Abstract:
A method of controlling output power of an electrosurgical generator apparatus that controls a variable output signal to a pair of electrodes includes setting the output power of the generator apparatus to a selected power output level. An impedance is measured across the electrodes when the electrodes are applied to an area of tissue. The output power of the generator apparatus is changed to a boost power output level greater than the selected power output level. The boost power output level corresponds to a calculation based at least in part on the measured impedance. The method further includes applying the output signal to the electrodes at the boost power output level for a first time duration and changing the power output to the selected power output level after the first time duration. An electrosurgical generator apparatus operating in accordance with the method is also described.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/037,794, filed on Mar. 19, 2008, entitled “Electrosurgical Generator Having Boost Mode Control Based on Impedance,” the entire contents of which are incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    An embodiment of the present invention relates generally to a method of providing a boost mode in an electrosurgical generator apparatus, and more particularly, to a method of providing a boost mode wherein the boost output power level is based on a measured impedance of tissue. 
         [0003]    Devices used for controlling monopolar and bipolar electrode tools are well known in the art. U.S. Pat. No. 5,318,563, the contents of which are incorporated by reference herein, relates to electrosurgical radio frequency (RF) generators. The electrodes in the prior art systems are used for cutting and coagulation of tissue. An RF current is generated between the electrodes and is applied to the tissue. Regarding bipolar tools in particular, cutting occurs by application of the concentrated RF current to destroy cells placed between the electrodes. 
         [0004]    It is found, however, that when the electrodes are placed in contact with the body prior to activation, the output voltage of the RF amplifier is decreased. As a result, the cutting ability of the electrosurgical tool is hindered. One solution has been to provide a short, initial boost to the power output level of the generator upon activation of the electrosurgical tool. The brief power output surge is enough to overcome the impedance caused by the tissue to allow cutting to begin. After the surge, the power output level returns to normal and cutting proceeds in the typical fashion. 
         [0005]    The general practice has been to set the boost voltage to a certain level and use the same level regardless of the conditions. This can lead to an increase in collateral damage in the tissue caused solely by the power surge. For example, the impedance of tissue between individuals may vary greatly, and even within the same individual, different tissues exhibit various impedance levels. The impedance is correspondingly proportional to an amount of cell destruction caused by the generator apparatus. Therefore, a constant boost voltage of, for example, 1100 V may cause more unintended damage in a patient or tissue with a lower impedance level than in a patient or tissue having a higher impedance level. 
         [0006]    It is desirable to provide a method of generating a boost voltage in an electrosurgical generator apparatus while minimizing the collateral damage to surrounding tissue when the boost voltage is applied. It is further desirable to provide an electrosurgical apparatus that provides a variable boost voltage for minimizing collateral damage to surrounding tissue. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Briefly stated, an embodiment of the present invention comprises a method of controlling output power of an electrosurgical generator apparatus that controls a variable output signal to a pair of electrodes. The method includes setting the output power of the generator apparatus to a selected power output level. An impedance is measured across the electrodes using an impedance monitoring circuit when the electrodes are applied to an area of tissue. The output power of the generator apparatus is changed to a boost power output level greater than the selected power output level. The boost power output level corresponds to a calculation based at least in part on the measured impedance. The method further includes applying the output signal to the electrodes at the boost power output level for a first time duration. The power of the output signal applied to the electrodes is changed to the selected power output level after the first time duration. 
         [0008]    Another embodiment of the present invention comprises an electrosurgical generator apparatus that controls a variable output signal to a pair of electrodes. The generator apparatus includes a controller for controlling the generator apparatus. An impedance monitoring circuit detects an impedance as measured across the electrodes when the electrodes are applied to an area of tissue. A memory stores predetermined values for calculating a boost power output level based at least in part on the measured impedance. The controller is configured to change a selected power output level to the boost power output level based at least in part on the measured impedance for a first time duration and change the boost power output level to the selected power output level after the first time duration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustration, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
           [0010]      FIG. 1A  is an elevational view of a front panel of an electrosurgical generator apparatus in accordance with a preferred embodiment of the present invention; 
           [0011]      FIG. 1B  is an elevational view of a rear panel of the electrosurgical generator of  FIG. 1A ; 
           [0012]      FIG. 2A  is a perspective view of an electrosurgical bipolar instrument for use in accordance with the electrosurgical generator of  FIG. 1A ; 
           [0013]      FIG. 2B  is a perspective view of an electrosurgical monopolar instrument for use in accordance with the electrosurgical generator of  FIG. 1A ; 
           [0014]      FIG. 3  is a control circuit block schematic diagram in accordance with a preferred embodiment of the present invention; 
           [0015]      FIG. 4  is a screenshot from a display of an electrosurgical generator apparatus in accordance with a preferred embodiment of the present invention; 
           [0016]      FIG. 5  is a flowchart depicting a method of supplying a boost power output level from an electrosurgical generator apparatus in accordance with a preferred embodiment of the present invention; and 
           [0017]      FIG. 6  is a table of multipliers for determining a boost power output level stored in memory of an electrosurgical generator apparatus in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower”, and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the apparatus and designated parts thereof. The terminology includes the above-listed words, derivatives thereof, and words of similar import. Additionally, the words “a” and “an”, as used in the claims and in the corresponding portions of the specification, mean “at least one.” 
         [0019]    Referring to the drawings in detail, wherein like reference numerals indicate like elements throughout, there is shown in  FIGS. 1A and 1B  a preferred embodiment of an electrosurgical RF generator apparatus or RF generator  50 .  FIG. 1A  is an elevational view of a front panel  52   a  of the RF generator  50 , and  FIG. 1B  is a perspective view of a rear panel  52   b  of the RF generator  50 . 
         [0020]    The RF generator  50  includes a housing  52 , a display screen  54 , such as a cathode ray tube (CRT), liquid crystal display (LCD), or the like, on the front panel  52   a  and a connector panel  56  on the rear panel  52   b.  The display screen  54  is preferably a touch panel. Control knobs  57   a,    57   b  on the front panel  52   a  may be used for selecting output power. A power cord (not shown) of the conventional type as is known in the art is connected to a power source to provide power to the RF generator  50  via a source power plug adapter  49 . Preferably, the RF generator  50  is supplied with between about 110-125 volts of alternating current (VAC) at 60 Hertz (Hz) or about 220-240 VAC at 50 Hz, and may be selected using the voltage supply switch  48 . But, other supply voltages and frequencies of AC voltage or other direct current (DC) voltages may be supplied without departing from the present invention. The RF generator  50  also includes an on/off power switch  53 . The RF generator  50  may also include one or more speakers or audio outputs (not shown) for generating indicator beeps and/or vocal instructions in one or more selectable languages. 
         [0021]    The RF generator  50  may be connected to either a monopolar electrosurgical tool (e.g., as shown in  FIG. 2B ) or a bipolar electrosurgical tool (e.g., as shown in  FIG. 2A ). Preferably, the RF generator  50  is used with a bipolar surgical pen  40 , shown in  FIG. 2A , having a cord  46  connected to an output adapter  58  (or alternate output adapter  58   a ) of the RF generator  50 . The bipolar surgical pen  40  is well known in the art and typically includes an instrument housing  42  having a distal end  42   a,  a proximal end  42   b,  and an elongated body  42   c  therebetween. The cord  46  from the output adapter  58  of the RF generator  50  attaches to the surgical pen  40  at the proximal end  42   b.  First and second cut/coagulate mode push buttons  45   a,    45   b  are located on the upper surface of the instrument housing  42 . Alternatively, mode selection between cut and coagulate may be placed on the RF generator  50  or a foot pedal (not shown). A pair of RF electrodes  44   a,    44   b  are located at the distal end  42   a  of the instrument housing  42 . The electrodes  44   a,    44   b  are each of opposite polarity such that one electrode is positively charged and the other electrode is negatively charged, alternately, during use. The electrodes  44   a,    44   b  can be of varying sizes, shapes and thicknesses depending upon the particular application. 
         [0022]    A monopolar electrosurgical tool  40   mp  is shown in  FIG. 2B , and may alternatively be used with the RF generator  50 . The monopolar electrosurgical tool  40   mp  comprises a pen  42   m  and an electrode pad  44   p.  A cord  46   m  of the pen  42   m  connects to the RF generator  50  through, for example, output adapter  58 . An electrode  44   m  of the pen  42   m  is applied to the tissue of a patient. The electrode pad  44   p  is applied to the patient and is separately connected to the RF generator  50  via a cord  46   p.  For simplicity, the preferred embodiments will be described as using the bipolar surgical pen  40 , but those skilled in the art will recognize that a monopolar electrosurgical tool  40   mp  may be substituted therefor. 
         [0023]    Referring to  FIG. 3 , an overall control circuit  59  for the RF generator  50  is shown in a general block diagram. The control circuit  59  is comprised of multiple sub-circuits forming an overall control system for the RF generator  50 . The control circuit  59  includes a main controller U 1  and high and low voltage power supplies  64 ,  66 . Preferably, the RF generator  50  includes a high voltage (HV) power supply  64  that is an off-line switching power supply to provide a high voltage DC output to an RF amplifier circuit  68 . The HV power supply  64  receives supply voltage (e.g., 120 VAC, 60 Hz) and serves as the power source for the RF amplifier  68 . The touch panel  54   a  is controlled by an LCD or simply display controller  60  and is powered by an LCD or simply display compact fluorescent lamp (CFL) HV inverter  61 . Inputs from the touchscreen  54   a  are interfaced through a touch pad controller  62 . The touch pad controller  62  interfaces with the main controller U 1 . The front panel controls  57  and rear panel connectors  56  provide input/output (I/O) to the control circuit  59 . The main controller U 1  controls the RF amplifier circuit  68 . The RF amplifier circuit  68 , which serves to modulate a carrier signal, in combination with the HV power supply  64  provide a variable signal output to the bipolar surgical pen  40 . Feedback from the bipolar surgical pen  40  may be sensed by an impedance monitor circuit  76 . 
         [0024]    The impedance monitor circuit  76  is connected in parallel with an RF output and filter of the RF amplifier  68 . Impedance is thereby detected using the electrodes  44   a,    44   b  of the surgical pen  40 , and the actual impedance of the tissue to be cut or coagulated may be calculated. The impedance value is used by the main controller U 1  to determine a boost voltage to apply at the initial cutting stage, as described in further detail below. The main controller U 1  further includes a partial short circuit detection monitor  75 , shown in  FIG. 3  as a “low-low” impedance monitor. The partial short circuit detection monitor  75  detects partial shorts that significantly drop measured impedance levels that may result in boost elevations that may present safety hazards, or damage or melt the tips of the electrodes  44   a,    44   b.  The partial short circuit detection monitor  75  is configured to limit boost current when the measured impedance is less than a predetermined or operator adjustable “low-low impedance” set point. 
         [0025]      FIG. 4  is a screenshot  100  displayed on touchscreen  54   a  that may be shown during a typical cut mode of the RF generator  50 . The screen  100  includes onscreen indicators  130   a - 130   c  for cut power output ( 130   a ), coagulate power output ( 130   b ), and measured impedance ( 130   c ). In particular, an operator may select the cutting power output of the RF generator  50  by adjusting the cut control knob  57   a  ( FIG. 1A ). Similarly, an operator may select the coagulating power output by adjusting the coagulate control knob  57   b.  An option panel  138   a  allows a user to select whether to irrigate the electrodes  44   a,    44   b  during operation. Option panels for adjusting tone volume ( 138   b ) and voice volume ( 138   c ) are provided, wherein the user may adjust the volume level for either setting using the volume selector panel  138   d.  A settings menu for adjusting further parameters of the RF generator  50  is provided to the user upon selection of the settings button  138   e.  The RF generator  50  may also provide the user with an option to “blend” cutting and coagulation operations, selectable at various levels by a blend control panel  138   f.    
         [0026]    It will be appreciated by those skilled in the art that the RF generator  50  need not utilize a touchscreen  54   a  for displaying and selection of information. For example, selections may be made by an operator using conventional knobs, switches, or the like. Further, information may be conveyed to the operator using alphanumeric light emitting diode (LED), LCD, or other displays known in the art. 
         [0027]      FIG. 5  is a flowchart illustrating a method in accordance with preferred embodiments of the present invention. At block  200 , a desired cutting power output level is set. The desired power output level may be set manually by the operator by, for example, adjusting the control knob  57   a.  Alternatively, the desired power output level may be a predetermined value associated with the cutting mode. In any event, the desired power output level is typically the power output level for the cutting operation of tissue under normal conditions. 
         [0028]    At block  202 , when the electrodes  44   a,    44   b  of the surgical pen are applied to an area of tissue, the impedance monitor circuit  76  measures an impedance. At block  204 , the value of the measured impedance is used by the main controller U 1  to determine a boost power output level that is greater than (or equal to) the desired power output level. In preferred embodiments, the controller U 1  additionally accounts for the desired power output level and determines the boost power output level as a multiple of the desired power output level. For example,  FIG. 6  shows a table  300  stored in a memory of the main controller U 1 . The table  300  considers the desired power output level and the measured tissue impedance and lists a number of multipliers associated with various combinations of the two values. For example, for a desired power output level of 15 W and a tissue impedance of 500 Ω, the main controller U 1  proceeds to block  302  and retrieves a multiplier of 1.7. The multiplier is applied to the desired output level to obtain the boost power output level, or in this instance, 15 W×1.7=25.5 W. It is noted that under certain conditions several of the multipliers in the table  300  are listed as 1.0. For such conditions, the desired power output level is already sufficient to overcome the tissue impedance and no boost is required. 
         [0029]    Returning to  FIG. 5 , having determined the boost power output level, the main controller U 1  increases the power output to the boost level and when the operator sends a signal to begin cutting, for example via foot pedal, push button, or the like, the increased power output is applied to the electrodes  44   a,    44   b  of the surgical pen  40 . The output signal provided by the RF amplifier  68  may be a sine waveform. However, during a boost time duration t b , the signal may have an amplitude that differs from the amplitude of the signal following the boost time duration t b . 
         [0030]    In preferred embodiments, other characteristics of the output signal supplied to the surgical pen  40  may additionally be altered. For example, the waveform supplied by the RF amplifier  68  during the boost time duration t b  may differ from the waveform supplied thereafter. A Malis waveform, described in U.S. Pat. No. 4,590,934, the contents of which are incorporated by reference herein, may be applied during the boost time duration t b . Periodic damping, a distinctive feature of the Malis waveform, provides further protection from collateral damage to the tissue. Once the boost time duration t b  has expired, RF amplifier  68  may return to a sine waveform. The peak amplitude of both the first and second waveforms may differ. Other waveforms (such as, for example, an impulse waveform) or combinations thereof may be used in keeping with preferred embodiments of the present invention. Other preferred embodiments of the present invention may include combinations of the signal variations described above or other variations such as to wavelength, frequency, or the like. 
         [0031]    The boost power output level is applied only for a short duration t b , long enough to overcome the tissue impedance and begin the cutting procedure. Preferably the boost power output voltage is applied for t b =200 ms. The main controller U 1  at block  208  therefore determines whether the boost time duration t b  has expired. If not, the electrodes  44   a,    44   b  continue to receive the boost power output from the RF generator  50 . Once the boost time duration t b  has expired, at block  210  the power output level is reduced to the initial desired power output level and cutting thereafter proceeds in the normal fashion. 
         [0032]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.