as a 1~ number of hours. Since many surgical procedures can extend a number of hours, this effect is also taken into consideration in the present invention. Accord-ingly, RIV is continuously monitored and any minima in REM impedance, i.e., a downward trend followed by a constant or upward trend in REM impedance, initiates a new five second timing 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. Qpera-tion 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 REM
alert and the generator is disabled. It will remain in alert until the RIV drops to 115% of 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 con-tinues un-til either the RIV exceeds 24 ohms (1~ = J seconds) or the system .i5 reinitialized. The hysteresis in the limits of the REM range (that is, the changing 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.

1(~
It should be noted in the example of Figure 4 the alert actually does not turn off when RIV re-turns 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 relocated 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 described above, this is the one time (whenever RIV enters the 20 to 120 ohms range (either as preset during power on or as reinitialized as at T = X seconds) for the first time) that the upper limit can be raised during the normal REM cycle~ Otherwise, it is continuall~
decreased to adapt to the decreasing RIV impedance with the passage of time.
The preferred implementa-tion of the foregoing Figure 4 operation of 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 INTEL 8048 for implementing the Figure 4 opera-tion, Reference should now be made to Figures 5A and 5B, which are a flow chart of the above-mentioned program.
As indicated at 100, the program ~s called by another program TIMINT (Timing Interrupt) which samples EREM
approximately 50 times every second. First, RIV is calculated at portions 102 of the program in accordance with the following equation:
RIV = EREM (1) Is nse ~ I hunt J~

where ISense is the constant current flowing through resistors 55 and 58 of Figure 3 and I h t is shunt cur-rent which flows through shunt paths in transformer 38 and through resistors 85 and 87. Ideally IShunt would not be present and EREM would onl~ be a func-tion of the variable RIV and the constant current Isense.
However, not all of IsenSe is employed to produce EREM
because 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 of which is to implement the function described at T = K seconds of Figure 4 whereby removal of electrode 18 or unplugging of cable 24 for more than approximately one second causes reinitialization of the s~stem. That is, as indicated at step 114, RNV is reset to 120 ohms, 115% RNV 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 initlally applied to the generatox. Another parameter LSTRIV
(I,AST RIV), which will be discussed below, is also preset to 120 ohms at the time of initial power appli-cation. At step 108, a determination is made as to whether RIV is greater than 150 ohms (that is, whether electrode 18 has been removed or cable 24 unplugged3.
If so, a one second counter is incremented at step 110. Fifty increments (corresponding to the 50 samples per second of EREM3 will cause the counter to overflow 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 whereby the program will pass from step 112 to step 114 to effect the resetting of RNV, 115~ RNV and 120% RNV as described above If RIV
is less than 150 ohms, the one second counter is cleared at step 115.
The program passes rom portion 106 to step 116 where the upper limit UL 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 illustrated at T = G or I of Figure 4. Thus, as will be described 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 LL will be reset to 24 ohms 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 :L30-136 where the actual determinations are made as to whether RIV has remained with the desired range extending between UL and LL. If RIV
is greater than UL (T = G), this is determined at step 130 and indicates the presence of a faul-t. Ac-cordingly, 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 of the program and the INHIBIT signal 3~

on line 46 of Figure 1 is generated to disable the generator. Rather than generating the INHIBIT
signal directly from the Figure 6 program, it ma~
also be done (and is done in the actual implementation of the invention) by communicating R~M 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 generator 10 not forming a part of this invention) via specific registers. These regis-ters are continually checked and if any REM fault bits are set, the generator is disabled.
Portion 137 includes steps 140-146. Step 140 turns on an REM alarm light. A sound alarrn may also be activated to provide a predetermined 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 by the alarm is determined at step 146 where, in this example, the 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 b~
the current value of UL and LL, the program passes to step 149 where any previous fault (which may have been set at steps 132, 136 or 180) is cleared, RF.M alert lights (which may have been turned on at step 140) are turned off and the bong flag lwhich 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 determina-tion is made as to whether any new minimum in RIV, resulting either from RIV entering the desir~d range for the first time as at T = B or L or from a decrease in value thereof as at T = D or E, should be disre~
reyarded as being a transient. If the minimum lasts more than five seconds, it is not disregarded and RNV
is updated to the RIV if RIV is lower as indicated at T = F. Thus, at step 152, a determina-tion 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 xegister for storing LSTRIV and thus becomes the last RIV for the next sample of EREM. If RIV is increasing in such a manner that it is moving out of the desired range, this will quickly be detected at step 130 as successive samples of EREM
are processed, at which time, portion 136 will be activated to 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 by the counter overflowing to zero. After starting the counter, the new lower RIV is moved to LSTRIV at step 156. Of course, if RIV ever becomes less than 20 ohms, this will be detected at step 1340 A check is next made at step 158 as 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 again, at step 162, a check is made to see if Eive seconds have elapsed on the c~unter. If not, the program returns to TIMINT. If it has, a check is made at step 164 to see if RIV is less than RNV. If RIV is not less than RNV, this indicates the downward trend initially detected in RIV was transient and is thus disregarded and the program returns to TIMINT. 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.
As stated above, the system is placed in its second mode of operation, when single ~oil electrode 53 of Figure lA is employed. Portion 170 of the program is used to assure continuity of 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 continuity is verified when the measured resistance between the two connector prongs is less than 20 ohmsO A resistance of greater than 20 ohms ~auses a REM alert, and the generator is inhibited over line 46. Causing the resistance to decrease to less than 16 ohms, typicall~ by replacing the cord/return electrode, will clear the REM fault condition.
Accordingly, 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 se-t 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 upp~r 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 1~
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. I ~IV is less than or equal to UL, all fault flags are cleared, the REM
alert light is turned off and the bong flag is cleared prior to returning to TIMINT.
Reference should now be made to Figure 6 which is a flow chart of a computer program which may be used 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 of the adaptive system as described hereinbefore are not availableO
However; 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 5 adaptive program. Hence, in Figure 6I there is no portion 106 to reinitialize the upper limit since the upper limit is not changed. 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~ining 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 6 program corresponds to that described above for the Eigure 5 program. Hence, the operation of the Figure 6 program will not be repeated here.

The software embodiment of Figure 6 is preferred for implementing a non-adaptive s~stem when a processor such as the INTEL 8048 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 221 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 output occurring at terminal 228 is used by the comparator circuits. If they are connected to terminal 228, the operational amplifier circuit~y 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. Hysteresis 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 of 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 signal at terminal 226 exceeds this upper limit signal established at the other input to comparator 222, an inhibit is applied to terminal 230 (connected to line 46 of Figure 1) via gates 228 and 232 and inverter 234 to thereby disable the generator.
If a split patient electrode is employed (mode one), the low resistance value of comparator 222 is employed as the lower limit and the high resistance 5 ~

value of comparator 220 is employed as the upper limit.
If either the input signal at terminal 224 exceeds the upper limi-t established at comparator 220 or the input signal at terminal 226 is less than the lower limit established at comparator 22~, an inhibit signal is applied to terminal 230. Appropriate visual and sound alarms may also be provided as needed upon occurrence of the inhibit signal.
It is to be understood -that the above de-tailed description of the various embodiments of the invention is provided by way of example only. Various details of design and conskruction may be modified without departing from the true spirit and scope of the invention as set forth in the appended claims.

Claims

1. A return electrode monitoring system for use with a split patient return electrode having two, electrically isolated electrode elements adapted for contacting a patient, said system comprising means responsive to the impedance between said two electrode elements for producing a signal which is a function of said impedance;
means for establishing a desired range having an upper limit and a lower limit for said impedance when the patient is in contact with the electrode elements;
determining means responsive to said signal for determining whether said impedance is within said desired range; and adjusting means for adjusting said upper limit to adapt said system to the particular impedance of said patient in response to said particular impedance occurring within the desired range.

2. A system as in Claim 1 including means for respectively presetting said upper and lower limits to 120 and 20 ohms.

3. A system as in Claim 1 where said adjusting means includes means for adjusting the upper limit to a value greater than the said patient's particular impedance by a predetermined percentage of the particular impedance.

4. A system as in Claim 3 where said predeter-mined percentage is 120%.

5. A system as in claim 1 including delay means for delaying the operation of said adjusting means until a predetermined amount of time has elapsed from the first occurrence of the patient's particular impedance within the desired range.

6. A system as in claim 5 where the said predetermined amount of time is 5 seconds.

7. A system as in claim 1 including means for detecting a change in the patient's particular impedance to a decreased value and where said adjusting means includes decreasing means for decreasing the upper limit of the desired range to a value related to said decreased value.

8. A system as in claim 7 where said decreasing means includes means for decreasing the upper limit to a value greater than the decreased value of the patient's particular impedance by a predetermined percentage of the decreased value.

9. A system as in claim 8 where said predetermined percentage is 120%.

10. A system as in claim 7 including delay means for delaying the operation of said adjusting means until the predetermined amount of time has elapsed from the occurrence of said decreased value of the patient's particular impedance.

11. A system as in claim 10 where said predetermined amount of time is 5 seconds.

12. A system as in Claim 1 including transition detection means for detecting a transition of said patient's particular impedance from within the desired range to outside the range and changing means for changing the one of said upper and lower limits in response to said transition.

13. A system as in Claim 12 where said transi-tion detection means includes means for detecting said transition of the patient's particular impedance to a value greater than the upper limit and where said changing means includes means for decreasing the upper limit in response to said transition.

14. A system as in Claim 13 where said means for decreasing the upper limit includes means for de-creasing the upper limit from 120% of the patient's particular impedance to 115% of the patient's partic-ular impedance.

15. A system as in Claim 12 where said transition detection means includes means for de-tecting said transition of the patient's particular impedance to a value less than the lower limit and where said changing means includes means for in-creasing the lower limit in response to said transi-tion.

16. A system as in Claim 15 where said means for increasing the lower limit includes means for in-creasing the lower limit from 20 to 24 ohms.

17. A system as in Claim 1 including means for presetting said upper limit to an initial value; means for detecting a transition of said patient's partic-ular impedance from within the desired range to a value greater than the upper limit; timing means for establishing a predetermined time interval after the occurrence of said transition; and means for resetting the upper limit to said initial value if the patient's particular impedance remains in excess of a maximum amount for a predetermined time interval.

18. A system as in Claim 17 where said pre-determined time interval is one second.

19. A system as in Claims 1, 3, or 8 including means for detecting the transition of said patient's particular impedance from within the desired range to outside the range and means for disabling the system in response to the occurrence of the transition.

20. A system as in Claims 1, 3, or 8 in-cluding means for detecting the transition of said patient's particular impedance from within the desired range to outside the range and means for generating an alarm signal in response to the occurrence of the transition.

21. A return electrode monitoring system for use with a common 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 microprocessor means including 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.

22. A system as in Claim 21 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.

23. A return electrode as in Claim 21 where said desired upper limit is 20 ohms.

24. A system as in Claim 21 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.

25. A system as in Claim 21 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.

26. A system as in Claim 21 including transition detecting means for detecting a transition of the impedance from a value less than said upper limit 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.

27. A system as in Claim 26 including means for disabling the system in response to the occurrence of the transition.

28. A system as in Claim 26 including means for generating an alarm signal in response to the occurrence of the transition.

29. A system as in Claim 26 where said initial value is 20 ohms and said lower value is 16 ohms.

30. A system as in Claim 17 including means for detecting the transition of said patient's particular impedance from within the desired range to outside the range and means for disabling the system in response to the occurrence of the transition.

31. A system as in Claim 17 including means for detecting the transition of said patient's particular impedance from within the desired range to outside the range and means for generating an alarm signal in response to the occurrence of the transition.

32. A system as in claim 3 including delay means for delaying the operation of said adjusting means until a predetermined amount of time has elapsed from the first occurrence of the patient's particular impedance within the desired range.

33. A system as in claim 32 where the said predetermined amount of time is 5 seconds.

34. A system as in claim 3 including means for detecting a change in the patient's particular impedance to a decreased value and where said adjusting means includes decreasing means for decreasing the upper limit of the desired range to a value related to said decreased value.

35. A system as in claim 34 where said decreasing means includes means for decreasing the upper limit to a value greater than the decreased value of the patient's particular impedance by a predetermined percentage of the decreased value.

36. A system as in claim 35 where said predetermined percentage is 120%.

37. A system as in claim 35 including delay means for delaying the operation of said adjusting means until the predetermined amount of time has elapsed from the occurrence of said decreased value of the patient's particular impedance.

38. A system as in claim 37 where said predetermined amount of time is 5 seconds.

39. A system as in claim 13 where said transition detection means includes means for detecting said transition of the patient's particular impedance to a value less than the lower limit and where said changing means includes means for increasing the lower limit in response to said transition.

40. A system as in claim 39 where said means for increasing the lower limit includes means for incresing the lower limit from 20 to 24 ohms.