Multiple RF return pad contact detection system

IS 2 275 136 T3

DESCRIPTION

Multiple RF flyback plate contact detection system.

Background

1. Technical field

The present disclosure is directed to electrosurgery, and in particular to circuits for measuring or sensing contact resistance or impedance between the patient and pairs of contacts or RF return plate electrodes used in said surgery.

2. Description of Related Art

One potential hazard in electrosurgery is the possibility of stray electrical currents causing excessive heating near the RF return plate contacts or patient return electrodes. The most common conditions thought to lead to excessive heating include:

(1) inflection: separation of the return electrode from the patient due to patient movement or inappropriate application. This situation can lead to excessive heating if the electrode-patient contact area is significantly reduced;

(2) wrong application site: applying a return electrode to a highly resistive body site (for example, excessive adipose tissue, scarring, erythema or lesions, excessive hair or hair) will lead to a greater and faster increase in temperature. However, if the electrode is not applied to the patient (that is, the electrode hangs freely or is attached to another surface), the current can seek an alternative return path such as the table or the monitoring electrodes; and (3) gel drying due to premature opening of the electrode bag or use of an electrode that has exceeded the recommended shelf life.

In times past, many surveillance or detection systems have been developed, but most of them cannot be directly protected from the three situations listed above. In order to protect against these potentially dangerous 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 circuits are known with which split (or double) patient electrodes are used and a direct current is passed (see German Patent No. 1,139,927, published November 22, 1962) or an alternating current (see US Patent Nos. 3,933,157 and 4,200,104) between the split electrodes to sense contact resistance or impedance between the patient and the electrodes. US Patent No. 3,913,583 describes circuits to reduce the current through the patient depending on 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 on the perceived impedance of the contact area.

The above systems are susceptible to at least one or more of the following drawbacks:

(a) lack of sensitivity or ability to adapt to different physiological characteristics of patients and (b) susceptibility to electrosurgical current interference when surveillance continues during electrosurgical excitation.

U.S. Patent Nos. 4,416,276 and 4,416,277 describe a split patient return electrode monitoring system that can be tailored to different physiological characteristics of patients, and a return electrode monitoring system that has low susceptibility, if it is that it has it, to the interference of the electrosurgical current when the control is continued during the electrosurgical excitation.

There is still a need for a detection or control system, which is: 1) adaptive to the different physiological characteristics of patients; 2) have little, if any, susceptibility to electrosurgical current interference (including interference or measurement interaction between detection system components); 3) can measure or sense the contact resistance or impedance between the patient and pairs of RF return plates or electrodes when using multiple pairs of RF return plates due to the high current frequently needed during electrosurgery, such as during tissue ablation; and 4) eliminate or minimize the risk of measurement interaction between pairs of RF return plates.

Accordingly, one aspect of the invention is to provide a multiple RF return plate contact detection system for use during electrosurgical excitation that achieves the above objectives.

IS 2 275 136 T3

Summary

A multiple RF return plate contact detection system is disclosed that can be tailored to different physiological characteristics of patients, without being susceptible to electrosurgical current interference. The detection system includes interference or measurement interaction between the components of the detection system that can measure or sense the contact resistance or impedance between the patient and pairs of plates or RF return electrodes when using multiple pairs of return plates RF. Due to the high current that is frequently needed during electrosurgery, such as during tissue ablation, the detection system advantageously eliminates or minimizes the risk of measurement interaction between the pairs of RF return plates.

The circuits of the multiple RF return plate contact detection system are preferably arranged within an electrosurgical generator to control the generator according to various measurements, such as contact resistance or impedance between the patient and pairs of return plates. or RF return electrodes. Advantageously, the system allows independent and simultaneous measurement of the plate contact impedance for each pair of RF return plates. If the impedance of any pair of plates is above a predetermined limit, the system advantageously turns off or reduces the electrosurgical output of the electrosurgical generator to avoid excessive heating.

The system eliminates or minimizes interference or measurement interaction between plate pairs by providing a different signal source frequency for each pair of plate contacts, but a frequency that matches an associated series resonant circuit frequency. The current flowing in the series resonant circuit is a direct reflection or function of the plate impedance of the corresponding pair of plates. Since the two resonant circuits are tuned to different frequencies, there is minimal, if any, interaction within the system, which advantageously reduces the chances of inaccurate measurements.

The system may advantageously include or be modified to include a multiplexer to multiplex the measurements for each pair of board contacts in order to eliminate or minimize measurement interaction and also minimize hardware resources.

More specifically, the present description relates to a return plate contact detection system for use with at least two pairs of patient return plates adapted to make contact with a patient, each pair of said at least two pairs of patient return plates two conductors attached to a corresponding patient return plate to connect the plate to a source of direct current power passing through the plate, said return plate contact detection system comprising: at least two signal sources for generating a drive current for a corresponding pair of said at least two pairs of patient return plates; means for applying drive current through said conductors to said at least two pairs of patient return plates; and at least two resonant circuits that each correspond to at least one pair of patient return plates and to one of said at least two signal sources, each of said at least two resonant circuits responding to said current of actuation to produce a signal that is a function of the impedance between said two corresponding patient return plates, said at least two resonant circuits being tuned to different frequencies and being tuned to substantially the same frequency as the corresponding signal source to substantially minimize the measurement interaction between the at least two pairs of patient return plates when said currents of actuation are applied simultaneously to said corresponding pairs of patient return plates. As can be appreciated from the present disclosure, in one embodiment the frequency of the electrosurgical current may be substantially different from the frequency of the drive current.

Preferably, the at least two resonant circuits are RCL series resonant circuits having a minimum impedance at the resonant frequency. Advantageously, the system may include means for establishing a desired range that has at least one upper limit for impedance. The system may also include determining means that responds to said signal to determine if the impedance falls within the desired range. Preferably, the means for establishing a desired range includes means for generating a reference signal corresponding to the upper limit and wherein the determining means includes comparing means for comparing the signal that is a function of impedance with the reference signal.

Advantageously, the system may further include means for generating a control signal to control the operation of the return plate contact detection system in accordance with the determination made by the comparator means.

The desired range may advantageously include a lower limit for impedance, wherein the means for establishing a desired range includes means for generating a reference signal corresponding to the lower limit and the determining means includes comparing means for comparing the signal that is a function of the impedance with the reference signal. Preferably, the lower limit for impedance is about 20 ohms and the upper limit for impedance is about 144 ohms.

Advantageously, the means for applying the driving current include at least two transformers each of them to couple the corresponding pair of patient return plates to the corresponding signal source.

ES 2 275 136 T3 tooth. The secondary winding of each transformer is connected to the corresponding pair of patient return plates and the primary winding thereof is in circuit with the corresponding signal source and resonant circuit.

Brief description of the drawings

Various embodiments of the invention will now be described with reference to the drawings, in which:

Figure 1 is a schematic diagram of the multiple RF return plate contact detection system in accordance with a preferred embodiment of the invention; Y

Figure 2 is a graph illustrating the operation of the plate contact impedance measurement subsystem of Figure 1.

Detailed description of preferred embodiments

Reference should be made to the drawings where like reference numerals refer to like elements. Referring to Figure 1, there is shown a schematic diagram of the multiple RF return plate contact detection system 100 of the present invention, in which an electrosurgical generator 10 includes known circuitry such as a radio frequency oscillator 12 and an output amplifier 14 that generates an electrosurgical current. This current is applied to a patient (not shown) through an active electrode 16. The electrosurgical current is returned to the generator 10 through pairs of plate contacts or pairs of return electrodes 18a, 18b having plates or electrodes. 20a, 20b and 22a, 22b and a corresponding two conductor patient cable 24a, 24b having conductors 26 and 28. Two capacitors 32 and 34 are connected across each of the secondary windings 40a, 40b of the transformer 38a, 38b.

Each primary winding 36a, 36b is connected to a corresponding direct current signal source 42a, 42b and to a series resonant circuit 44a, 44b. The purpose of each series resonant circuit 44a, 44b is to produce a current (ie, left and right current directions) that is a function of the impedance between the plates or electrodes 20a, 20b and 22a, 22b.

System 100 eliminates or minimizes interference or measurement interaction between plates 20a, 20b and 22a, 22b, while allowing independent and simultaneous measurement of plate contact impedance of each pair of return plates RF signal by having each AC signal source 42a, 42b provide a different signal source frequency for its corresponding pair of plate contacts. The frequency of each series resonant circuit 44a, 44b is tuned to match the frequency of the current produced by its associated AC signal source 42a, 42b.

Consequently, the frequency of one of the series resonant circuits 44a is different from the frequency of the other series resonant circuit 44b. Accordingly, there is minimal interaction, if any, between the left and right circuits of system 100, especially the two pairs of contact plates 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 the frequency of the current produced by the alternating current signal sources 42a, 42b.

The current flowing in each series resonant circuit 44a, 44b, i.e. left and right current directions, is a direct reflection or function of the plate impedance of the corresponding pair of plate contacts 18a, 18b according to physics of a series resonant circuit. Each series resonant circuit 44a, 44b is an RCL circuit or a combination of R (resistance), L (inductance), and C (capacitance). In a preferred embodiment of the series resonant circuits 44a, 44b, the inductive component of each circuit is integrated into the respective transformer 38a, 38b.

The frequency response of a series resonant circuit has a maximum resonant frequency f_R. At resonant frequency, the series resonant circuit has the minimum impedance, as opposed to a parallel resonant circuit 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 circuit is Z_T+ jX_L-jX_C= R + j (X_L-X_C). A resonance: X_L= X_C, f_R= 1 / (2nsqrtLC), Z_T= r, and V_L= V_C. Resonance in a series resonant circuit occurs when the inductive and capacitive reactances are of equal magnitude but cancel each other out because they are 180 degrees out of phase.

The left and right current directions are applied to the plate contact impedance measurement subsystem 46 which determines whether the impedance measurements between plates or return electrodes 20a, 20b and 22a, 22b fall within a desired range. The margin is preferably adaptable to the physiological characteristics of the patient. If at least one of the impedance measurements did not fall within a desired range, an inhibit signal would be applied on a line 48 to internally disable the electrosurgical generator 10 (or reduce the RF output from it) to prevent excessive heating. .

IS 2 275 136 T3

US Patents 4,416,276 and 4,416,277 describe a method for determining the desired range according to the physiological characteristics of the patient, the full contents of these patents being incorporated herein by reference.

Preferably, the desired range within which the impedance between return electrodes 20a, 22b and 22a, 22b has to fall is from 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 set at the nominal value of 20 ohms, thus reducing the initiation of injury to the patient as a result of stray current paths that may appear if a plate or plate is applied. contact electrode to a different surface of the patient. The upper limit is set to avoid problems such as those mentioned above, that is, inflection, wrong application site, gel drying, etc.

In accordance with an important aspect of the invention, the upper limit is adjustable from absolute maximum (typically about 144 ohms) down to as low as typically 20 ohms to thereby provide automatic adaptability to physiological characteristics. of the patient. This endows the multiple RF return plate contact detection system 100 of the present invention to a significant degree with more control over the integrity of the RF plate contact or electrode connections without limiting the range of patient types with which RF 100's multi-touch plate contact detection system can be used or burden the operator with additional concerns.

That is, the physiological characteristics can vary significantly from one patient to another and from one location for the pairs of plates to another. Thus, patients can vary their respective amounts of adipose tissue (which is a determining factor in measuring impedance between the various plates) without affecting the detection system. Also, for a particular patient, one site may be more greasy, hairy, or scarred than another. Again, this does not reduce the efficiency of the system, that is, all of these factors typically affect the impedance measured between plates 20a, 20b and 22a, 22b and thus concern the operator as to which location is optimal for a particular patient. Such concerns are eliminated in accordance with the present invention by providing automatic adaptability to the physiological characteristics of the patient.

You should now refer to Figure 2 which is a graph illustrating the operation of the plate contact impedance measurement subsystem 46.

During operation, the desired impedance range (that is, the acceptable range of impedance detected between plates 20a, 20b and 22a, 22b), is pre-set when the current is turned on to an upper limit of, for example, 120 ohms and to a lower limit of, for example, 20 ohms as can be seen at time T = 0 seconds in figure 2. If it is determined that the monitored impedance for any pair of plate contacts lies outside this range (T = A seconds) by comparing the current direction signal (or a signal derived from it) with a reference signal (for example, a signal equal to 120 ohms or 20 ohms) using comparator circuits (for example, when a pair of plates or any single contact plate is not attached to the patient) an alert will be maintained and the electrosurgical generator 10 on line 48 will be disabled.

The impedance between two contact plates of a pair of contact plates at any time is designated as the Return RF Electrode Monitor Instantaneous Value (RIV) in Figure 2. When a REM impedance enters the range (T = B seconds) limited 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 REM Nominal Value (RNV). The upper limit is then reset as 120% of this amount. The 80 ohm RIV shown in Figure 2 puts the upper limit at 96 ohms. This characteristic of the invention is particularly important because it is at this time (T = C seconds) that the adaptation to the physiological characteristics of the patient is initially made. Note that if the RIV were to exceed 96 ohms in a time between T = C and TF seconds (even if the upper limit was 96 ohms) the alert will remain and the electrosurgical generator 10 will be disabled.

However, if the upper limit has not been set to 96 ohms, the alert will not be maintained until after the RIV exceeds the initial upper limit of 120 ohms determined by the comparator circuits, thus possibly heating up one or both boards 20a, 20b and 22a, 22b. This situation will naturally be aggravated if the patient's initial IVR within the previously set range of 20 to 120 ohms is 30 ohms.

A starting RIV of 10 ohms within the previously set 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 the contact plates of pairs of contact plates decreases over a relatively long period, such as several hours. Since many surgical procedures can take several hours, this effect is also taken into consideration in the present invention. Consequently, the RIV is continuously monitored and any minima in REM impedance (for example, a decreasing trend followed by a constant or increasing trend in REM impedance) start a new time interval of five seconds (T = E seconds) at end of which the RNV is updated to the RIV if the RIV is lower (T = F seconds). The upper limit of the REM of 120% of the RNV is reset in this

ES 2 275 136 T3 time. The 5 second interval causes any temporary negative change in REM impedance to be ruled out (T = D seconds). Operation will continue in this manner as long as the RIV does not exceed the upper limit of 120% of the RNV or falls below the lower limit of 20 ohms. Exceeding the upper limit (T = G seconds) produces an alert and the electrosurgical generator 10 is disabled. It will remain on alert until the RIV drops to 115% of RNV or less (T = H seconds) or until system 100 is reset. Dropping the RIV to less than 20 or less (T = I seconds) produces a similar alert which continues until the RIV exceeds 24 ohms (T = J seconds) or the system is reset 100. Hysteresis at REM range limits (ie, change from upper limit to 115% RNV and lower limit to 24 ohms in the examples above) prevents an erratic alert when RIV is marginal.

It should be noted in the example in figure 2 that the alert is not actually deactivated when the RIV returns to a value greater than 24 ohms because the pairs of plates separate within 5 seconds after T = J seconds . Thus, the alarm continues until the separation of the pairs of plate contacts 18a, 18b.

Separation of patient plate contact pairs 18a, 18b or unplugging of leads 26, 28 from electrosurgical generator 10 (T = K seconds) for more than one second causes system 100 to reset to original limits 120 and 20 ohms. This allows a plate to be reattached or replaced (T = L seconds) without disconnecting the electrosurgical generator 10. The IVR at the new location is 110 ohms and 120% of the RNV is 132 ohms. Thus, as described above, this is the time (as long as the RIV falls within the range of 20 to 120 ohms (as previously set during power-up or as when resetting as at T = K seconds) for the first time) in which the upper limit can be raised during the normal cycle of REM. Otherwise, it is continuously lowered to accommodate the decreasing impedance of the RIV over time.

The preferred implementation of the above operation of FIG. 2 of the plate contact impedance measurement subsystem 46 is accomplished 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 pair of board contacts 18a, 18b in order to eliminate or minimize the measurement interaction and also minimize hardware resources.

Other plate contact pair arrangements may be envisioned in the system 100 of the present invention in addition to the plate pair arrangements shown in FIG. 1. For example, ten pairs of plate contacts 18 may be provided which can be connected to electrosurgical generator 10 via leads 26 and 28, wherein the corresponding AC signal source 42 and the series resonant circuit 44 corresponding to each pair of contacts Boards 18 are tuned to the same frequency that is different from the frequency of the other AC signal sources 42 and series resonant circuits 44.

The system 100 of the present invention is intended to allow impedance comparisons to be made between pairs of plates. Consequently, if pairs of plates are placed symmetrically on the patient, ie, on the left leg and the right leg, the contact impedance comparison can provide another degree of detection and safety.

Although the present apparatus has been described with respect to preferred embodiments, it will be readily apparent to those of ordinary skill in the art to which it belongs that changes and modifications can be made therein without departing from the spirit of the present apparatus which is defined by the claims. attached.