Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B18/00—Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body
  • A61B18/04—Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by heating
  • A61B18/12—Surgical instruments, devices or methods for transferring non-mechanical forms of energy to or from the body by heating by passing a current through the tissue to be heated, e.g. high-frequency current
  • A61B18/14—Probes or electrodes therefor
  • A61B18/16—Indifferent or passive electrodes for grounding
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/08—Accessories or related features not otherwise provided for
  • A61B2090/0813—Accessories designed for easy sterilising, i.e. re-usable
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S128/00—Surgery
  • Y10S128/908—Patient protection from electric shock

Definitions

• the Field of the Invention relates to electrosurgery and, more particularly, to return electrodes adapted for providing effective and safe electrosurgical energy return without conducting or dielectric gels or polymers, which may be reusable and/or disposable.
• the active electrode at the point of contact with the patient must be small in size to produce a high current density in order to produce a surgical effect of cutting or coagulating tissue.
• the return electrode which carries the same current as the active electrode, must be large enough in effective surface area at the point of communication with the patient such that a low density current flows from the patient to the return electrode. If a relatively high current density is produced at the return electrode, the temperature of the patient's skin and tissue will rise in this area and can result in an undesirable patient burn.
• AAMI Advanced Medical Instrumentation
• an Electrode Contact Quality Monitoring System that would monitor the contact area of the electrode that is in contact with the patient and turn off the electrosurgical generator whenever there was insufficient contact area.
• Such circuits are shown, for example, in United States patent 4,231,372, issued to Newton, and entitled “Safety Monitoring Circuit for Electrosurgical Unit,” the disclosure of which is incorporated by this reference.
• This system has resulted in an additional reduction in patient return electrode burns, but requires a special disposable electrode and an added circuit in the generator that drives the cost per procedure even higher. Fifteen years after this system was first introduced, fewer than 40 percent of all the surgical operations performed in the United States use this system because of its high costs.
• the present invention overcomes the problems of the prior art by providing a return electrode that eliminates patient burns without the need for expensive disposable electrodes and monitoring circuits in specialized RF generators.
• the improved return electrode according to the preferred embodiment of the invention hereof includes an effective surface area that is larger than other return electrodes that have been disclosed or used in surgery previously. It is so large and so adapted for positioning relative to the body of a patient that it eliminates the need for conductive or dielectric gels. Moreover, the exposed surface is of a material that is readily washable and/or sterilizable so as to facilitate easy and rapid conditioning for repeated reuse. It employs geometries and materials whose impedance characteristics, at typically used electrosurgical frequencies, are such that it self-limits the current densities (and corresponding temperature rises) to safe thresholds, should the effective area of the working surface of the electrode be reduced below otherwise desirable levels. Accordingly, the need for the foregoing expensive monitoring circuits in specialized RF generators is eliminated.
• Figure 2 A is a top view of a wide-area distributed electrosurgical return electrode illustrating the principles of the invention.
• Figure 2B is an enlargement of a segment of the electrosurgical return electrode of Figure 2 A;
• Figure 2C is a cross section taken along the section lines 2C-2C of Figure 2B and illustrating the effective circuit impedance represented by the segment of 2B;
• Figure 3 is a chart illustrating in graphical form the relationships between effective surface area of the return electrode and the effective radio frequency current density developed at the electrode;
• Figure 4 is a perspective view showing an operating table with the electrosurgical return electrode according to the invention disposed on the upper surface thereof;
• Figure 5 is a front view illustrating a surgical chair with an electrosurgical return electrode according to the invention disposed on the surface of the seat thereof;
• Figure 6 is a top view of an electrosurgical return electrode according to the invention.
• Figure 7 is a section taken along the lines 7-7 of Figure 6;
• Figure 8 is a section similar to that of Figure 7 but illustrating the capacitance presented by a patient's surgical gown;
• Figure 9 is a perspective view of a cover adapted for encasing any of the embodiments of Figures 6-8;
• Figure 10 is a view illustrating one of the embodiments of Figures 6-8 encased within the cover of Figure 9;
• Figure 11 is a perspective view illustrating, for the purpose of analysis, the circuit equivalent of a patient in operative association with the ohmic and capacitive regions of a pad according to the invention;
• Figure 12 is a simple electronic schematic circuit equivalent to Figure 11;
• Figure 13 is a graph depicting percent capacitive power conduction as a function of bulk resistivity of the resistive layer for different electrosurgical operating frequencies
• Figure 14 is a perspective view of a pad according to the invention illustrating a simulated condition when the effective contact area with a patient is substantially less than the physical pad size;
• Figure 15 is a view illustrating current flow density within the pad when the effective patient contact area is much smaller than the total pad area
• Figure 16 is a graph showing minimum bulk resistivity of the resistive layer as a function of pad thickness for different electrosurgical generator frequencies.
• Impedance z 2 is provided to represent the impedance presented by the patient's tissue lying between the operation site and the return electrode.
• the initial embodiment, hereof, is that of an electrode operating in a combined resistive and/or capacitive mode. Accordingly, if the relatively small stray capacitive and inductive reactances are disregarded, the total effective impedance of the circuit will be equal to the sum of the individual impedances z,, z, and z 3 ; and since essentially the same current will pass through all three, the voltage generated by RF generator 10 will be distributed across impedances z l3 T ⁇ and z 3 in direct proportion to their respective values. Thus, the energy released in each of such components will also be directly proportional to their values.
• the resistive component of the impedance represented by z be substantial and that current passing therethrough (and consequent energy release) be concentrated in a very small region. The latter is accomplished by making the region of contact with the patient at the operative site very small.
• the surface 20a of return electrode 20 is preferably smooth and homogeneous and includes a thin resistive and/or dielectric layer 21a (Fig. 2C).
• surface 20a of return electrode 20 may include a capacitive and/or inductive layer, depending on the particular operation of return electrode 20.
• electrode 20 may be thought of as including a plurality of uniformly-sized regions or segments as represented by regions 21, 21a, 21b, 21c 21 n. It will be appreciated by one skilled in the art, however, that return electrode may or may not include discontinuous regions or segment, it being preferred that electrode 20 have continuous segments.
• Region/segment 21 is shown larger in Figure 2B in order to be similar in scale to the resistive impedance z 3 ' it represents. It thus will now be evident that each of the segments of electrode 20 corresponding to segments 21 . . . 21n inherently has the capability of presenting an impedance similar to that of impedance z 3 '. However, the number of such segments which are effectively active in parallel within the circuit is a direct function of the surface area of the patient that overlies the electrode.
• the effective contact area between the patient and electrode were to be reduced to the surface of only one of the segments 21-2 In, then the effective impedance (combined capacitive reactance and resistance in the example under consideration) would increase to 100 ohms; and at some point of reduction in contact area, the effective impedance would rise to a level relative to the impedance presented at the site of the electrosurgical instrument so as to diminish the electrosurgical effect of the surgical instrument or otherwise prevent effective use of the instrument by the surgeon, thus signaling the surgeon that the patient should be repositioned so as to present a greater surface area in contact with the return electrode.
• the total circuit impedance would be increased so that the total current that would flow if the surgeon attempted to employ his instrument without repositioning the patient would be reduced to a value below that which would cause undesired trauma to the patient. Accordingly, there is provided a self-limiting feature that enhances safety in use without the need for the aforementioned separate circuit monitoring and control circuits.
• Figure 2C is a cross section taken along the section lines 2C-2C of Figure 2B and illustrating the effective circuit impedance z 3 ' represented by the segment 21 of 2B.
• each of the impedances represented by the remaining segments are connected at their lower extremities in parallel to terminal 22; whereas, if such highly conductive layer is absent, then, in addition to the impedance represented by the material lying between the upper and lower regions of each segment, there will be an additional impedance (not shown) that is represented by the material through which current would have to pass transversely or laterally through the electrode in order to get to terminal 22.
• Figure 3 is a chart generally illustrating in graphic form the relationships between the effective surface area of the return electrode and the effective radio frequency current densities developed at the electrode.
• the chart is simplified so as to illustrate the principles underlying die invention and does not represent actual data that may vary substantially.
• RF Current Density versus Electrode Effective Surface Area the latter (as should now be evident to those skilled in the art) being that part of the surface of the return electrode that makes effective electrical contact with the body of a patient.
• Various embodiments of the present invention may have substantially simultaneous changes in current density and available current, while other embodiments of the present invention may include a lag period therebetween.
• the parameters selected for the materials and electrode dimensions are chosen so that current density and corresponding tissue temperature elevation adjacent the return electrode do not exceed the limits mentioned in the introduction hereof. It will now be seen that by a proper selection of such parameters the return electrode is made self-limiting, thereby obviating the need for the additional monitoring circuits to which reference is made above.
• impedances whose principal components are resistances and capacitive reactances.
• the principles of the invention are also applicable to other embodiments in which the impedances include any combination of resistive, capacitive and/or inductive impedances.
• an effective dielectric layer is represented by a physical dielectric layer on the upper surface of the electrode, by the material of a surgical gown worn by the patient, by a bed sheet or other operating room linens interposed between the patient and the return electrode, by the material of a protective sleeve fitted over the return electrode, or any combination thereof.
• FIG 4 illustrates in perspective an operating table 40 with an electrosurgical return electrode 41 according to the invention disposed on the upper surface thereof, an edge of which is identified by the numerals 42.
• the operating table is shown to have conventional legs 44a-44d that may be fitted with wheels or rollers as shown.
• Table 40 is one structure that is capabe of performing the function of supporting means for supporting a patient during treatment. It may be appreciated by one skilled in the art, however, that various other configurations of support means are possible and capable of performing this function.
• supporting means may include, but not limited to, chairs, plates, beds, carts, and the like.
• the effective working surface area will vary depending on the material used, in some geometrical configurations, and in instances where various layers of operating room linens are placed over the electrode. The principles hereof may be successfully employed and the effective working surface area of the return electrode determined in such circumstances by routine experimentation. Under certain conditions, the effective working surface may be as small as about seven square inches (or about 45 square centimeters).
• the electrode be configured so that when the electrode is used: (1) the return current density on the surface of the patient is sufficiently low; (2) the electrical impedance between the electrode and the patient is sufficiently low so that electrical energy is not concentrated sufficiently to heat the skin of the patient at any location in the electrical return path by more than six degrees (6°) Celsius; and (3) the characteristics of the materials and geometries are such that if the effective area of the electrode is reduced below a selected threshold level, there will be insufficient energy dissipated at the surgeon's implement for him to continue effectively using the implement in its electrosurgical mode.
• C capacitance in Farads
• K is the dielectric constant of the material lying between the effective plates of the capacitor
• A is the area of the smallest one of the effective plates of the capacitor in square meters
• t is separation of the surfaces of the effective plates in meters
• e 0 is the pen ttivity of air in Farads per meter.
• a return electrode according to the invention hereof would need a minimum effective area of between about 7 and about 11 square inches (or about 45 cm 2 to about 70cm 2 ) with a relatively small separation from the slcin of the patient such as that provided by a surgical gown or no interposing gown at all. Such an effective area is easy to obtain if the patient is positioned on an electrode that is the size of their upper torso or larger.
• the characteristics of the desired dielectric for the present embodiment are sufficiently comparable to those of selected rubbers, plastics and other related materials that the latter may be satisfactorily employed as materials for the return electrode.
• the results would be that the current flow from the electrosurgical generator would be reduced to a level making it difficult for the surgeon to perform surgery.
• the features described above will continue to occur.
• Figure 5 is a front view illustrating a surgical chair 50 with an electrosurgical return electrode 51 according to the invention disposed on the upper surface of the seat thereof. Accordingly, when a patient is sitting in the chair, the buttocks and upper part of the thighs overlie and are in sufficiently close proximity to the return electrode so that coupling there between presents an impedance meeting the foregoing criteria; namely, that the electrical impedance between it and the patient is sufficiently low to allow the surgeon to perform the procedure while providing that current density is sufficiently low and that insufficient electrical energy is developed across the return impedance to heat the slcin of the patient at any location in the electrical return path by more than six degrees (6°) Celsius.
• FIG. 6 is a top view of another electrosurgical return electrode according to the invention. It will be observed that the upper exposed, or working, surface of the electrode again is expansive so as to meet the foregoing criteria for low impedance. Although it is not necessary that the electrode cover the entire surface of an operating table or the entire seat surface of a dental or other patient chair, it has been found advantageous in some instances to provide a greater surface area than that of the projected area of the buttocks or torso of a patient so that if a patient moves position during the course of a procedure, a sufficient portion of the patient will remain in registration with the electrode surface so that the effective impedance will remain less than the above-described level.
• the electrode does not need to be in direct contact with a patient, either directly or through intervening conductive or nonconductive gel.
• the electrode does not need to be in direct contact with a patient, either directly or through intervening conductive or nonconductive gel.
• the electrode because of its expansive size, there is no need for tailoring the electrode to fit physical contours of a patient.
• the self-correcting and self-limiting principles hereof could be achieved in an electrode as small as seven square inches (or 45 square centimeters) in working surface area, the preferable range of exposed upper working surface area of the electrode lies in the range of from about 11 to 1,500 square inches (or about 70 to 9,680 square centimeters).
• the electrode according to the invention hereof, as illustrated in Figure 6, may be made of conductive plastic, rubber or other flexible material which, when employed in the electrode will result in an effective dc resistance presented by each square centimeter of working surface to be greater than about 8000 ohms. Silicone or butyl rubber have been found to be particularly attractive materials as they are flexible, as well as readily washable, sterilizable, and disinfectable.
• the main body of the return electrode may be made of inherently relatively high resistance flexible material altered to provide the requisite conductivity.
• a preferred example of the latter is that of silicone rubber material in which there are impregnated conductive fibers, such as carbon fiber, or in which there have been distributed quantities of other conductive substances such as carbon black, quantities of gold, silver, nickel, copper, steel, iron, stainless steel, brass, aluminum, or other conductors.
• Connector 54 is another structure capable of performing the function of connecting means for making electrical connection to the sheet.
• Connector 54 is only illustrative of one possible structure for performing the desired function; it being appreciated by one skilled in the art that various other structures are capable of performing this function.
• Figure 7 is a section taken along the lines 7-7 of Figure 6.
• Figure 7 shows an electrode 46 similar to electrode 20 of Figures 2A-2C, except that electrode 46 includes a thin highly-conductive lower stratum 46c to facilitate conduction of current outwardly to terminal 54.
• the thickness of the electrode lies in a range from about 1/32 inch to 1/4 inch (about 0.08 cm to 0.64 cm), which, with the aforementioned range of impedance of the main body of material and the capacitive reactance of the upper dielectric layer, provides the required impedance together with desired physical flexibility for ease of use and handling.
• Figure 8 is a section similar to that of Figure 7, but presenting a multiple layer embodiment illustrating the separation presented by a patient's gown according to the invention hereof.
• a layer 46a similar to layer 46 of Figure 7
• an overlying effectively capacitive layer 47 representing an insulating dielectric layer, a patient's surgical gown, an operating room linen, a protective sleeve or sheath, or any combination thereof.
• a conductive layer 47a of Figure 8 could comprise a sheet or screen of gold, brass, aluminum, copper, silver, nickel, steel, stainless steel, conductive carbon, conductive fluids, gels, saline, and the like.
• Further reference ' to Figure 8 reveals another dielectric layer 47b covering the lower surfaces of layer 46a.
• Figure 9 is a perspective view of a sleeve 50 adapted for encasing any one of the embodiments of Figures 6-8.
• a sleeve 50 adapted for encasing any one of the embodiments of Figures 6-8.
• such a sleeve may preferably be made of any of a variety of known materials, such as vinyl plastics, polyester or polyethylene.
• Figure 10 is a view illustrating one of the embodiments of Figures 6-8 encased within the sleeve of Figure 9. There, it will be seen, is outer surface 50a of sleeve 50; and shown encased within sleeve 50 for illustrative purposes is electrode 41 of Figure 6.
• Figures 11-16 are set forth to define the geometries and characteristics of materials employed to obtain the foregoing self-limiting action. Discussion will be made hereinafter to an illustrative electrode that may be used for electrosurgical procedures utilizing capacitive conduction while still remaining self- limiting. Although discussion is made herein with respect to an electrosurgical electrode functioning under capacitive conduction, similar illustrative information and examples may be provided for resistive and inductive conduction, as described herein and known by one skilled in the art.
• Figure 11 depicts an electrosurgical electrode 60 consisting of a conductive metal backing 61 and a semi-insulating layer 62 of material with bulk resistivity p, thickness t and area A.
• the electrode is in contact with another conducting layer 63 which represents a patient thereupon.
• the circuit can be modeled as a resistor R in parallel with a capacitor C ( Figure 12).
• the resistance R is related to the bulk resistivity p, area A, and thickness t by the formula
• the ratio Y is independent of the electrode area and thickness, depending only upon K and p .
• Y For principally capacitive coupling, Y»l , whereas for principally resistive power conduction, Y «1
• K ranges from 3 to 5.
• Commercially available electrosurgical generators presently have operating frequencies ranging from 200 kHz to 4 MHz.
• Figure 13 illustrates the percentage (%) of capacitive coupling for various frequency electrosurgical generators.
• a minimum bulk resistivity of 100,000 Ohm-cm is required for the majority of the power to be passed through capacitive coupling.
• This minimum bulk resistivity number is greater than required by the available prior art. Consequently, the capacitive coupling electrode grounding pad according to the invention hereof appears to be neither taught nor suggested by known prior art.
• a product according to the invention hereof can be easily distinguished from previous art through a simple test of the bulk resistivity of the insulating material, independent of pad area or pad thickness.
• the self-limiting feature of the electrosurgical return electrode arises due to the impedance of the electrode material.
• This impedance may arise from resistive, inductive, or capacitive components, or a combination thereof. For example ⁇ a single layer of insulative material placed between a conductive surface and the patient presents an impedance equivalent to a resistor in parallel with a capacitor.
• the total impedance of the electrosurgical electrode should be less than 75 ⁇ under normal operating conditions. It is preferred, therefore, that
• I max may vary from patient to patient due to changes in the amount of time that the electrode is in contact with the patient, the electrical characteristics of the patient's skin (i.e., resistivity and the like), the amount of heat being conducted by the patient, the patients initial skin temperature, and the like.
• an electrosurgical return electrode designed according to the prior art, in the event that the contact area with the patient reduces below the A C0Macl (mjn) , while maintaining the I max , a burn may result because (I/A) crilica , is greater than lOOmA/cm 2 , which is the burn tlireshold.
• the present invention limits the possibility of a burn caused from a reduction of the contact area below A conlacl(min) , while also preventing electrosurgical procedures when the contact area is significantly reduced. Therefore, by selecting the appropriate impedance of the electrode, the current I is always reduced below I max when
• a.atA conlacl(min have a value from about 7 cm 2 to about 22 cm 2 , and more preferably about 10 cm 2 .
• ⁇ range from about 10 to about 50, and more preferably have a value of about 10.
• the factor of 1.2 is included within the resistivity and reactance terms of the equation; however, it may be appreciated by one skilled in the art that the factor of 1.2 is geometry dependent for both the resistive and reactance terms and may vary. Additionally, the value of 1.2 is based on the illustrative geometry of the presently described self limiting electrode and when the geometry of the electrode varies, the factor value will also vary to account for the different edge effects.
• the maximum electrode thickness one could imagine using would range from about 0.5 to about 4 inches (about 1.3 cm to about 10.2 cm), and more preferably about 1 inch (2.5 cm) thick. At this thickness the electrode may become unwieldy to use and uncomfortable for the patient.
• the minimum bulk resistivity for a electrode of such thickness is about 4000 ⁇ -cm to be self-limiting in a resistive mode as previously noted.

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