# Patent

## Title
Multiple RF return pad contact detection system

## Abstract

A multiple RF return pad contact detection system is provided which is adaptive to different physiological characteristics of patients without being susceptible to electrosurgical current interference (e.g., interference or measurement interaction between components of the detection system). The detection system can measure or sense the contact resistance or impedance between the patient and pairs of RF return pads or return electrodes where multiple pairs of RF return pads are utilized due to the high current frequently needed during electrosurgery while eliminating or minimizing the risk of measurement interaction between the RF return pad pairs. The system allows for the independent and simultaneous measurement of the pad contact impedance for each pair of RF return pads. If the impedance of any pad pair is above a predetermined limit, the system turns off or reduces the electrosurgical output of the electrosurgical generator to prevent excess heating. The system eliminates or minimizes interference or measurement interaction between the pad pairs by providing a different signal source frequency for each pad contact pair, but a frequency which matches an associated series resonant network frequency. The current that flows in the series resonant network is a direct reflection or function of the pad impedance of the corresponding pad pair.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 10/918,984, filed Aug. 16, 2004 by Sturm et al., entitled “MULTIPLE RF RETURN PAD CONTACT DETECTION SYSTEM”, now U.S. Pat. No. 7,160,293, which is a continuation of U.S. application Ser. No. 10/254,956, filed Sep. 25, 2002 by Sturm et al., entitled “MULTIPLE RF RETURN PAD CONTACT DETECTION SYSTEM”, now U.S. Pat. No. 6,860,881.


BACKGROUND
1. Technical Field
The present disclosure is directed to electrosurgery and, in particular, to circuitry for measuring or sensing the contact resistance or impedance between the patient and pairs of RF return pad contacts or electrodes employed in such surgery.
2. Description of the Related Art
One potential risk involved in electrosurgery is the possibility of stray electrical currents causing excess heating proximate the RF return pad contacts or patient return electrodes. The most common conditions which are thought to lead to excess heating include:
(1) Tenting: Lifting of the return electrode from the patient due to patient movement or improper application. This situation may lead to excess heating 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 (e.g., excessive 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 (i.e. electrode hangs freely or is attached to another surface), the current may seek an alternate return path such as the table or monitoring electrodes; and
(3) Gel drying either due to premature opening of the electrode pouch or use of an electrode which has exceeded the recommended shelf life.
Many monitor or detection systems have been developed in the past, but most cannot directly guard against all three of the above listed situations. In order to protect against these potentially hazardous situations, the contact resistance or impedance between the return electrode and the patient 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 Pat. No. 1,139,927, published Nov. 22, 1962) or an AC current (see U.S. Pat. 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. Pat. No. 3,913,583 discloses circuitry for reducing the current passing through the patient depending upon the area of contact of the patient with a solid, patient plate. A saturable reactor is included in the output circuit, the impedance 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 characteristics of patients and (b) susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation.
U.S. Pat. Nos. 4,416,276 and 4,416,277 describe a split-patient return electrode monitoring system which is adaptive to different physiological characteristics of patients, and a return electrode monitoring system which has little, if any, susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation. The entire contents of both U.S. Pat. Nos. 4,416,276 and 4,416,277 are incorporated herein by reference.
Still a need exists for a detection or monitoring system, which is: 1) adaptive to different physiological characteristics of patients; 2) has little, if any, susceptibility to electrosurgical current interference, (including interference or measurement interaction between components of the detection system); 3) can measure or sense the contact resistance or impedance between the patient and pairs of RF return pads or electrodes where multiple pairs of RF return pads are utilized due to the high current frequently needed during electrosurgery, such as during tissue ablation; and 4) eliminates or minimizes the risk of measurement interaction between the RF return pad pairs.
Therefore, it is an aspect of the invention to provide a multiple RF return pad contact detection system for use during electrosurgical activation which achieves the above objectives.
SUMMARY
A multiple RF return pad contact detection system is disclosed which is adaptive to different physiological characteristics of patients, without being susceptible to electrosurgical current interference. The detection system includes interference or measurement interaction between components of the detection system which can measure or sense the contact resistance or impedance between the patient and pairs of RF return pads or electrodes when multiple pairs of RF return pads are utilized. Due to the high current frequently needed during electrosurgery, such as during tissue ablation, the detection system eliminates or minimizes the risk of measurement interaction between the RF return pad pairs.
The circuitry of the multiple RF return pad contact detection system is preferably provided within an electrosurgical generator for controlling the generator according to various measurements, such as the contact resistance or impedance between the patient and pairs of RF return pads or return electrodes. The system allows for the independent and simultaneous measurement of the pad contact impedance for each pair of RF return pads. If the impedance of any pad pair is above a predetermined limit, the system turns off or reduces the electrosurgical output of the electrosurgical generator to prevent excess heating.
The system eliminates or minimizes interference or measurement interaction between the pad pairs by providing a different signal source frequency for each pad contact pair, but a frequency which matches an associated series resonant network frequency. The current that flows in the series resonant network is a direct reflection or function of the pad impedance of the corresponding pad pair. Since the two resonant networks are tuned to different frequencies, there is minimal interaction, if any, within the system, thus reducing the chances of inaccurate measurements.
The system could be modified by providing a multiplexer to multiplex the measurements corresponding to each pad contact pair to eliminate or minimize measurement interaction and also minimize hardware resources.
Further features of the multiple RF return pad contact detection system of the invention will become more readily apparent to those skilled in the art from the following detailed description of the apparatus taken in conjunction with the drawing.



BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the invention will be described herein below with reference to the drawings wherein:
FIG. 1 is a schematic diagram of the multiple RF return pad contact detection system in accordance with a preferred embodiment of the invention; and
FIG. 2 is a graph illustrating the operation of the pad contact impedance measurement subsystem of FIG. 1.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference should be made to the drawings where like reference numerals refer to similar elements. Referring to FIG. 1, there is shown a schematic diagram of the multiple RF return pad contact detection system 100 of the present invention wherein electrosurgical generator 10 includes known circuitry such as 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 pad contact pairs or return electrode pairs 18a, 18b having pads or electrodes 20a, 20b and 22a, 22b and a corresponding two conductor patient cable 24a, 24b having leads 26 and 28. Two capacitors 32 and 34 are connected across each of the secondary windings 40a, 40b of transformer 38a, 38b. 
Each primary winding 36a, 36b is connected to a corresponding a.c. signal source 42a, 42b and a series resonant network 44a, 44b. The purpose of each series resonant network 44a, 44b is to produce a current (i.e., left and right current senses) which is a function of the impedance between pads or electrodes 20a, 20b and 22a, 22b. 
The system 100 eliminates or minimizes interference or measurement interaction between the pads 20a, 20b and 22a, 22b, while allowing for the independent and simultaneous measurement of the pad contact impedance for each pair of RF return pads by having each a.c. signal source 42a, 42b provide a different signal source frequency for its corresponding pad contact pair. The frequency of each series resonant network 44a, 44b is tuned to match the frequency of the current produced by its associated a.c. signal source 42a, 42b. 
Accordingly, the frequency of one of the series resonant networks 44a is different from the frequency of the other series resonant network 44b. Hence, there is minimal interaction, if any, between the left and right circuitry of the system 100, especially the two contact pad pairs 18a, 18b. This essentially eliminates inaccurate or confusing measurements.
Additionally, the frequency of the electrosurgical current produced by the electrosurgical generator 10 is substantially different from that of the current produced by the a.c. signal sources 42a, 42b. 
The current that flows in each series resonant network 44a, 44b, i.e., left and right current senses, is a direct reflection or function of the pad impedance of the corresponding pad contact pair 18a, 18b according to the physics of a series resonant network. Each series resonant network 44a, 44b is an RCL network or a combination of R (resistance), L (inductance) and C (capacitance). In a preferred embodiment of the series resonant networks 44a, 44b, the inductive component for each network is integrated into the respective transformer 38a, 38b. 
The frequency response of a series resonant network has a maximum resonant frequency fR. At the resonant frequency, the series resonant network has the minimum impedance, as opposed to a parallel resonant network which has the maximum impedance at the resonant frequency, and the phase angle is equal to zero degrees. The total impedance of a series resonant network is ZT+jXL−jXC=R+j(XL−XC). At resonance: XL=XC, fR=1/(2πsqrtLC), ZT=R, and VL=VC. The resonance of a series resonant network occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase.
The left and right current senses are applied to pad contact impedance measurement subsystem 46 which determines whether the impedance measurements between pads or return electrodes 20a, 20b and 22a, 22b are within a desired range. The range is preferably adaptable to the physiological characteristics of the patient. If at least one of the impedance measurements is not within a desired range, an inhibit signal is applied over a line 48 to internally disable the electrosurgical generator 10 (or reduce the RF output therefrom) to prevent excess heating.
U.S. Pat. Nos. 4,416,276 and 4,416,277 describe a method for determining the desired range according to the physiological characteristics of the patient, the entire contents of these patents is incorporated herein by reference.
Preferably, the desired range for which the impedance must fall between return electrodes 20a, 20b and 22a, 22b is about 20 to about 144 ohms. If not, the electrosurgical generator 10 is disabled. Thus, in one method of operation of the present invention, the lower limit is fixed at the nominal value of 20 ohms, thus reducing the onset of patient injury as a result of stray current paths which may surface if a contact pad or electrode is applied to a surface other than the patient. The upper limit is set to avoid such problems as those mentioned hereinbefore, i.e., tenting, incorrect application site, gel drying, etc.
In accordance with an important aspect of the invention, the upper limit is adjustable from the absolute maximum (typically about 144 ohms) downward to as low as typically 20 ohms to thereby provide for automatic adaptiveness to the physiological characteristics of the patient. This provides the multiple RF return pad contact detection system 100 of the present invention with significantly more control over the integrity of the RF pad contact or electrode connections without limiting the range of patient types with which the multiple RF return pad contact detection system 100 may be used or burdening the operator with additional concerns.
That is, the physiological characteristics can vary significantly from patient to patient and from one location site for the pad pairs to another. Thus, patients may vary in their respective amounts of adipose tissue (which is one determining factor in the impedance measurement between the various pads) without effecting the detection system. Further, for a particular patient, one location site may be more fatty, hairy or scarred than another. Again, this does not reduce the effectiveness of the system, i.e., all of these factors typically affect the impedance measured between pads 20a, 20b and 22a, 22b and thus concern the operator as to which site is optimal for a particular patient. Such concerns are eliminated in accordance with the present invention by providing for automatic adaptability to the physiological characteristics of the patient.
Reference should now be made to FIG. 2 which is a graph illustrating the operation of pad contact impedance measurement subsystem 46.
During operation, the desired impedance range (that is, the acceptable range of the impedance detected between pads 20a, 20b and 22a, 22b) is preset when the power is turned on to an upper limit of, for example, 120 ohms and a lower limit of, for example, 20 ohms as can be seen at time T=0 seconds in FIG. 2. If the monitored impedance for any pad contact pair is determined to be outside of this range (T=A seconds) by comparing the current sense signal (or a signal derived there from) with a reference signal (e.g., a signal equal to 120 ohms or 20 ohms) using comparator circuitry (e.g., when a pad pair or any single contact pad is not affixed to the patient) an alert will be asserted and the electrosurgical generator 10 will be disabled over line 48.
The impedance between two contact pads of a contact pad pair at any instant is designated the return RF electrode monitor (REM) Instantaneous Value (RIV) in FIG. 2. When the REM impedance enters the range (T=B seconds) bounded by the Upper Limit (UL) and the Lower Limit (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 REM Nominal Value (RNV). The upper limit is then reestablished as 120% of this amount. The 80 ohm RIV shown in FIG. 2 causes the upper limit to be at 96 ohms. This feature of the invention is particularly important because it is at this time (T=C seconds) that adaptation is initially made to the physiological characteristics of the patient. Note if the RIV were to exceed 96 ohms at a time between T=C and T=F seconds (while the upper limit is 96 ohms), the alert will be asserted and the electrosurgical generator 10 disabled.
However, if the upper limit had not been adjusted to 96 ohms, the alert would not have been asserted until after the RIV exceeded the initial 120 ohms upper limit as determined by the comparator circuitry, thus possibly heating one or both of the pads 20a, 20b and 22a, 22b. This situation is of course exacerbated if the patient's initial RIV within the preset 20 to 120 ohm range is 30 ohms.
An initial RIV of 10 ohms within the preset range of 20 to 120 ohms sets an upper limit of 144 ohms.
In accordance with another aspect of the invention, it has been observed that the impedance between contact pads of contact pad pairs decreases over a relatively long period, such as a number of hours. Since many surgical procedures can extend a number of hours, this effect is also taken into consideration in the present invention. Accordingly, RIV is continuously monitored and any minima in REM impedance (e.g., 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 temporary negative change in REM impedance (T=D seconds) to be disregarded. Operation will continue in this manner provided RNV does not exceed the upper limit of 120% RNV or drop below the lower limit of 20 ohms. Exceeding the upper limit (T=G seconds) causes an alert and the electrosurgical generator 10 is disabled. It will remain in alert until the RIV drops to 115% of RNV or less (T=H seconds) or until the system 100 is reinitialized. 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 the system 100 is 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.
It should be noted in the example of FIG. 2 that the alert actually does not turn off when RIV returns to a value greater than 24 ohms because the pad pairs are removed before 5 seconds after T=J seconds elapse. Thus, the alarm stays on due to the removal of the pad contact pairs 18a, 18b. 
Removing the pad contact pairs 18a, 18b from the patient or unplugging the cables 26, 28 from the electrosurgical generator 10 (T=K seconds) for more than one second causes the system 100 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 electrosurgical generator 10 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=K seconds) for the first time) 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 FIG. 2 operation of the pad contact impedance measurement subsystem 46 is effected by a set of programmable instructions configured for execution by a microprocessor.
The system 100 could be modified by providing a multiplexer to multiplex the measurements corresponding to each pad contact pair 18a, 18b to eliminate or minimize measurement interaction and also minimize hardware resources.
Other pad contact pair arrangements can be provided in the system 100 of the present invention besides the pad pair arrangements shown in FIG. 1. For example, ten pad contact pairs 18 can be provided and connected to electrosurgical generator 10 by cables 26 and 28, where the corresponding a.c. signal source 42 and series resonant network 44 corresponding to each pad contact pair 18 are tuned to the same frequency which is different from the frequency of the other a.c. signal sources 42 and series resonant networks 44.
It is provided that the system 100 of the present invention allows for impedance comparisons to be performed between pad pairs. Therefore, if the pad pairs are placed symmetrically on the patient, i.e., left leg and right leg, comparison of the contact impedance can provide another degree of detection and safety.
Although the subject apparatus has been described with respect to preferred embodiments, it will be readily apparent to those having ordinary skill in the art to which it appertains that changes and modifications may be made thereto without departing from the spirit or scope of the subject apparatus.

## Claims

1. A method for adaptive impedance monitoring of at least two patient return pads configured for contacting a patient and transmitting electrosurgical energy back to an electrosurgical generator, the method comprising the steps of:
generating operating currents for the at least two patient return pads;
tuning first and second resonant circuits in operative communication with the electrosurgical generator to respective first and second frequencies, the first and second resonant circuits responsive to the operating currents for producing a signal as a function of an instantaneous impedance between the at least two patient return pads;
selecting a desired impedance range having a lower limit and an upper limit;
recording the instantaneous impedance value;
determining whether the instantaneous impedance is within the impedance range;
updating the upper limit as a function of the instantaneous impedance value according to the determination of the determining step; and
monitoring the impedance of the at least one patient return pad to determine if the impedance is between the lower limit and the updated upper limit.

2. A method according to claim 1, further comprising the step of:
measuring initial impedance of the at least two patient return pads to determine whether the initial impedance is within the desired impedance for a predetermined interval of time.

3. A method according to claim 2, further comprising the step of: generating a control signal for controlling the operation of an electrosurgical generator according the determination made by the monitoring impedance step.

4. A method according to claim 3, wherein the control signal of the generating step signals the electrosurgical generator to perform an operation selected from the group consisting of issuing an alert and adjusting supply of electrosurgical energy.

5. A method according to claim 1, wherein the function of the updating step for updating the upper limit is multiplication of the instantaneous impedance by a factor larger than 1.0.

6. A method according to claim 1, further comprising the step of:
detecting a termination of a downward impedance trend to determine whether the instantaneous impedance value is a minimum impedance value.

7. A method according to claim 6, further comprising the step of:
updating the upper limit as a function of the instantaneous impedance value according to the determination of the detecting step.

8. An electrosurgical system comprising:
an electrosurgical generator configured to generate electrosurgical energy, the electrosurgical generator coupled to at least two patient return pads configured for contacting a patient and transmitting electrosurgical energy back to the electrosurgical generator, the electrosurgical generator including at least two signal sources for generating an operating current for the at least two patient return pads, the at least two signal sources in operative communication with at least two resonant circuits tuned to different frequencies, the at least two resonant circuits responsive to the operating current for producing a signal as a function of the impedance between the at least two patient return pads;
an impedance measurement subsystem coupled to the at least two patient return pads and adapted to record the impedance between the at least two patient return pads to obtain an instantaneous impedance value; and
a microprocessor configured to:
select a desired impedance range having a lower limit and an upper limit;
update the upper limit as a function of the instantaneous impedance value according to the determination whether the instantaneous impedance is within the impedance range; and
monitor the impedance of the at least one patient return pad to determine if the impedance is between the lower limit and the updated upper limit.

9. An electrosurgical system according to claim 8, wherein the impedance measurement subsystem measures initial impedance of the at least two patient return pads and the microprocessor is further configured to determine whether the initial impedance is within the desired impedance for a predetermined interval of time.

10. An electrosurgical system according to claim 9, wherein the microprocessor is configured to generate a control signal which controls the operation of the electrosurgical generator according the determination made by the monitoring impedance step.

11. An electrosurgical system according to claim 10, wherein the control signal of the microprocessor signals the electro surgical generator to perform an operation selected from the group consisting of issuing an alert and adjusting the supply of electrosurgical energy.

12. An electrosurgical system according to claim 8, wherein the upper limit is updated by multiplying the instantaneous impedance by a factor larger than 1.0.

13. An electrosurgical system according to claim 8, wherein the microprocessor is configured to detect a termination of a downward impedance trend to determine whether the instantaneous impedance value is a minimum impedance value.

14. An electrosurgical system according to claim 13, wherein the microprocessor is configured to update the upper limit as a function of the instantaneous impedance value if the instantaneous impedance value is the minimum impedance value.