Abstract:
A system for monitoring and/or controlling tissue modification during an electrosurgical procedure is disclosed. The system includes a sensor module and a control module operatively coupled to the sensor module and configured to control the delivery of electrosurgical energy to tissue based on information provided by the sensor module. The sensor module further includes at least one optical source configured to generate light and at least one optical detector configured to analyze a portion of the light transmitted through, and/or reflected from, the tissue.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. application Ser. No. 11/585,506, filed on Oct. 24, 2006, by Wham et al., entitled “METHOD AND SYSTEM FOR CONTROLLING OUTPUT OF RF MEDICAL GENERATOR”, now U.S. Pat. No. 8,105,323, which is a divisional application of U.S. application Ser. No. 10/427,832, filed on May 1, 2003, by Wham et al., entitled “VESSEL SEALING SYSTEM,”, now U.S. Pat. No. 7,137,980, which is a continuation-in-part of U.S. application Ser. No. 10/073,761, filed on Feb. 11, 2002, by Wham et al., entitled “VESSEL SEALING SYSTEM”, now U.S. Pat. No. 6,796,981, which is a continuation-in-part of U.S. Ser. No. 09/408,944, now U.S. Pat. No. 6,398,779, filed on Sep. 30, 1999 by Buysse et al., entitled “VESSEL SEALING SYSTEM”, which claims the benefit of the priority date for provisional application No. 60/105,417, filed on Oct. 23, 1998, the entire contents of all of these applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The present invention is directed to electrosurgical surgery and, in particular, to a closed loop control system for an electrosurgical generator. 
     Technical Field 
     Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical instrument to cut, coagulate, dessicate and/or seal patient tissue. High frequency electrical energy, e.g., radio frequency (RF) energy, is produced by the electrosurgical generator and applied to the tissue by the electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures. 
     Electrosurgical techniques and instruments can be used to coagulate small diameter blood vessels or to seal large diameter vessels or tissue, e.g., soft tissue structures, such as lung, brain and intestine. A surgeon can either cauterize, coagulate/desiccate and/or simply reduce or slow bleeding, by controlling the intensity, frequency and duration of the electrosurgical energy applied between the electrodes and through the tissue. For the purposes herein, the term “cauterization” is defined as the use of heat to destroy tissue (also called “diathermy” or “electrodiathermy”). The term “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. “Vessel sealing” is defined as the process of liquefying the collagen and elastin in the tissue so that it reforms into a fused mass with significantly-reduced demarcation between the opposing tissue structures (opposing walls of the lumen). Coagulation of small vessels is usually sufficient to permanently close them. Larger vessels or tissue need to be sealed to assure permanent closure. 
     In order to achieve one of the above desired surgical effects without causing unwanted charring of tissue at the surgical site or causing collateral damage to adjacent tissue, e.g., thermal spread, it is necessary to control the output from the electrosurgical generator, e.g., power, waveform, voltage, current, pulse rate, etc. 
     It is known that measuring the electrical impedance and change thereof across the tissue at the surgical site provides a good indication of the state of desiccation or drying of the tissue, e.g., as the tissue dries or looses moisture, the impedance across the tissue rises. This observation has been utilized in some electrosurgical generators to regulate the electrosurgical power based on a measurement of tissue impedance. For example, commonly owned U.S. Pat. No. 6,210,403 relates to a system and method for automatically measuring the tissue impedance and altering the output of the electrosurgical generator based on the measured impedance across the tissue. The entire contents of this patent is hereby incorporated by reference herein. 
     It has been determined that the particular waveform of electrosurgical energy can be tailored to enhance a desired surgical effect, e.g., cutting, coagulation, sealing, blend, etc. For example, the “cutting” mode typically entails generating an uninterrupted sinusoidal waveform in the frequency range of 100 kHz to 4 MHz with a crest factor in the range of 1.4 to 2.0. The “blend” mode typically entails generating an uninterrupted cut waveform with a duty cycle in the range of 25% to 75% and a crest factor in the range of 2.0 to 5.0. The “coagulate” mode typically entails generating an uninterrupted waveform with a duty cycle of approximately 10% or less and a crest factor in the range of 5.0 to 12.0. In order to effectively and consistently seal vessels or tissue, a pulse-like waveform is preferred. Energy may be supplied in a continuous fashion to seal vessels in tissue if the energy input/output is responsive to tissue hydration/volume through feedback control. Delivery of the electrosurgical energy in pulses allows the tissue to cool down and also allows some moisture to return to the tissue between pulses which are both known to enhance the sealing process. 
     It is further known to clamp or clip excess voltage output from the electrosurgical generator by the use of avalanche devices, such as diodes, zener diodes and transorbs, resulting in absorption and dissipation of excess energy in the form of heat. 
     Commonly owned U.S. Pat. No. 6,398,779 discloses a sensor which measures the initial tissue impedance with a calibrating pulse which, in turn, sets various electrical parameters based on a look-up table stored in a computer database. The transient pulse width associated with each pulse measured during activation is used to set the duty cycle and amplitude of the next pulse. Generation of electrosurgical power is automatically terminated based on a predetermined value of the tissue impedance across the tissue. 
     Thus a need exists to develop an electrosurgical generator having improved control circuitry and/or processing for providing continuous control of various electrical parameters (e.g., pulse frequency and intensity, voltage, current, power) of the electrosurgical generator based upon sensing information obtained from the surgical site relating to tissue impedance, changes in tissue impedance, tissue temperature, changes in tissue temperature, surgical intent (e.g., cutting, coagulating, sealing), tissue type, leakage current, applied voltage, applied current, tissue hydration levels, tissue compliance, and/or tissue optic transmission. 
     SUMMARY 
     A closed-loop control system is disclosed for use with an electrosurgical generator that generates electrosurgical energy. The closed loop control system includes a user interface for allowing a user to select at least one pre-surgical parameter, such as the type of surgical instrument operatively connected to the generator, the type of tissue and/or desired surgical effect. A sensor module is also included for continually sensing at least one of electrical and physical properties proximate a surgical site and generating at least one signal relating thereto. The closed loop control system also includes a control module for continually receiving the selected at least one pre-surgical parameter from the user interface and each of the signals from the sensor module, and processing each of the signals in accordance with the at least one pre-surgical parameter using a microprocessor, computer algorithm and/or a mapping (e.g., look-up table, continuous mapping and equivalent). The control module generates at least one corresponding control signal relating to each signal from the sensor module, and relays the control signal to the electrosurgical generator for controlling the generator. 
     A method is also disclosed for performing an electrosurgical procedure at a surgical site on a patient. The method includes the steps of applying at least one electrical pulse (pulsed or continuous) to the surgical site; continually sensing electrical and physical properties proximate the surgical site; and varying pulse parameters of the individual pulses of the at least one pulse in accordance with the continually-sensed properties. 
     In another embodiment, a control system is provided, which includes a sensor module for sensing at least one property associated with a surgical site prior to a surgical procedure (pre-surgical), during the surgical procedure and/or after the surgical procedure (post-surgical). The sensor module generates at least one signal relating to the property back to the control module. A control module which is executable on a processor receives each signal and processes the signals utilizing a computer algorithm and/or a mapping and generates one or more control signals relating thereto. The control signal is then communicated to the electrosurgical generator for controlling the generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments will be described herein below with reference to the drawings wherein: 
         FIG. 1  is a schematic diagram of a closed-loop control system for use with an electrosurgical generator according to the present disclosure; 
         FIG. 2  is a schematic diagram of a sensor module for use with the closed-loop control system of  FIG. 1 ; 
         FIG. 3  is a flowchart illustrating a method of operation of the closed-loop control system according to the present disclosure; and 
         FIG. 4  is a block diagram of a dual loop control system in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference should be made to the drawings where like reference numerals refer to similar elements throughout the various figures. Referring to  FIG. 1 , there is shown a schematic diagram of one embodiment of the presently disclosed closed loop control system  100  for use with an electrosurgical generator  101 . Control system  100  includes a control module  102 , user interface  108  and sensor module  110 . The control module  102  is operatively connected to the electrosurgical generator  101 . The electrosurgical generator  101  preferably includes electrosurgical energy output stage  104  and a power supply  106 , where the output stage  104  receives power from the power supply  106  and delivers RF energy to a patient  112  via at least one electrode (not shown). As can be appreciated one or more electrodes may be used with the electrosurgical instrument for performing monopolar or bipolar surgery. 
     The sensor module  110  senses various electrical and physical parameters or properties at the operating site and communicates with the control module  102  to regulate the electrosurgical output from the output stage  104 . It is envisioned that the sensor module  110  may be configured to measure or “sense” various electrical or electromechanical conditions at the operating site such as: tissue impedance, changes in tissue impedance, tissue temperature, changes in tissue temperature, leakage current, applied voltage and applied current. Preferably, the sensor module  110  measures one or more of these conditions continuously or in “real time” such that the control module  102  can continually modulate the electrosurgical output according to a specific purpose or desired surgical intent. More particularly, analog signals provided by the sensor module  110  are converted to digital signals via an analog-to-digital converter (ADC)  114 , which in turn are provided to the control module  102 . 
     The control module  102 , thereafter, regulates the power supply  106  and/or the output stage  104  according to the information obtained from the sensor module  110 . The user interface  108  is electrically connected to the control module  102  to allow the user to control various parameters of the electrosurgical energy output to the patient  114  during surgery to manually set, regulate and/or control one or more electrical parameters of the delivered RF energy, such as voltage, current, power, frequency, amplified, and/or pulse parameters, e.g., pulse width, duty cycle, crest factor, and/or repetition rate depending upon a particular purpose or to change surgical intent. 
     The control module  102  includes at least one microprocessor capable of executing software instructions for processing data received by the user interface  108  and the sensor module  110  for outputting control signals to the output stage  104  and/or the power supply  106 , accordingly. The software instructions executable by the control module are stored in an internal memory in the control module  102 , an internal or external memory bank accessible by the control module  102  and/or an external memory, e.g., an external hard drive, floppy diskette, CD-ROM, etc. Control signals from the control module  102  to the electrosurgical generator  101  may be converted to analog signals by a digital-to-analog converter (DAC)  116 . 
     The power supply  106  is preferably a high voltage DC power supply for producing electrosurgical current, e.g., radiofrequency (RF) current. Signals received from the control module  102  control the magnitude of the voltage and current output by the DC power supply. The output stage  104  receives the output current from the DC power supply and generates one or more pulses via a waveform generator (not shown). As can be appreciated, the pulse parameters, such as pulse width, duty cycle, crest factor and repetition rate are regulated in response to the signals received from the control module  102 . Alternatively, the power supply  106  may be an AC power supply, and the output stage  104  may vary the waveform of the signal received from power supply  106  to achieve a desired waveform. 
     As mentioned above, the user interface  108  may be local to or remote from the control module  102 . A user may enter data such as the type of electrosurgical instrument being used, the type of electrosurgical procedure to be performed, and/or the tissue type upon which the electrosurgical procedure is being performed. It is envisioned that the closed loop control system  100 , in particular the sensor module, may include one or more smart sensors which provide feedback to the surgeon relating to one or more of these physical parameters. Furthermore, the user may enter commands, such as a target effective voltage, current or power level to be maintained, or a target response e.g., change in regulation of the power supply  106  and/or output stage  104 , to changes in sensed values, such as an effective change in voltage, current and/or power level as a function of the changes. Preferably, the user may also enter commands for controlling electrical parameters of the RF energy, delivered by the electrosurgical generator  101 , as described above. It is envisioned that default values are provided for the above target levels and target responses. 
     The sensor module  110  includes a plurality of sensors (not shown) strategically located for sensing various properties or conditions at or proximate points “A” and “B”. Sensors positioned at or proximate point “A” (hereinafter referred to as at point “A”) sense properties and/or parameters of electrosurgical output from output stage  104 , and/or properties, parameters or conditions prior to surgical effect of the currently administered electrosurgical energy during the surgical procedure. For example, sensors positioned at point “A” may be provided with or attached proximate the generator  101 . 
     Sensors positioned at or proximate point “B” (hereinafter referred to as at point “B”) sense parameters, properties and/or conditions at or across the operating site prior to the surgical procedure and/or in response to surgical effect during the surgical procedure. Preferably, one or more of these sensors may be included with the electrosurgical instrument, (e.g., on one end effector or opposing end effectors) or attached proximate the operating site. For example, optical sensors, proximity sensors, temperature sensors may be used to detect certain tissue characteristics, and electrical sensors may be employed to sense other parameters of the tissue or operating effects. It is noteworthy that point “A” may be located proximate the surgical site “B” at a location where the signals outputted by the generator  101  are propagated before they are applied or approximately when they are applied to the surgical site “B”. 
     The sensors are provided with leads or wireless means for transmitting information to the control module, where the information is provided directly to the control module  102 , and/or provided to the control module  102  via the sensor module  110  and/or the ADC  114 . The sensor module  110  may include means for receiving information from multiple sensors, and providing the information and the source of the information (e.g., the particular sensor providing the information) to the control module  102 . 
     With reference to  FIG. 2 , the inner-working components of the sensor module  110  are shown in greater detail. More particularly, the sensor module  110  preferably includes a real-time voltage sensing system  202  and a real-time current sensing system  204  for sensing real-time values for applied voltage and current at the surgical site “B”. The sensor module  110  also preferably includes a real-time voltage sensing system  206  and a real-time current sensing system  208  for sensing real-time values of signals returned from the patent at a point “A”. An RMS voltage sensing system  210  and an RMS current sensing system  212  are also included for sensing and deriving RMS values for applied voltage and current at the surgical site “B”, and an RMS voltage sensing system  214  and an RMS current sensing system  216  are included for sensing and deriving RMS values of signals at point “A”. A temperature sensing system  218  is preferably included for sensing tissue temperature at the surgical site “B”. Real-time and RMS current and voltage sensing systems are known in the art. The sensor module  110  may further include sensors (not shown) for sensing voltage and current output by the generator. 
     The measured or sensed values are further processed, either by circuitry and/or a processor (not shown) in the sensor module  110  and/or by the control module  102 , for deriving changes in sensed values and tissue impedance at the surgical site “B”. Tissue impedance and changes in tissue impedance may be determined by measuring the voltage and/or current across the tissue and/or calculating changes thereof over time, and comparing the voltage and current values to known and/or desired values associated with various tissue types for use by the control system  100  to drive electrical output to achieve desired impedance and/or change in impedance values. As can be appreciated, these known and/or desired values, tissue types and ranges may be stored in an internal look-up table, “a continuous value map” or in an external searchable memory. Commonly owned U.S. Pat. No. 6,398,779, U.S. Pat. No. 6,203,541, U.S. Pat. No. 5,827,271 and U.S. application Ser. No. 10,073,761 disclose methods for measuring tissue impedance, and are incorporated by reference herein in their entirety. 
     It is envisioned that deriving tissue impedance (or other physical and electrical parameters) from real-time value(s) provides the benefit of monitoring real-time tissue impedance and/or changes in tissue impedance. As the surgical procedure proceeds, it is believed that the tissue impedance fluctuates in response to removal and restoration of liquids from the tissue at the surgical site “B”. As the control module  102  monitors the tissue impedance and changes in tissue impedance (or other physical and electrical parameters) the control module  102  regulates the power supply  106  and output stage  104  accordingly for achieving the desired and optimal electrosurgical effect. 
     Before beginning an electrosurgical procedure, an operator of the electrosurgical instrument enters information via the user interface  108 . Information entered includes, for example, the type of electrosurgical instrument being used, the type of procedure being performed (i.e., desired surgical effect), the type of tissue, relevant patient information, and a control mode setting. The control mode setting determines the amount of or type of control that the control module  102  will provide. As mentioned above, one or more sensors (not shown) may also be included to automatically provide information to the control module  102  relating to tissue type, initial tissue thickness, initial tissue impedance, etc. 
     Exemplary modes include, but are not limited to, one or a combination of one or more of the following modes: a first mode wherein the control module  102  maintains a steady selected output power, current and/or voltage value at site “A”; a second mode wherein the control module  102  maintains a steady selected output power, current and/or voltage value at site “B”; a third mode wherein the control module  102  maintains a variable selected output power, current and/or voltage values at site “A” which is dependent upon (i.e., a function of) time value(s) and/or sensed parameter(s) or changes in sensed parameter(s) during the procedure; a fourth mode wherein the control module  102  maintains a variable selected output power, current and/or voltage values at site “B”, which is dependent upon (i.e., a function of) time value(s) and/or sensed parameter(s) or changes in sensed parameter(s) during the procedure. Functions performed on the time value(s) and sensed properties(s) include operations such as calculations and/or look-up operations using a table or map stored by or accessible by the control module  102 . The control module  102  processes the selected output power, current and voltage values, such as by performing calculations or table look up operations, to determine power control signal values and output control values. 
     It is also envisioned that, the control module  102  determines initial settings for control signals to the power supply  106  and the output stage  104  by using and/or processing operator-entered data or settings, performing calculations and/or accessing a look-up table stored by or accessible by the control module  102 . Once the electrosurgical procedure begins, the sensors of sensor module  110  sense various physical and electrical properties and provide feedback to the control module  102  through the ADC  114  as needed. The control module  102  processes the feedback information in accordance with the pre selected mode, as well as any additional operator-entered commands entered during the procedure. The control module then sends control information to the power supply  106  and the output stage  104 . It is contemplated that the generator  101  may be provided with override controls, to allow the operator to override the control signals provided by the control module  102 , if needed, e.g., by entering override commands via the user interface  108 . 
       FIG. 3  shows a flow chart illustrating a method for controlling operation of the closed loop control system  100  during an electrosurgical procedure in accordance with an embodiment of the present disclosure. At step  302 , the method includes continually sensing various physical and electrical properties at the surgical site. At step  304 , the sensed properties are continually processed. At step  306 , power supply control signals are continually generated for controlling the magnitude of the signals output by the electrosurgical generator and output stage control signals are continually generated, for controlling pulse parameters of the output signals in accordance with the continually-processed sensed properties. 
     It is contemplated that the sensor module  110  further includes a proximity sensor for sensing (measuring) tissue thickness proximate the surgical site “B”, and generating a tissue thickness value. An initial tissue thickness value may be provided to the control module  102  as a pre-surgical parameter. Sensed real time tissue thickness values and/or changes in tissue thickness values over time (.DELTA.[difference] thickness/.DELTA.[difference] time) may further be provided to the control module  102  during the surgical procedure, where the control module  102  modulates the electrical surgical output in accordance with the sensed real time tissue thickness values and/or changes in tissue thickness values over time. 
     It is further contemplated that the sensor module  110  further includes an additional sensor module (or the same sensor module  110  with additional capabilities) for sensing (measuring) tissue moisture (which is often indicative of tissue type) and generating a moisture content value and/or determining tissue type. It is envisioned that moisture content is determined from tissue compliance data or optical clarity. The additional sensor module may include an infrared, optical, or light sensor  220  for sensing (measuring) light or energy generated by a source  118 , such as an infrared or other light source, which is transmitted through or reflected from the tissue, where the sensed value is indicative of tissue moisture content and/or tissue type of tissue proximate the surgical site “B”. An initial tissue moisture content value and/or tissue type may be provided to the control module  102  as a pre-surgical parameter. Sensed real time moisture content values and/or changes in moisture content over time (Δ(difference) moisture content/Δ·(difference) time) may further be provided to the control module  102  during the surgical procedure, where the control module  102  modulates the electrical surgical output in accordance with the sensed real time moisture content values and/or changes in moisture content values over time. 
     Accordingly, the present disclosure provides a closed loop control system  100  for providing continual control of the power supply  106  and the output stage  104  in response to “sensed” physical or electrical properties at the surgical site and/or proximate the output stage. 
     In an additional embodiment according to the present disclosure and in particular reference to  FIG. 4 , the control module  102  is provided with two control loops, an inner loop controlled by inner loop control module  402  and an outer loop controlled by outer loop control module  404 . Preferably, the inner and outer loop control modules  402 ,  404  are software modules executable by a processor of the control module  102 . The inner and outer loop control modules  402 ,  404  both receive signals generated by sensor module  110 . 
     The inner loop control module  402  controls the amount of current, voltage and/or power delivered to the tissue for controlling a variable, e.g., I, V or P, sensed at the tissue and/or calculated from sensed values, until a desired event occurs (a rapid dz/dt or impedance rise is achieved), e.g., an impedance value is reached preferably in the range of about 200 ohms to about 400 ohms. The control variable is controlled to change during the course of the seal cycle according to impedance value (or other sensed and/or derived values), as determined by generator limitations (power, current, voltage) and surgical limitations (maximum limits for application of energy to tissue). 
     The inner loop control module  402  continually receives real time sensed values, such as current I and voltage V, from the sensor module  110  and may perform calculations on the received values for deriving additional real time values, such as power P and impedance Z. A desired inner loop value for I, V, and/or P are obtained by accessing at least one stored inner mapping of continuous values  408 , look-up table or equivalent, where preferably the inner mapping  408  is in accordance with a function of impedance. Preferably, the inner loop control module  402  consults the inner mapping  408  for obtaining the desired inner loop value for the impedance currently being sensed and derived. 
     An algorithm is used to compare the real time value of I, V and/or P to the respective desired inner loop value and output an RF command to the electrosurgical generator  101  accordingly for achieving the desired inner loop value without exceeding the desired inner loop value, e.g., the RF command raises the target current, voltage and/or power output by the electrosurgical generator  101  when the real time value for I, V and/or P is lower than the respective desired inner loop value for I, V and/or P, and vice versa. It is contemplated that the RF command controls waveform parameters of electrosurgical energy output by the electrosurgical generator  101 , including current, power, voltage, duty cycle, frequency, waveshape, etc. It is further contemplated that the inner loop is used without the outer loop for achieving the desired tissue effect. 
     The outer loop control module  404 , layered over the inner loop control module  402 , provides additional control of a variable for reaching a desired output value or effect. For example, control of the variable may monitor/regulate the rate of change of impedance of the tissue (sensed and calculated). In different embodiments, the variables controlled may include temperature, rate of change of temperature, and/or the energy input to the tissue. Outer loop control module  404  continually receives sensed values, such as I, V and temperature T from the sensor module  110  at a time “t” and performs calculations on the sensed values and preferably stored values for deriving values such as rate of change of impedance and/or rate of change in temperature. For example, the value for change in impedance (dz/dt) is obtained in accordance with:
 
 dz/dt =( Z−Z _OLD)/( t−t _OLD);
 
 Z _OLD= Z;   (1)
         where Z is the impedance in accordance with values measured at time t; and   Z_OLD is the stored impedance in accordance with values measured at a previous time interval at time t_OLD       

     An outer loop desired value for the control variable is obtained by accessing a stored outer mapping of continuous values  406 , or alternatively a table or equivalent. The desired rate of change according to outer mapping  406  may be steady, or may depend on the stage of the seal cycle and change over time. The tissue is in a dynamic state during the seal procedure, and the outer loop monitors the rate of change throughout the procedure to determine the degree to which the desired rate of change is being achieved. When the control variable is temperature, a temperature map may be used for outer mapping  406  in which desired temperature is plotted versus time. When the control variable is rate of change in temperature, a rate of change in temperature map may be used for outer mapping  406  in which desired temperature is plotted versus time. Energy may be applied in a similar fashion, where an energy function can be calculated using equations derived for specific tissue types or using sensed values. 
     An algorithm is used to compare the real time sensed/calculated value of rate of change of impedance, temperature, rate of change of temperature and/or energy at time “t” to the respective desired outer value at time “t” obtained from the outer mapping  406  for determining if the desired outer value is met, and if not, for determining the ratio of the difference between the real time value and the desired outer value to the desired outer value. If the desired outer value is not being met, the outer loop module  406  generates a set point value which is provided to the inner loop module  402 . The set point value is raised when the real time value for rate of change of impedance, temperature and/or rate of change of temperature is lower than the respective desired outer value for rate of change of impedance, temperature and/or rate of change of temperature, and vice versa. 
     The set point value is preferably a ratio signal for altering the inner mapping  408  by raising or lowering a plotted curve of the inner mapping  408  along the y-axis. Preferably, the ratio signal is a proportional integral derivative (PID) control signal, as is known in the art. The inner loop control module  402  responds instantaneously by accessing the altered inner mapping  408  for obtaining a desired inner value from the outer loop, comparing the real time value of the control variable, generating an RF command for achieving the desired inner value without exceeding the desired inner value, and outputting the RF command accordingly to the electrosurgical generator  101  for controlling voltage, current and/or power needed for achieving a desired tissue effect. 
     Preferably the outer loop control module  404  uses the real time value of rate of change of impedance, temperature, rate of change of temperature, and/or total energy delivered to determine if a desired outer value has been reached which indicates completion of a seal. Upon determination of seal completion, a stop signal is generated for stopping the sealing process. Otherwise, the outer loop continues to monitor, receive and process sensed values from the sensor module  110 . 
     Control of I, V and/or P by the inner loop control module  402  improves system stability and control capabilities in low impedance ranges, e.g., 0-20 ohms, which are critical for seal initiation, particularly by avoiding a low-end impedance break point which induces oscillation and lack of system control. The outer loop control enhances the control module&#39;s ability to control sealing in accordance with desired trends or events, to change seal intensity by changing the rate of change of impedance, and to enhance uniform sealing of tissue, i.e., normalize tissue in terms of variability, including tissue hydration, volume and composition. With feedback control and continuous sensing of the tissue&#39;s condition, there is not a need to switch control variables (i.e., low/high end break points), which improves system stability as explained above. 
     It is contemplated that the control module  102  controls a module for producing resistive heat for regulating heat applied to the tissue for achieving the desired tissue effect instead of or in addition to controlling the electrosurgical output stage  104  and/or the power supply  106 . The control module  102  responds to sensed tissue temperature or other sensed properties indicative of tissue temperature, accesses at least one mapping, data table or equivalent using the sensed values for obtaining desired output current or resistivity values, and outputs a command signal for controlling output heat resistivity. Preferably, the module for producing resistive heat includes a current source and/or a variable resistor which are responsive to the command signal for outputting a desired current or providing a desired resistance, respectively. 
     It is envisioned that in another embodiment of the invention the control system includes a sensor module for sensing at least one property associated with a surgical site during at least one of a pre-surgical time prior to a surgical procedure, the surgical procedure and a post-surgical time following the surgical procedure for generating at least one signal relating thereto; and a control module executable on a processor for receiving said at least one signal and processing each of said signals using at least one of a computer algorithm and a mapping and generating at least one control signal in accordance with the processing, and providing the at least one control signal to the electrosurgical generator for controlling the generator. Preferably, the processing includes determining tissue type of tissue proximate the surgical site. 
     In an additional preferred embodiment, the sensor module  110  (or an additional sensor module (not shown)) senses at least one property as a pre-surgical condition, as a concurrent surgical condition and/or as a post-surgical condition. Preferably, the sensor module  110  senses at least two surgical conditions (or changes in surgical conditions over time) selected from pre-surgical, concurrent surgical and post-surgical conditions. Pre-surgical conditions include: degree of opaqueness of tissue proximate the surgical site; moisture content level of the tissue; and/or thickness of the tissue. Concurrent conditions include: degree of opaqueness of the tissue proximate the surgical site; moisture content level of the tissue; thickness of the tissue; temperature of the tissue; impedance of the tissue; current across the tissue; voltage across the tissue; power across the tissue; changes in degree of opaqueness of the tissue; changes in moisture content level of the tissue; changes in thickness of the tissue; changes in temperature of the tissue; changes in impedance of the tissue; changes in current across the tissue; changes in voltage across the tissue; and changes in power across the tissue. The post-surgical conditions include: degree of opaqueness of tissue; proximate the surgical site; moisture content level of the tissue; thickness of the tissue: temperature of the tissue; and impedance of the tissue. 
     Preferably, at least one property sensed during the post-surgical condition is indicative of the quality of a tissue seal formed during the surgical procedure. In a preferred embodiment the sensor module  110  includes a light sensor  220  for detecting light generated by a light source  118  and transmitted through (or reflected from) the tissue proximate the surgical site. A proximity sensor having sensing elements placed at opposite surfaces of the tissue may also be included for sensing the distance between the elements which is indicative of the tissue thickness. 
     Although this disclosure has been described with respect to preferred embodiments, it will be readily apparent to those having ordinary skill in the art to which it appertains that changes and modifications may be made thereto without departing from the spirit or scope of the disclosure. For example, it is contemplated that the control module  102  may include circuitry and other hardware, rather than, or in combination with, programmable instructions executed by a microprocessor for processing the sensed values and determining the control signals to be sent to the power supply  106  and the output stage  104 . 
     While several embodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosures 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 preferred embodiments.