Patent Publication Number: US-2009234352-A1

Title: Variable Capacitive Electrode Pad

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application Ser. No. 61/037,243 entitled “VARIABLE CAPACITIVE ELECTRODE PAD” filed Mar. 27, 2008 by Robert Behnke et al, which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to electrosurgical apparatuses, systems and methods. More particularly, the present disclosure is directed to electrosurgical systems utilizing one or more capacitive return electrode pads configured to monitor contact quality thereof. 
     2. Background of Related Art 
     Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, heat, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, the active electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator and safely disperse current applied by the active electrode. 
     The return electrodes usually have a large patient contact surface area to minimize heating at that site. Heating is caused by high current densities which directly depend on the surface area. A larger surface contact area results in lower localized heat intensity. Return electrodes are typically sized based on assumptions of the maximum current utilized during a particular surgical procedure and the duty cycle (i.e., the percentage of time the generator is on). 
     The first types of return electrodes were in the form of large metal plates covered with conductive jelly. Later, adhesive electrodes were developed with a single metal foil covered with conductive jelly or conductive adhesive. However, one issue with these adhesive electrodes was that if a portion peeled from the patient, the contact area of the electrode with the patient decreased, thereby increasing the current density at the adhered portion and, in turn, increasing the heating at the tissue. This risked burning the patient in the area under the adhered portion of the return electrode if the tissue was heated beyond the point where circulation of blood could cool the skin. 
     To address this problem various return electrodes and hardware circuits, generically called Return Electrode Monitors (REMs) and Return Electrode Contact Quality Monitors (RECQMs), were developed. Such systems relied on measuring impedance at the return electrode to calculate a variety of tissue and/or electrode properties. These systems were only configured to measure changes in impedance of the return electrodes to detect peeling. Further, the systems were only designed to work with conventional resistive return electrode pads. Still, other systems are configured to detect a fault (e.g., defect) in the return electrode pad (e.g., in the dielectric material) based on a detected phase difference between the current and the voltage of the electrosurgical energy. One such system is disclosed in U.S. patent application [Ser. No. to be determined] entitled “SYSTEM AND METHOD FOR DETECTING A FAULT IN A CAPACITIVE RETURN ELECTRODE FOR USE IN ELECTROSURGERY,” which is being filed with the United States Patent and Trademark Office concurrently herewith, 
     SUMMARY 
     The present disclosure relates to an electrosurgical variable capacitive return electrode pad. The variable capacitive pad includes one or more pairs of split return electrodes arranged in a capacitive configuration. This allows a generator having a return electrode monitoring system to measure capacitance between the split return electrodes and map the initial capacitance with full adherence of the variable capacitive pad to the patient. The return electrode monitoring system then monitors the capacitance between the split return electrodes and correlates the change therein with the contact quality of the variable capacitive pad based on the initial mapping. 
     According to one aspect of the present disclosure an electrosurgical system is disclosed. The system includes one or more variable capacitive pads including a pair of split electrodes arranged in a capacitive configuration, wherein the pair of split electrodes is configured to return electrosurgical energy to a generator. The system also includes a return electrode monitoring system coupled to the at least one pair of split electrodes and is configured to map an initial capacitance between the at least one pair of split electrodes with substantially full adherence of the at least one variable capacitive pad to the patient and determines an adherence factor of the variable capacitance pad as a function of a change in capacitance between the at least one pair of split electrodes with respect to the map of the initial capacitance indicative of substantially full adherence. 
     According to another aspect of the present disclosure a return electrode monitoring system is disclosed. The system includes a variable capacitive pad having a pair of split electrodes arranged in a capacitive configuration, wherein the pair of split electrodes is configured to return electrosurgical energy to a generator. The system also includes a detection circuit coupled to the pair of split electrodes and configured to measure capacitance therebetween. The detection circuit also maps initial capacitance between the at least one pair of split electrodes with substantially full adherence of the variable capacitive pad to the patient and determines contact quality of the variable capacitive pad based on the change in capacitance between the pair of split electrodes. 
     A method for monitoring a variable capacitive pad is also contemplated by the present disclosure. The method includes the step of providing a monitor signal waveform to a variable capacitive pad having a pair of split electrodes arranged in a capacitive configuration, wherein the pair of split electrodes is configured to return electrosurgical energy to a generator. The method also includes the steps of measuring a property of the monitor signal waveform and determining capacitance between the pair of split electrodes based on the property of the monitor signal waveform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present disclosure are described herein with reference to the drawings wherein: 
         FIG. 1  is a schematic block diagram of an electrosurgical system according to the present disclosure; 
         FIG. 2  is a schematic block diagram of a generator according to one embodiment of the present disclosure; 
         FIGS. 3A-3D  are top cross-sectional views of multi-sectioned capacitive return electrode pads in accordance with the present disclosure; and 
         FIG. 4  is a flow chart diagram of a method according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
     A capacitive return electrode pad can safely return more current than a return electrode pad incorporating a resistive design. However, conventional capacitive return electrode pads are not configured to couple with a return electrode monitoring (“REM”) system. The REM system monitors the adherence of the return electrode pad to the patient by measuring the impedance and/or current between one or more split pads. Split pad designs have been incorporated into resistive return electrode pads but previously were not included in capacitive return electrode designs due to the increased impedance of these electrode pads. 
     Other Methods Incorporate 
     The present disclosure provides for a variable capacitive return electrode pad incorporating capacitive and return electrode monitoring technologies. More specifically, the capacitive return pad according to embodiments of the present disclosure includes a plurality of split electrodes that create a capacitance therebetween. When the capacitive return pad is in contact with the patient, capacitance between the split electrodes increases. This capacitance, and ally changes therein, is used to monitor the contact area between the patient and the capacitive return pad. 
       FIG. 1  is a schematic illustration of an electrosurgical system according to one embodiment of the present disclosure. The system includes an electrosurgical instrument  2  having one or more electrodes for treating tissue of a patient P. The instrument  2  is a monopolar instrument including one or more active electrodes (e.g., electrosurgical cutting probe, ablation electrode(s), etc.). Electrosurgical RF energy is supplied to the instrument  2  by a generator  20  via an electrosurgical cable  4 , which is connected to an active output terminal, allowing the instrument  2  to coagulate, ablate and/or otherwise treat tissue. The energy is returned to the generator  20  through a return electrode pad  6  via a return cable  8 . The system may include a plurality of return electrodes pads  6  that are arranged to minimize the chances of tissue damage by maximizing the overall contact area with the patient P. In addition, the generator  20  and the return electrode  6  may be configured for monitoring so-called “tissue-to-patient” contact to insure that sufficient contact exists therebetween to further minimize chances of tissue damage. 
     The generator  20  includes input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator  20 . In addition, the generator  20  may include one or more display screens for providing the user with variety of output information (e.g., intensity settings, treatment complete indicators, etc.). The controls allow the user to adjust power of the RF energy, waveform, and other parameters to achieve the desired waveform suitable for a particular task (e.g., coagulating, tissue sealing intensity setting, etc.). The instrument  2  may also include a plurality of input controls that may be redundant with certain input controls of the generator  20 . Placing the input controls at the instrument  2  allows for easier and faster modification of RE energy parameters during the surgical procedure without requiring interaction with the generator  20 . 
       FIG. 2  shows a schematic block diagram of the generator  20  having a controller  24 , a high voltage DC power supply  27  (“HVPS”) and an RE output stage  28 . The HVPS  27  provides high voltage DC power to an RF output stage  28 , which then converts high voltage DC power into RF energy and delivers the RF energy to the active electrode. In particular, the RF output stage  28  generates sinusoidal waveforms of high RF energy. The RF output stage  28  is configured to generate a plurality of waveforms having various duty cycles, peak voltages, crest factors, and other suitable parameters. Certain types of waveforms are suitable for specific electrosurgical modes. For instance, the RF output stage  28  generates a 100% duty cycle sinusoidal waveform in cut mode, which is best suited for ablating, fusing and dissecting tissue, and a 1-25% duty cycle waveform in coagulation mode, which is best used for cauterizing tissue to stop bleeding. 
     The controller  24  includes a microprocessor  25  operably connected to a memory  26 , which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microprocessor  25  includes an output port that is operably connected to the HVPS  27  and/or RF output stage  28  that allows the microprocessor  25  to control the output of the generator  20  according to either open and/or closed control loop schemes. The microprocessor  25  may be substituted by any suitable logic processor (e.g., control circuit) adapted to perform the calculations discussed herein. 
     The generator  20  may include a sensor circuit (not explicitly shown) having suitable sensors for measuring a variety of tissue and energy properties (e.g., tissue impedance, tissue temperature, output current and/or voltage, etc.) and to provide feedback to the controller  24  based on the measured properties. Such sensors are within the purview of those skilled in the art. The controller  24  then signals the HVPS  27  and/or RF output stage  28 , which then adjust DC and/or RF power supply, respectively. The controller  24  also receives input signals from the input controls of the generator  20  or the instrument  2 . The controller  24  utilizes the input signals to adjust power outputted by the generator  20  and/or performs other control functions thereon. 
     Referring now to  FIG. 3A , the return electrode pad  6  is embodied as a variable capacitive pad (“VCP”)  50  for providing a return path for electrosurgical current and monitoring surface impedance and capacitance according to the present disclosure. While the VCP  50  is depicted as having a general rectangular shape, it is within the scope of the disclosure for the VCP  50  to have any suitable regular or irregular shape. 
     VCP  50  includes a carrier layer  53  having one or more layers, such as, a backing layer  52 , a heat distribution layer  54 , a passive cooling layer  55 , and an attachment layer  56 . An active cooling layer (not explicitly shown) or any other suitable insulating layer or combination above layers  53 ,  54 ,  55 , and  56  may also be included. 
     The attachment layer  56  is disposed on a patient-contacting surface of the VCP  50  and may be formed from an adhesive material (not explicitly shown) which may be, but is not limited to, a polyhesive adhesive, a Z-axis adhesive, a water-insoluble, hydrophilic, pressure-sensitive adhesive, or any combinations thereof, such as POLYHESIVE™ adhesive manufactured by Valleylab, a division of Tyco Healthcare of Boulder, Colo. The adhesive may be conductive or dielectric. The attachment layer  56  ensures an optimal surface contact area between the electrosurgical return electrode pad  6  and the patient “P,” which minimizes the risk of damage to tissue. In another embodiment, the VCP  50  may be reusable and have a sufficiently large surface area so that the VCP  50  may be used without the attachment layer  56 , allowing the VCP  50  to be cleaned and sanitized between uses. 
     The backing layer  52  supports a pair of split return electrodes  52   a  and  52   b  for positioning under a patient during electrosurgery. The backing layer  52  may be made of cloth, cardboard, non-woven or any suitable material. In one embodiment, the backing layer  52  may be formed from a dielectric material such as flexible polymer materials to enhance capacitive properties of the VCP  50 . The polymer materials may be polyimide film sold under a trademark KAPTON™ and polyester film, such as biaxially-oriented polyethylene terephthalate (boPET) polyester film sold under trademarks MYLAR™ and MELINEX™. In another embodiment the backing layer  52  may act as an insulating layer between the pair of split return electrodes  52   a  and  52   b  and the attachment layer  56 . 
     The split return electrodes  52   a  and  52   b  may be made from materials that include aluminum, copper, mylar, metalized mylar, silver, gold, stainless steel or other suitable conductive material and may be of various shapes and may be arranged in various configurations and orientations. The split configuration of the split return electrodes  52   a  and  52   b  create a measurable capacitance therebetween which may be measured by generator  20  to determine adherence of the VCP  50  to the patient “P.” 
     More specifically, capacitive coupling between the split return electrodes  52   a  and  52   b  increases upon the initial placement of the VCP  50  in contact with the patient “P.” This capacitance corresponds to full adherence of the VCP  50  to the patient. During the procedure, the VCP  50  may peel from the patient “P,” thereby decreasing the adherence factor thereof. The decrease in adherence directly affects the capacitance between the split return electrodes  52   a  and  52   b.  Measuring the change in capacitance between the split return electrodes  52   a  and  52   b,  therefore, provides an accurate measurement of adherence of the VCP  50  to the patient “P.” The amount of capacitance coupling or the change in capacitance coupling then may be used to insure positive patient contact or to determine adequate patient coverage of the VCP  50 . 
     With returned reference to  FIG. 2 , the generator  20  includes a return electrode monitoring (“REM”) system  70  having a detection circuit  22  which is coupled to the split return electrodes  52   a  and  52   b.  The VCP  50  is in contact with the patient “P” and returns the electrosurgical energy to the generator  20  via the split return electrodes  52   a  and  52   b  that are coupled to leads  41  and  42  respectively. In one embodiment, the VCP  50  may include a plurality of pairs of split electrode pads which are coupled to a corresponding number of leads. The leads  41  and  42  are enclosed in a return cable  8  and are terminated at a secondary winding  44  of a transformer  43 . The leads  41  and  42  are interconnected by capacitors  45  and  46 . A return lead  48  is coupled between the capacitors  45  and  46  and is adapted to return the electrosurgical energy to the RF output stage  28 . The transformer  43  of the REM system  70  also includes a primary winding  47  which is connected to the detection circuit  22 . 
     Components of the REM system  70 , e.g., the transformer  43 , the split return electrodes  52   a  and  52   b,  the capacitors  45  and  46 , along with the detection circuit  22  form a resonant system which is adapted to resonate at a specific interrogation frequency from the controller  24 . Namely, the controller  24  provides an interrogation signal at a specific interrogation frequency to the detection circuit  22 . The detection circuit  22  then rectifies the interrogation signal to generate a monitor signal. The monitor signal is a constant, physiologically benign waveform (e.g., 140 kHz, 2 mA) which the detection circuit  22  applies to the split return electrodes  52   a  and  52   b.  The monitor signal thereafter passes through the patient and is returned to the circuit  22  via the split return electrodes  52   a  and  52   b.    
     The returning monitor signal is modified by the capacitance of the split return electrodes  52   a  and  52   b.  More specifically, as the capacitance between the split return electrodes  52   a  and  52   b  changes due to peeling of the VCP  50  from the patient, the resonance of the detection circuit  22  with respect to other components changes as well. The change in the resonance, in turn, affects the change in amplitude of the monitor signal. Thus, the detection circuit  22  determines the magnitude of the capacitance between the split return electrodes  52   a  and  52   b  by monitoring changes in amplitude of the monitor signal waveform. The detection circuit  22  then supplies the capacitance measurement to the controller  24  which determines whether the capacitance is within a predetermined range. Initially, the controller  24  may determine an initial capacitance value corresponding to full adherence of the VCP  50 . The initial capacitance value may be used as a baseline measurement of capacitive coupling between the split return electrodes  52   a  and  52   b  to determine the contact area between the patient “P” and the VCP  50 . Additional measurements may be made after the VCP  50  is placed in contact with the patient “P” and prior to initiating the delivery of electrosurgical energy to patient tissue. Subsequent measurements may be made after commencement of electrosurgical energy delivery to determine any degradation in contact quality or change in a characteristic of patient contact. If the capacitance is out of range, thereby indicating excessive peeling of the return electrode pad  6 , the generator  20  issues an alarm (e.g., audibly, visually, etc. via the controller  24 ) and/or the controller  24  adjusts the output of the generator  20  (e.g., terminates RF supply). 
       FIGS. 3B-3D  illustrate additional embodiments of the VCP  150 ,  250 , and  300 , respectively, and the corresponding arrangement of the split return electrodes. More specifically,  FIGS. 3B and 3C  illustrate that the split return electrodes  152   a,    152   b  and  252   a,    252   b  may be of various shapes and sizes designed to maximize capacitive coupling between the VCP  150 ,  250  and the patient “P.” In  FIG. 3B , the VCP  150  includes L-shaped split return electrodes  152   a  and  152   b  arranged in a reverse interlocking configuration to maximize contact area with the patient “P.” In  FIG. 3C , the VCP  250  includes oval shaped split return electrodes  252   a  and  252   b.    
     In another embodiment illustrated in  FIG. 3D , the VCP  350  may include a plurality of split return electrodes arranged in an interweaving pattern. The VCP  350  includes three (3) split return electrodes for each pair, namely,  352   a   1 , 352   a   2 , and  352   a   3  and  352   b   1 ,  352   a   2 , and  352   b   3 . The split return electrodes  350   a   1 - 350   a   3  are coupled to the return lead  41  and the split return electrodes  350   b   1 - 350   b   3  are coupled to the return lead  42  with each pair of the split electrodes being arranged in a sequential manner. Having multiple pairs of return electrode pads within the VCP  350  allows for fault protection in case one or more pairs of the return electrode pads fail. 
       FIG. 4  illustrates a method for monitoring adherence of the VCP  50  to the patient “P.” In step  100 , the controller  24  supplies a interrogation signal to the REM system  70 . The controller  24  rectifies the interrogation signal and supplies a monitor signal waveform across the split return electrodes  52   a  and  52   b.  In step  102 , the detection circuit  22  measures the current and voltage of the monitor signal waveform, which are used by the controller  24  to determine the phase of the monitor signal waveform with respect to frequency. In step  104 , the controller  24  determines the reactance as a function of the voltage, current and phase values with respect to frequency. The phase of the monitor signal waveform may be determined by sweeping the interrogation signal across the resonance range of the REM system  70 . This allows for correlation of phase responses with respect to multiple frequency interrogation signals. 
     The reactance is used to determine the capacitance of the split return electrodes  52   a  and  52   b.  In step  106 , prior to the start of the electrosurgical procedure, the capacitance is mapped with respect to full adherence of the return electrode pad  6 . In step  108 , as the procedure commences, the capacitance is monitored and is used to determine the adherence of the VPC  50 . 
     Other methods for monitoring contact quality of the return pad to the patient include utilizing a sensor to communicate parameters such as capacitance, to the electrosurgical generator. One such method is disclosed in U.S. patent application Ser. No. 11/800,687 entitled “CAPACITIVE ELECTROSURGICAL RETURN PAD WITH CONTACT QUALITY MONITORING,” filed May 7,2007. 
     While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.