Abstract:
This invention is an improved method and apparatus for tissue electrical impedance determination and electrical power control in a surgical device. 
     In an embodiment of the invention an apparatus for controlling power delivery in an electro-surgical instrument is disclosed. The electro-surgical instrument includes a first channel and a second channel for delivery of energy to a surgical site. The apparatus includes: a switch, a measuring unit, a processor and a drive unit. The switch electrically isolates the second channel during a first measurement interval and the first channel during a second measurement interval. The measuring unit is coupled to the first and the second channel. The measurement unit measures a first power level of the first channel during a first measurement interval and a second power level of the second channel during a second measurement interval. The processor is coupled to the measuring unit and to the switch. The processor adjusts the first power level and the second power level to minimize a difference between a measured value of a control parameter and a target value of the control parameter. The drive unit is controlled by the processor. The drive unit delivers the adjusted first and second power levels to the surgical site via respectively the first channel and the second channel during a heating interval. 
     In an alternate embodiment of the invention a method for controlling power delivery in an electro-surgical instrument is disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of prior filed copending Provisional Application No. 60/061,213, filed on Oct. 6, 1997, entitled  Method And Apparatus For Impedance Measurement In A Multi - Channel Electro - Surgical Generator,  Provisional Application No. 60/062,458, filed on Oct. 6, 1997, , entitled  Linear Power Control With Digital Phase Lock,  Provisional Application No. 60/061,193, filed on Oct. 6, 1997, , entitled  Linear Power Control With PSK Regulation,  Provisional Application No. 60/061,197, filed on Oct. 6, 1997, entitled  Memory for Regulating Device Utilization and Behavior,  Provisional Application No. 60/061,714, filed on Oct. 6, 1997, entitled  Dual Processor Architecture For Electro Generator,  and Provisional Application No. 60/062,543, filed on Oct. 6, 1997, entitled  Method And Apparatus For Power Measurement In Radio Frequency Electro - Surgical Generators.    
     The present application is related to copending U.S. patent application No. 09/167,717, filed Oct. 6, 1998, entitled  Linear Power Control With Digital Phase Lock,  U.S. patent application No. 09/167,412, filed Oct. 6, 1998, entitled  Linear Power Control With PSK Regulation,  U.S. patent application No. 09/167,508, filed Oct. 6, 1998, entitled  Memory for Regulating Device Utilization and Behavior,  U.S. patent application No. 09/167,508, filed Oct. 6, 1998, entitled  Dual Processor Architecture For Electro Generator,  U.S. patent application No. 09/167,505, filed Oct. 6, 1998, entitled  Method And Apparatus For Power Measurement In Radio Frequency Electro - Surgical Generators,  International Application No. PCT/US98/21065 filed Oct. 6, 1998, entitled  Linear Power Control With Digital Phase Lock,  and International Application No. PCT/US98/21066, filed October 1998, entitled Dual Processor Architecture For Electro Generator. 
     Each of the above-cited applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to the field of electro-surgical medical devices. More particularly, this invention relates to devices that deliver energy in the form of radio-frequency electrical current to tissue in order to perform surgical functions. 
     Description of Related Art 
     Various medical procedures rely on high-frequency electrical currents to deposit energy and thus heat human and animal tissues. During such procedures, a high-frequency current is passed through the tissue between electrodes. One electrode is located at the tip of a surgical probe. Another electrode is located elsewhere, and may be a ground pad or another surgical probe tip. The tissue to be treated lies between the electrodes. 
     When the electrode circuit is energized, the electric potential of the electrodes at the probe tips oscillates at radio frequencies about a reference potential. If one is used, a ground pad remains at a floating reference potential. As the electric potential of the probe electrodes varies, a motive force on charged particles in the tissue is established that is proportional to the gradient of the electric potential. This electromotive force causes a net flow of electric charge, a current, to flow from one electrode, through the tissue, to any other electrode(s) at a lower potential. In the course of their flow, the charged particles collide with tissue molecules and atoms. This process acts to convert electrical energy to sensible heat in the tissue and is termed Joule heating. 
     Upon heating, surgical functions such as cutting, cauterizing and tissue destruction can be accomplished. For example, tissues can be cut by heating and eventually vaporizing the tissue cell fluids. The vaporization causes the cell walls to rupture and the tissue to cleave. When it is beneficial to destroy tissue, comparatively higher rates of energy deposition can cause tissue ablation. 
     Ablation of cellular tissues in situ is used in the treatment of many diseases and medical conditions either alone or combined with surgical removal procedures. Surgical ablation is often less traumatic than surgical removal procedures and may be the only alternative where other procedures are unsafe. 
     Tissue ablation devices commonly utilize electromagnetic (microwave, radio frequency (RF), lasers) or mechanical (acoustic) energy. In the category of electro-surgical devices, microwave ablation systems utilize a microwave antenna which is inserted into a natural body opening through a duct to the zone of treatment. Electromagnetic energy then radiates from the antenna through the duct wall into the target tissue. However, there is often severe trauma to the duct wall in this procedure since there is a significant microwave energy flux in the vicinity of the intended target. The energy deposition is not sufficiently localized. To reduce this trauma, many microwave ablation devices use a cooling system. However, such a cooling system complicates the device and makes it bulky. Laser ablation devices also suffer the same drawback as microwave systems. The energy flux near the target site, while insufficient to ablate the tissue, is sufficient to cause trauma. 
     Application of RF electric currents emanating from electrode tips offers the advantage of greater localization of the energy deposition since the electrode tip is nearly a point source. However, these devices require consideration and monitoring of the effect of the energy deposition on the tissue since the electrical dissipation and storage characteristics of the tissue carrying the current may vary with time as a result of the current-induced heating. As a result, the tissue heating response could vary over the time of treatment due to changing values of the tissue&#39;s electrical properties. 
     In addition, the localization of energy flux in an RF electro-surgical device may require a number of electrodes to be included in the surgical probe to provide adequate area coverage. In the case of multiple probe electrodes, each electrode may not be at the same electric potential at each instant due to amplitude, frequency, or phase variations in their RF oscillations. In this instance, an electric current would flow between the probe electrodes, coupling them to an extent primarily determined by the difference in electric potential between the probe electrodes and the electrical properties of the tissue between the electrode tips. This coupling can confuse monitoring of applied power and tissue response. 
     With an electro-surgical device, the tissue heating response depends largely on the electrical impedance since impedance is a representation of energy dissipation and storage properties. As described, the impedance of the tissue lying between the electrodes is an important parameter in both in the case of a single electrode, as well as in the case of devices with multiple electrodes. In fact, tissue electrical impedance is often displayed to the medical practitioner during a procedure since large changes in tissue impedance are indicative of tissue drying, ablation, etc.. Thus, the efficacy of these devices is critically affected by the methods of tissue electrical impedance determination and power control. 
     Prior art methods for determining the electrical impedance of the tissue in the context of a device for electro-surgery are of questionable accuracy since the measurements are made at a comparatively low electric current. In the prior art methods, the electric current utilized to determine the impedance is insufficient to damage the tissue. However, the resulting measurements are prone to error since the electrical signals are not strong relative to the noise in the measurement circuit. Prior art methods also do not adequately eliminate electric coupling between the electrodes, termed crosstalk, in the case of a multiple electrode probe. Therefore, there is a need in the field of electro-surgical devices for improved methods and apparatae for tissue electrical impedance determination and electrical power control. 
     SUMMARY OF THE INVENTION 
     This invention is an improved method and apparatus for tissue electrical impedance determination and electrical power control in a surgical device. The tissue electrical impedance determination and associated power control is improved relative to the prior art methods in that the uncertainties in determining the actual tissue impedance are reduced in two ways. First, this invention makes the impedance determining measurements less prone to noise-related uncertainties. Second, this invention eliminates measurement uncertainties due to electrical cross-talk between multiple electrode channels. 
     In an embodiment of the invention, an apparatus for controlling power delivery in an electro-surgical instrument is disclosed. The electro-surgical instrument includes a first channel and a second channel for delivery of energy to a surgical site. The apparatus includes: a switch, a measuring unit, a processor and a drive unit. The switch electrically isolates the second channel during a first measurement interval and the first channel during a second measurement interval. The measuring unit is coupled to the first and the second channel. The measurement unit measures a first power level of the first channel during a first measurement interval and a second power level of the second channel during a second measurement interval. The processor is coupled to the measuring unit and to the switch. The processor adjusts the first power level and the second power level to minimize a difference between a measured value of a control parameter and a target value of the control parameter. The drive unit is controlled by the processor. The drive unit delivers the adjusted first and second power levels to the surgical site via respectively the first channel and the second channel during a heating interval. 
     In an alternate embodiment of the invention a method for controlling power delivery in an electro-surgical instrument is disclosed. The electro-surgical instrument includes a first channel and a second channel for delivery of energy to a surgical site. The method for controlling power comprises the acts of: 
     determining a target value for a control parameter for the first channel and the second channel; 
     measuring a first power level of the first channel during a first measurement interval in which the first channel is electrically isolated from the second channel; 
     measuring a second power level of the second channel during a second measurement interval in which the second channel is electrically isolated from the first channel; and 
     adjusting the first power level and the second power level to minimize a difference between a measured value of the control parameter and the target value of the control parameter, to deliver the energy to the surgical site during an heating interval. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts throughout. 
     FIG. 1 shows the apparatus for a typical embodiment of the RF electro-surgical device. 
     FIG. 2 shows a block diagram showing elements of the system hardware architecture. 
     FIG. 3A shows a graph of power versus time for a single electrode probe illustrating the time sub-intervals for impedance determination, tissue heating and tissue temperature measurement time. 
     FIG. 3B shows a graph of RF power supply output versus time for a single electrode probe illustrating the time sub-intervals for impedance determination, tissue heating and tissue temperature measurement time. 
     FIG. 4 shows a schematic illustrating electrical hardware elements of the surgical apparatus in a multi-channel embodiment. 
     FIG. 5 shows a graph of power versus time and RF power supply output versus time for a multiple electrode probe illustrating the impedance determination, tissue heating and tissue temperature measurement time subintervals. 
     FIG. 6 shows a graph of power versus treatment time illustrating a power time schedule, target power and power delivered by the power control system. 
     FIG. 7 shows a schematic illustrating the electrical hardware elements of a power drive element. 
     FIG. 8 shows a process flow chart illustrating the method for tissue impedance determination and power control. 
    
    
     DETAILED DESCRIPTION 
     Accurate determination of the tissue electrical impedance is important in RF electro-surgery since it is a guiding parameter for the surgeon and relates directly to the intended medical benefit. This invention is an improved method and apparatus for tissue electrical impedance determination and electrical power control for such surgeries. The tissue electrical impedance determination and associated power control is improved relative to the prior art methods in that the uncertainties in determining the actual tissue impedance are reduced in two ways. First, this invention makes the impedance determining measurements less prone to noise-related uncertainties. Second, this invention eliminates measurement uncertainties due to electrical cross-talk between multiple electrode channels. 
     FIG. 1 shows the apparatus for a typical embodiment of the RF electro-surgical device. The system comprises an RF power supply  100  with a user input and display panel  102 , a foot switch  104 , a surgical handset  106  with a surgical probe  108  and an electrical grounding pad  110 . 
     The RF power supply  100  converts the low frequency electrical energy supplied by a wall connection (not shown) into the high frequency or RF energy necessary for surgery. The user input and display panel  102  displays relevant parameters and provides buttons and switches for user input to the control systems. The foot switch  104  connected to the power supply provides means for switching the unit on and off. The surgical handset  106  is also connected to the power supply and is the means for delivering the RF energy to the surgical probe  108 . The probe has one or more probe electrodes. The electrical grounding pad  110  is also connected to the power supply and floats at a reference electric potential. Other embodiments of this invention have no electrical grounding pad. 
     FIG. 2 shows a block diagram showing elements of the system hardware architecture of an exemplary embodiment. FIG. 2 shows a block diagram of the RF power supply  100 , surgical probe  108  and grounding pad  110 . Within the power supply, the user input and display panel  102 , micro-controller, a.k.a. processor  202 , first and second electrode channels  204  and  206 , temperature measurement system  208 , memory unit  210 , memory files  212 , control parameter schedule  214 , and RF oscillator  203  are indicated. Electrode channels  204  and  206  are identical, each comprising a control system  220 A-B, waveform generator  222 A-B, an isolation switch  224 A-B, a power drive  226 A-B, a transformer  228 A-B, a filter  230 A-B, current and voltage sensors  232 A-B, and power measurement system  234 A-B. 
     In FIG. 2, the user input and display panel  102  is connected to the microcontroller  202  which is connected to the memory unit  210  including memory files  212 , including a control parameter schedule  214 . The control parameter schedule, a.k.a. profile contains data correlating target control parameters, e.g. temperature and power as a function of time. Exemplary control parameters are power and tissue temperature at the surgical site. Other control parameters are apparent to persons skilled in the art. The micro-controller is connected with the identical electrode channels  204  and  206  and also to the tissue temperature measurement system  208  and the RF oscillator  203 . Within each electrode channel, the control systems  220 A-B are connected to the microcontroller as well as to the RF oscillator and the tissue temperature measurement system. The control system also connects to the waveform generators  222 A-B. The waveform generators are connected to the power drive  226 A-B through the isolation switches  224 A-B. The RF signals from the transformer  228 A-B feed into filters  230 A-B. The current and voltage sensors  232 A-B connect to the filter, grounding pad  110 , surgical probe  108  and the power measurement systems  234 A-B. 
     The micro-controller  202  implements control programs and logic contained in memory files  212 , providing the principal intelligence of the control system including the selection of values for time scales and power levels. To act as a means for control, the microcontroller is in two way communication with the user through user input and display panel  102  as well as receives input from the RF oscillator  203 , and power and tissue temperature measurement systems  234 A-B,  208 A-B. The microcontroller is also coupled to memory  210  from which it can obtain the control parameter schedule  214 . Control variables are passed to control systems  220 A-B to achieve the desired amplitude, frequency, and phase of the electrode potentials. 
     The RF oscillator and waveform generator  222 A-B generate RF oscillations that modulate the output of the power drive  226 A-B. Power is coupled through transformer  228 A-B by the principle of induction, isolating the patient from direct current (DC). Further frequency filtering is accomplished by filter  230 A-B. Collectively components  220 A-B through  226 A-B constitute drive units for which there are numerous alternate embodiments known to those skilled in the art. Numerous substitutions are possible for the above described components without departing from the teachings of this invention. Current and voltage sensors  232 A-B provide required signals for the power measurement systems  234 A-B to determine the actual power transferred to the tissue by the current passing between the surgical probe  108  to grounding pad  110 . 
     FIG.  3 A and FIG. 3B show graphs of power versus time and RF power supply drive parameter versus time for a single electrode probe. FIGS. 3A-B illustrate the time interval multiplexing of an overall system timing cycle, showing the impedance determination, tissue heating and tissue temperature measurement time sub-intervals within an overall system timing cycle. The power control system governs the circuit power on significantly smaller time scales. 
     As shown in FIG.  3 A and FIG. 3B, the overall system timing cycle is typically one second in duration. Over this timing cycle, time markers  310 ,  312 ,  316 , and  318  bound several time sub-intervals. The sub-interval bounded by time markers  310  and  312  is devoted to determining the electrical impedance of the tissue. This sub-interval is typically 10 milliseconds in duration. Another sub-interval, bounded by time markers  312  and  318 , is devoted to the application of RF energy to heat the tissue. This tissue heating sub-interval is typically 900-1000 milliseconds in duration. It is obvious to those skilled in the art that the total time sub-interval for determination of the tissue impedance may be further subdivided into a number of time sub-intervals for sequentially determining the tissue impedance at each of several electrode locations in a multi-channel embodiment. 
     During each tissue impedance measurement interval, all of the electrodes except the selected electrode are electrically isolated from the system by isolation switches  224 . When isolated, no current flows through the electrode channel. Once all electrodes except that of interest are isolated, a comparatively high RF power  308  (typically 5 Watts) is applied to the single electrode and a tissue impedance determination is made from the current and voltage measurements made with the current and voltage sensors  232 . The micro-controller repeats the measurements on each electrode channel in succession until the impedances of the tissue along the current paths from each electrode have been determined. 
     As described, a comparatively high RF power is applied through the probe electrode on each channel during the time sub-interval for tissue impedance determination. Typically, the power control system holds this first power level constant during the sub-interval by comparing a measured power to a target power level. While powerful currents pass through the tissue, the period of time during which high power is applied is sufficiently brief that no significant tissue heating or other undesirable effects occur. The application of a comparatively high current is necessary during this time interval to ensure a signal to noise ratio that is compatible with an accurate impedance determination. This feature, along with the mitigation of inter-electrode coupling during the measurement time, are major advantages of this invention over prior art methods. 
     The tissue heating time interval is bounded by time markers  312 ,  318 . Typically, it is 900-1000 ms in duration. During this interval, a much smaller second power level is applied to the tissue  306 , typically 0.5 Watt. Control is applied to the circuit to maintain a desired power for each electrode channel throughout. Typically, the power is held constant over this time sub-interval. Although the power for each electrode channel is typically held constant, the system allows for different power levels amongst the electrode channels. 
     FIG. 3A shows, as an example, the maintenance of a constant power level during the period of tissue heating. The envelope of the RF output necessary to deliver the constant power  320  is in FIG.  3 B. The drive parameter envelope varies during the tissue heating period due to changes in the tissue impedance caused by Joule heating. 
     FIG.  3 A and FIG. 3B also show the time subinterval for tissue temperature measurement. This subinterval is bounded by time markers  316 ,  318  and occurs near the end of the tissue heating sub-interval. As described, the tissue temperature measurements are made immediately prior to tissue impedance determination in the subsequent system timing cycle. This reduces the time interval between the tissue temperature measurement and the application of the power level based on that temperature. 
     FIG. 4 shows a system with two probe electrodes. This two channel system includes current and voltage sensors  232 A-B for each channel, surgical probe  108  electrodes  406 A-B and electrical grounding pad  110 . Within each current and voltage sensor  232 A-B, there is a current sensor  402 A-B and voltage sensor  404 A-B. Within the surgical probe  108 , there is an electrode  406 A and  406 B for each channel. The electrodes are in contact with tissue  408  and the tissue is in contact with the electrical grounding pad. The equivalent electrical circuit representing the tissue impedance from the electrodes to the grounding pad  410 A-B and the inter-electrode coupling (cross-talk) impedance  412  is also shown. 
     In FIG. 4, each of the current and voltage sensors  232 A-B are connected to their respective electrodes  406 A-B in the surgical probe  108  as well as to the common electrical grounding pad  110 . The tissue is connected to the electrodes and the grounding pad. The grounding pad is connected to the tissue and the current and voltage sensors  232 A-B. 
     When each electrode  406 A-B is connected to the RF generator (not shown), a RF electrical current flows through the tissue to the grounding pad  110 . As this occurs, current and voltage sensors  402 A-B,  404 A-B act as a means to determine the RF power applied to the tissue, as well as the electrical impedance of the tissue between the electrode tip and the grounding pad. The relationships between current, voltage, power and impedance are well known to persons skilled in the art. Note that with all of the electrodes except the one of interest isolated by switches  224 A-B (see FIG. 2) there is no significant coupling between the electrodes in the surgical probe causing current flow between them. This is beneficial for an accurate tissue impedance determination. It is obvious to those skilled in the art that the isolating switch may be located elsewhere than shown in FIG.  2 . 
     FIG. 5 shows a graph of power versus time and RF power supply output versus time for a multiple electrode probe. FIG. 5 illustrates the impedance determination, tissue heating and tissue temperature measurement time sub-intervals within an overall system timing cycle and is similar to the case of a single channel described in FIG.  3 . As in the case of a single channel, the power control systems govern the circuit power on significantly smaller time scales. 
     As in the case of the single channel, each of the multiple channels has several sub-intervals within the overall system timing cycle of approximately one second duration. The sub-intervals are defined by time markers  500 ,  502 ,  504 ,  506 ,  508 ,  512 ,  514 . For each channel, there is an tissue heating time sub-interval bounded by time markers  508  and  514 . However, as seen in FIG. 5, the tissue heating time sub-intervals for all channels coincide. Each channel also has an impedance determination sub-interval  520 A-C that follow each other in sequence. The total impedance determination sub-interval for all channels is bounded by time markers  500  and  508  and is typically 100 milliseconds in duration. In this interval time division multiplexing allows each electrode to be electrically isolated from all others while its impedance is measured. The impedance measurement is carried out at a high power level. The high power level allows an accurate determination of impedance. Such a determination would be more difficult at the relatively low power levels used during the tissue heating time subinterval. Also shown in FIG. 5 is a tissue temperature measurement time sub-interval bounded by time markers  512 ,  514  near the end of the overall timing cycle. The tissue temperature sub-interval is approximately 100 milliseconds in duration. During all of the above time sub-intervals, the power control systems operate on significantly shorter time scales to maintain the desired power  530 A-C on each channel by varying the RF drive parameters  532 A-C. 
     As was the case for a single channel, significantly different power levels are applied to the tissue for the impedance determination and tissue heating. In order to have the signal to noise ratio necessary for an accurate impedance determination a comparatively high power is applied to each electrode channel during that time interval. However, as previously described, the duration of this high power application is sufficiently short so that no significant tissue heating occurs. During the subsequent tissue heating interval, a much lower RF power is applied to the tissue on all channels. While the power level during this interval is comparatively low, the application persists over a time interval several orders of magnitude longer than that for the tissue impedance determination. During the period of tissue heating, the power circuits of each channel are controlled to maintain a constant power under a varying impedance. A tissue temperature measurement is made near the end of the heating interval. 
     FIG. 6 shows a graph of power versus treatment time illustrating a power time schedule, target power and power delivered by the power control system under a exemplary control law. It illustrates the use of the power control system to accomplish the intended medical function by delivering prescribed power to the tissue site. FIG. 6 shows a control parameter schedule  214 , with power as the control parameter. Three overall system timing cycles  602 A-C of one second duration each are shown. During each overall system timing cycle, the micro-controller  202  (see FIG. 2) receives inputs from power and temperature measurements and executes control laws based on those and other system parameters. Under an exemplary control law, the micro-controller calculates a target value of power  604  and control is applied to each electrode channel  204 A-B (see FIG. 2) to maintain a constant delivered value  606  of power over the timing interval. The target value of the control parameter may be updated as desired to follow the control parameter schedule to a desired accuracy. This is illustrated by the comparative frequency of target value updates in timing cycles  602 A-C. It is obvious to those skilled in the art that the system can be configured to follow other appropriate control parameters, such as tissue temperature. It is equally obvious to those skilled in the art that the system can be configured to follow other power control laws. 
     FIG. 7 shows a schematic illustrating the electrical hardware elements of a power drive element  226 . The power drive contains two transformers  702  and  704 , two transistors  706  and  708 , positive voltage supply  710  and an decoupling capacitor  712 . 
     In FIG. 7, transformer  702  is connected to the RF oscillator on one side and the transistors  706  and  708  on its other side. The center tap of the transformer is also connected to the transistors  706  and  708 . The winding of the second transformer  704  are connected to the positive voltage supply  710  and decoupling capacitor  712  on one side and on the other side they form the output of the device. 
     The transformer  702  serves to isolate the unit from the RF waveform generator  222  (see FIG. 2) that provides its input. Through the principal of electrical induction, radio frequency oscillations are induced in the RF power supply from the RF waveform generator. The positive voltage supply  710  in conjunction with the second transformer  704  act to modulate the amplitude of the RF voltage in the circuit. Through the principal of electrical induction the RF signals are transferred to the output across the transformer  704 . 
     FIG. 8 shows a process flow chart for this method of tissue electrical impedance determination and electrical power control. The process shown in FIG. 8 is implemented by micro-controller  202  (see FIG.  2 ). In alternate embodiments, the process implementation is divided between the micro-controller and analog hardware in control system  220 A-B (see FIG.  2 ). The process begins by startup and initialization of the device in process block  798 . During startup, the system initializes itself, performs several self-tests and uploads information from the memory and receives information input by the user from the front panel. Clocks and other variables requiring initialization are set in block  801 . 
     Within the overall system timing cycle illustrated, control first passes to sequence  850  where the tissue impedance determinations are accomplished. In  802 , the micro-controller  202  electrically isolates all but the first channel and then applies a comparatively high RF power only to that channel. Control then passes to process block  804  where the electrical impedance in the tissue is determined from measurements of current and voltage in the energized channel. The resulting the value is stored. Control then passes to process block  806  where the value of the tissue impedance is displayed to the user at the user input and display panel  102  (see FIG.  1 ). Control then passes to decision block  808  where the system can repeat the preceding process for subsequent electrode channels or proceed once the tissue impedance across all of the electrodes is determined. 
     Once the tissue impedance has been measured at each electrode channel, control passes to sequence  860  were the tissue heating is accomplished. Sequence  860  begins with block  810  where the micro-controller  202  (see FIG. 2) determines the elapsed time from the start of the treatment. This corresponds to the abscissa shown in FIG.  6 . Following this, control passes to process block  812  where the micro-controller calculates a target value of the control parameter. In this exemplary embodiment, power is the control parameter. Thus, the micro-controller calculates a target power from the control parameter schedule  214  (see FIG.  2 ). Control then passes to decision block  814 . 
     At decision block  814 , the system chooses a power control protocol. In one embodiment, with power as the control parameter, there are two power control protocols to choose from. Under the first power control protocol, the power delivered to the tissue site from an electrode is calculated from the measurements of current and voltage in process block  816 . Control then passes to process block  820 , where the fractional difference between the actual power delivered and the target power, or fractional error, is calculated. Control then passes to process block  824  where a RF drive parameter is adjusted, altering the RF power to minimize the fractional error and maintain the constant target power delivered through the electrode. This value is communicated to the power control system  220  (see FIG.  2 ). Control then passes to decision block  828  where an evaluation is made as to whether an update of the targeted value of power is desired. If a new target value for the power is not desired, control passes to decision block  832 . At decision block  832 , an evaluation is made as to whether the tissue heating time limit is over. If it is, then the system returns to  850  and another overall system timing cycle begins with tissue impedance measurements on each electrode channel. 
     An alternate power control protocol proceeds from decision block  814  by retrieving the stored tissue impedance at process block  818 . Control then passes to process block  822  where the RF drive parameter required to deliver the target value of power is calculated directly, assuming the stored tissue impedance value from the previous impedance determination interval. Control then passes to process block  826  where the RF drive parameter is adjusted to achieve the target value. This value is communicated to the power control system  220  (see FIG.  2 ). Control then passes to decision block  830  where an evaluation is made as to whether an update of the targeted value of power is desired. If a new target value for the power is not desired, control passes to decision block  834 . At decision block  834 , an evaluation is made as to whether the tissue heating time period is over. If it is, then the system returns to  850  and another overall system timing cycle begins with tissue impedance measurements on each electrode channel. 
     In an alternate embodiment of the invention temperature rather than power constitutes the control parameter in the control parameter schedule  214 . In that embodiment the protocol followed compares current, previous temperature, and target temperature and impedances of each electrode and determines the amount of error in the desired versus actual temperature of the surgical site. Using this determination power is adjusted accordingly and the appropriate “heating” voltages for maintaining the target temperature at the surgical site are imposed by the waveform generator  222 A-B and the power drive  226 A-B. 
     While this invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.