Abstract:
A method for controlling energy applied to tissue as a function of at least one detected tissue property includes the initial step of applying energy to tissue. The method also includes the steps of initially adjusting the energy applied to tissue and determining a direction of change of the at least one detected tissue property. The method also includes the steps of subsequently adjusting the energy applied to tissue in the same direction as the initially adjusting step if the at least one detected tissue property is changing in a first direction and in the opposite direction to the initially adjusting step if the at least one detected tissue property is changing in a second direction and further adjusting the energy applied to the tissue in the same direction as the initially adjusting step if the at least one detected tissue property is changing in the second direction and in the opposite direction to the initially adjusting step if the at least one detected tissue property is changing in the first direction.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to electrosurgical apparatuses, systems and methods. More particularly, the present disclosure is directed to an algorithm that controls the application of energy to tissue. 
         [0003]    2. Background of Related Art 
         [0004]    Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical instrument to cut, coagulate, desiccate 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. 
         [0005]    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. In order to achieve one of these 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. 
         [0006]    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 loses 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. 
         [0007]    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. 
       SUMMARY 
       [0008]    The present disclosure relates to a method for controlling energy applied to tissue as a function of at least one detected tissue property. The method includes the initial step of applying energy to tissue. The method also includes the steps of initially adjusting the energy applied to tissue and determining a direction of change of the at least one detected tissue property. The method also includes the steps of subsequently adjusting the energy applied to tissue in the same direction as the initially adjusting step if the at least one detected tissue property is changing in a first direction and in the opposite direction to the initially adjusting step if the at least one detected tissue property is changing in a second direction and further adjusting the energy applied to the tissue in the same direction as the initially adjusting step if the at least one detected tissue property is changing in the second direction and in the opposite direction to the initially adjusting step if the at least one detected tissue property is changing in the first direction. 
         [0009]    In another embodiment, a method for controlling energy applied to tissue includes the initial step of applying energy to tissue. The method also includes the steps of setting a variable corresponding to at least one of a detected tissue property and a detected energy property to a first of two states and initially adjusting the energy applied to the tissue. The method also includes the steps of generating a control curve as a function of at least one of the detected properties and determining a direction of change of the control curve. The method further includes the steps of subsequently adjusting the energy applied to the tissue based on the first state if the control curve is changing in a first direction and subsequently setting the variable to the first state. The method further includes the steps of adjusting the energy applied to the tissue opposite to that of the subsequently adjusting step based on the first state if the control curve is changing in a second direction and subsequently setting the variable to the second state. The method further includes the steps of further adjusting the energy applied to the tissue opposite to that of the subsequently adjusting step based on the second state if the control curve is changing in the first direction and subsequently setting the variable to one of the first and second states. The method further includes the steps of adjusting the energy applied to the tissue in the same direction as the subsequently adjusting step based on the second state if the control curve is changing in the second direction and subsequently setting the variable to the first state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various embodiments of the present disclosure are described herein with reference to the drawings wherein: 
           [0011]      FIG. 1  is a schematic block diagram of a monopolar electrosurgical system according to the present disclosure; 
           [0012]      FIG. 2  is a schematic block diagram of a generator control system according to one embodiment of the present disclosure; 
           [0013]      FIG. 3  illustrates a relationship between a tissue conductivity vs. temperature curve and a tissue impedance vs. temperature curve for tissue undergoing treatment; 
           [0014]      FIG. 4A  is a schematic block diagram of a control algorithm according to embodiments of the present disclosure; 
           [0015]      FIG. 4B  is a schematic block diagram of a control algorithm according to an embodiment of the present disclosure; 
           [0016]      FIG. 5  is a schematic block diagram of a dual loop control system for use with the generator of  FIG. 2 ; 
           [0017]      FIG. 6A  is a schematic block diagram of a normal priority task algorithm according to embodiments of the present disclosure; 
           [0018]      FIG. 6B  is a schematic block diagram of a high priority task algorithm according to embodiments of the present disclosure; 
           [0019]      FIG. 6C  is a schematic block diagram of a low priority task algorithm according to embodiments of the present disclosure; 
           [0020]      FIG. 7A  is a schematic block diagram of a software system for use with the generator of  FIG. 2 ; 
           [0021]      FIG. 7B  is a schematic block diagram of a normal priority task algorithm for use with the software system of  FIG. 7A ; 
           [0022]      FIG. 7C  is a schematic block diagram of a high priority task algorithm for use with the software system of  FIG. 7A ; 
           [0023]      FIG. 7D  is a schematic block diagram of a normal priority task algorithm for use with the software system of  FIG. 7A ; and 
           [0024]      FIG. 8  is a schematic block diagram of a user interface for use with the generator and software system according to embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Particular embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Those skilled in the art will understand that the present disclosure may be adapted for use with either an endoscopic instrument or an open instrument. 
         [0026]    A generator according to the present disclosure can perform monopolar and bipolar electrosurgical procedures, including tissue ablation procedures. The generator may include a plurality of outputs for interfacing with various electrosurgical instruments (e.g., a monopolar active electrode, return electrode, bipolar electrosurgical forceps, footswitch, etc.). Further, the generator includes electronic circuitry configured for generating radio frequency power specifically suited for various electrosurgical modes (e.g., cutting, blending, division, etc.) and procedures (e.g., monopolar, bipolar, vessel sealing). 
         [0027]      FIG. 1  is a schematic illustration of a monopolar electrosurgical system according to one embodiment of the present disclosure. The system includes a monopolar electrosurgical instrument  2  including one or more active electrodes  3 , which can be electrosurgical cutting probes, ablation electrode(s), etc. Electrosurgical RF energy is supplied to the instrument  2  by a generator  20  via a supply line  4 , which is connected to an active terminal  30  ( FIG. 2 ) of the generator  20 , allowing the instrument  2  to coagulate, ablate and/or otherwise treat tissue. The energy is returned to the generator  20  through a return electrode  6  via a return line  8  at a return terminal  32  ( FIG. 2 ) of the generator  20 . The active terminal  30  and the return terminal  32  are connectors configured to interface with plugs (not explicitly shown) of the instrument  2  and the return electrode  6 , which are disposed at the ends of the supply line  4  and the return line  8 , respectively. 
         [0028]    The system may include a plurality of return electrodes  6  that are arranged to minimize the chances of tissue damage by maximizing the overall contact area with the patient P. In addition, the generator  20  and the return electrode  6  may be configured for monitoring so-called “tissue-to-patient” contact to insure that sufficient contact exists therebetween to further minimize chances of tissue damage. 
         [0029]    Not explicitly shown in  FIG. 1 , the generator  20  includes suitable input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator  20 , as well as one or more display screens for providing the surgeon with variety of output information (e.g., intensity settings, treatment complete indicators, etc.). The controls allow the surgeon to adjust power of the RF energy, waveform, and other parameters to achieve the desired waveform suitable for a particular task (e.g., tissue ablation). Further, the instrument  2  may include a plurality of input controls which may be redundant with certain input controls of the generator  20 . Placing the input controls at the installment  2  allows for easier and faster modification of RF energy parameters during the surgical procedure without requiring interaction with the generator  20 . 
         [0030]      FIG. 2  shows a schematic block diagram of the generator  20  having a controller  24 , a power supply  27 , an RF output stage  28 , and a sensor module  22 . The power supply  27  may provide DC power to the RF output stage  28  which then converts the DC power into RF energy and delivers the RF energy to the instrument  2 . The controller  24  includes a microprocessor  25  having a memory  26  which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microprocessor  25  includes an output port connected to the power supply  27  and/or RF output stage  28  which allows the microprocessor  25  to control the output of the generator  20  according to either open and/or closed control loop schemes. 
         [0031]    A closed loop control scheme generally includes a feedback control loop wherein the sensor module  22  provides feedback to the controller  24  (i.e., information obtained from one or more sensing mechanisms for sensing various tissue parameters such as tissue impedance, tissue temperature, output current and/or voltage, etc.). The controller  24  then signals the power supply  27  and/or RF output stage  2 S which then adjusts the DC and/or RF power supply, respectively. The controller  24  also receives input signals from the input controls of the generator  20  and/or instrument  2 . The controller  24  utilizes the input signals to adjust the power output of the generator  20  and/or instructs the generator  20  to perform other control functions. 
         [0032]    The microprocessor  25  is capable of executing software instructions for processing data received by the sensor module  22 , and for outputting control signals to the generator  20 , accordingly. The software instructions, which are executable by the controller  24 , are stored in the memory  26  of the controller  24 . 
         [0033]    The controller  24  may include analog and/or logic circuitry for processing the sensed values and determining the control signals that are sent to the generator  20 , rather than, or in combination with, the microprocessor  25 . 
         [0034]    The sensor module  22  may include a plurality of sensors (not explicitly shown) strategically located for sensing various properties or conditions, e.g., tissue impedance, voltage at the tissue site, current at the tissue site, etc. The sensors are provided with leads (or wireless) for transmitting information to the controller  24 . The sensor module  22  may include control circuitry which receives information from multiple sensors, and provides the information and the source of the information (e.g. the particular sensor providing the information) to the controller  24 . 
         [0035]    More particularly, the sensor module  22  may include a real-time voltage sensing system (not explicitly shown) and a real-time current sensing system (not explicitly shown) for sensing real-time values related to applied voltage and current at the surgical site. Additionally, an RMS voltage sensing system (not explicitly shown) and an RMS current sensing system (not explicitly shown) may be included for sensing and deriving RMS values for applied voltage and current at the surgical site. 
         [0036]    The measured or sensed values are further processed, either by circuitry and/or a processor (not explicitly shown) in the sensor module  22  and/or by the controller  24 , to determine changes in sensed values and tissue impedance. Tissue impedance and changes therein may be determined by measuring the voltage and/or current across the tissue and then calculating changes thereof over time. The measured and calculated values may be then compared with known or desired voltage and current values associated with various tissue types, procedures, instruments, etc. This may be used to drive electrosurgical output to achieve desired impedance and/or change in impedance values. As the surgical procedure proceeds, tissue impedance fluctuates in response to adjustments in generator output as well as removal and restoration of liquids (e.g., steam bubbles) from the tissue at the surgical site. The controller  24  monitors the tissue impedance and changes in tissue impedance and regulates the output of the generator  20  in response thereto to achieve the desired and optimal electrosurgical effect. 
         [0037]    In general, the system according to the present disclosure regulates the application of energy to achieve the desired tissue treatment based on properties (e.g., electrical and/or physical) of tissue. In embodiments, the application of energy to tissue is regulated based on the electrical conductivity of that tissue as a function of the tissue temperature. Tissue conductivity as a function of tissue temperature may be represented as a conductivity vs. temperature curve. Tissue conductance is inversely related to tissue impedance if the material tissue properties (e.g., length of tissue, area of tissue, etc.) remain constant. Specifically, tissue conductance and tissue impedance are related by the following equation: 
         [0000]        Z=L /(σ* A ); 
         [0038]    where Z is impedance of tissue undergoing treatment: 
         [0039]    L is the length of tissue undergoing treatment; 
         [0040]    σ is electrical conductance of tissue undergoing treatment; and 
         [0041]    A is the surface area of tissue undergoing treatment. 
         [0042]      FIG. 3  illustrates the relationship between a typical conductivity vs. temperature curve and a corresponding (i.e., over the same temperature range) impedance vs. temperature curve for tissue undergoing ablation (e.g. utilizing electrosurgical instrument  2 ). The illustrated curves demonstrate that, for tissue undergoing ablation, the lowest impedance value on the impedance vs. temperature curve corresponds to the highest conductance value on the conductance vs. temperature curve. 
         [0043]    The conductance vs. temperature curve for tissue undergoing ablation may be dynamically changing due a variety of factors such as, for example, the changes in energy applied to tissue. The present disclosure provides for a control algorithm that actively tracks this curve to allow for the application of energy to maintain an optimal positioning on the curve (e.g., peak tissue conductance) despite the dynamic nature of the curve. 
         [0044]      FIG. 4  shows a flow chart illustrating a control algorithm  200  for regulating the application of energy to tissue, according to one embodiment of the present disclosure. In embodiments, algorithm  200  may be a software application residing in the memory  26  and executable by the controller  24  (e.g., via the microprocessor  25 ). 
         [0045]    The control algorithm defines a state variable (SV) to express a real-time value of one or more physical properties of the tissue undergoing ablation (e.g., tissue impedance, voltage across tissue, current through tissue) and/or one or more electrical properties related to the applied energy (e.g., amplitude and/or phase of power applied to tissue, etc.). In embodiments, the SV may be defined in any one or more so-called “states”. For example, the SV may represent the real-time state of tissue resistance as being either “decreasing” or “rising”. 
         [0046]    In the embodiment illustrated in  FIG. 4A , the algorithm  200  initially defines the SV as decreasing and increases the application of energy to tissue (e.g., the controller  24  increases the output of the generator  20 ). Subsequently, the control algorithm  200  enters a switch loop  210  wherein the algorithm  200  continuously monitors the SV to be in any one of two states (e.g., decreasing or rising). Based on the detected state of the SV, the algorithm  200  switches between two control loops to control the application of energy to tissue. 
         [0047]    In the illustrated embodiment, the algorithm  200  enters one of two control loops  220  and  230  to correspond to decreasing and rising states of the SV, respectively, as detected by the algorithm  200  via the switch loop  210 . More specifically, the algorithm  200  enters a decreasing case control loop  220  if the switch loop  210  detects the state of the SV as decreasing. Upon entering control loop  220 , the algorithm  200  continuously detects (e.g., via the sensor module  22 ) the slope of the control curve (e.g., the impedance vs. temperature curve of  FIG. 3 ). If the detected slope of the control curve is negative, the algorithm  200  increases the application of energy to tissue (e.g., the controller  24  increases the output of the generator  20 ) and subsequently defines the SV as decreasing. In this manner, the decreasing case control loop  220  is repeated as long as the SV is defined as decreasing and the slope of the control curve is negative. 
         [0048]    Conversely, if the detected slope of the control curve is not negative (e.g., slope=0 or slope&gt;0), the algorithm  200  decreases the application of energy to tissue and subsequently defines the SV as rising. In this manner, the switch loop  210  detects the SV as rising and, thus, triggers the algorithm  200  to enter a rising case control loop  230 . 
         [0049]    Upon entering the rising case control loop  230 , the algorithm  200  continuously detects the slope of the control curve. The rising case control loop  230  is configured such that the response to the detected slope of the control curve is directly opposite to that of the decreasing case control loop  220 . More specifically, if the detected slope of the control curve is negative during the rising case control loop  230 , the algorithm  200  continues to decrease the application of energy to tissue (e.g., the controller  24  further decreases the output of the generator  20 ) and subsequently defines the SV as decreasing. Continuing to decrease the application of energy to tissue in this scenario allows the algorithm  200  to effectively track the optimal point on the control curve (e.g., lowest possible tissue impedance as a function of temperature). Conversely, if the detected slope of the control curve is not negative (e.g., slope=0 or slope&gt;0), the algorithm  200  increases the application of energy to tissue and subsequently defines the SV as decreasing. Increasing the application of energy to tissue in this scenario allows the algorithm  200  to effectively deliver the maximum energy to tissue. In either scenario (i.e., slope&lt;0; and slope&gt;0) of the rising case control loop  230 , the SV is reset to decreasing such that the algorithm  200  enters or re-enters the decreasing case control loop  220 . In this way, the algorithm  200  aggressively applies energy to tissue to achieve maximal tissue heating while tracking the optimal point on the control curve (e.g., the lowest possible tissue impedance). 
         [0050]    In embodiments wherein a tissue impedance vs. temperature curve (e.g.,  FIG. 3 ) is utilized as the control curve, the decreasing case control loop  220  recognizes that a slope detected as negative corresponds to the tissue impedance as decreasing and, thus, the algorithm  200  increases the application of energy to tissue accordingly and re-enters the decreasing case control loop  220 . Conversely, the decreasing case control loop  220  recognizes that a slope detected as not negative corresponds to the tissue impedance as rising and, thus, the algorithm  200  decreases the application of energy to tissue accordingly and enters the rising case control loop  230 . The rising case control loop  230  recognizes that a slope detected as negative corresponds to the tissue impedance as decreasing and, thus, the algorithm  200  further decreases the application of energy to tissue to ensure the algorithm  200  finds the lowest possible tissue impedance. Conversely, the rising case control loop  230  recognizes that a slope detected as not negative corresponds to the tissue impedance as not changing or continuing to rise and, thus, the algorithm  200  increases the application of energy to tissue to ensure that the maximum energy is delivered to tissue. 
         [0051]    In embodiments, in the case of the SV being defined as rising, if the slope of the control curve is negative, energy applied to tissue is decreased and the SV is reset to rising rather than decreasing, as is the case in the embodiment illustrated in  FIG. 4A .  FIG. 4B  shows a flow chart illustrating an alternative algorithm  300  according to embodiments of the present disclosure. The algorithm  300  operates similarly to the algorithm  200  illustrated in  FIG. 4A  and is only described to the extent necessary to illustrate the differences between the embodiments. The algorithm  300  utilizes the identical initialization as that of the algorithm  200  illustrated in  FIG. 4A . Further, the algorithm  300  includes a switch loop  310  configured to switch between two control loops, namely, a decreasing case control loop  320  and a rising case control loop  330  corresponding to the SV being defined as decreasing and rising, respectively. 
         [0052]    As illustrated in  FIGS. 4A and 4B , the difference between the algorithms  200  and  300  lies in the respective rising case control loops  230  and  330 . In the case of the SV being defined as rising in the switch loop  310  of the algorithm  300 , if the slope of the control curve is negative, the algorithm  300  decreases the energy applied to tissue and maintains the SV as rising rather than reset to decreasing, as is the case in the algorithm  200  embodied in  FIG. 4A . In this manner, the rising case control loop  330  will continue to loop until the slope of the control curve is not negative (e.g., slope=0 or slope&gt;0). In embodiments wherein a tissue impedance vs. temperature curve (e.g.,  FIG. 3 ) is utilized as the control curve, if the tissue impedance is decreasing (e.g., slope of the control curve&lt;0), the rising case control loop  330  will continue until the algorithm  300  detects that the tissue impedance is not negative (i.e. slope of the control curve&gt;0). Upon detection that the tissue impedance is not negative, the algorithm  300  increases the application of energy to tissue and resets the SV to decreasing. 
         [0053]    In embodiments, a high priority control loop may be layered over the algorithms  200  and  300  to run concurrently therewith. During the ablation of tissue, conditions may exist that lead to continued energy increases. Such energy increases may cause tissue properties (e.g., impedance) to rise and/or fall outside of the peak conductance range or into a so-called “runaway state.” The high priority control loop monitors the control curve for the runaway state and adjusts the application of energy (e.g., the controller  24  decreases the output of the generator  20 ) accordingly. More specifically, the high priority loop interrupts the algorithm (e.g., algorithm  200  and  300 ) to check for the runaway state, and decreases the application of energy in the event that such a state is detected. In embodiments wherein an impedance vs. temperature curve ( FIG. 3 ) is utilized as the control curve, the high priority loop continually interrogates whether tissue impedance is rising more than a pre-determined threshold value. The pre-determined threshold value may be pre-determined by the surgeon via the generator  20  input controls and/or reside in the memory  26  for execution by the microprocessor  25 . 
         [0054]    With reference to  FIG. 5 , another embodiment of the present disclosure is shown. In the illustrated embodiment, energy application is regulated by the controller  24  pursuant to a closed loop control system  400  stored within the memory  26 . The system  400  continuously monitors tissue impedance as an indicator of tissue conductance and automatically adjusts output to create the lowest possible tissue impedance and/or the highest possible tissue conductance. Upon the initialization of a given procedure or at some predetermined time delay thereafter, the system  400  processes and stores a baseline impedance Z BASE  determined by the sensor  24 . The system  400  determines deviations in average tissue impedance from the baseline impedance Z BASE  as a function of time and adjusts generator  20  output in response to such deviations. This allows peak tissue conductance to be maintained independent of tissue changes, variations in generator  20  output, and device accessory selection. 
         [0055]    Further, the system  400  continually interrogates whether detected impedance has risen above a threshold value, and reduces generator output in response to any such threshold breaches. Finally, the system  400  may commence a treatment termination sequence upon detection of specific tissue conditions which indicate a completed treatment. Treatment completion may be indicated by an equilibrium between the level of energy applied to the tissue (e.g., via the forceps  10 ) and the level of energy dissipated from the tissue. Based on this equilibrium, the system  400  determines that the detected tissue impedance has achieved its lowest sustainable level and has remained at that level without change for a substantial amount of time. 
         [0056]    Accordingly, the closed loop control system  400  of the present disclosure provides continual control of the power supply  27  and/or the output stage  28  ( FIG. 2 ) in response to so-called “sensed” physical or electrical properties at the surgical site and/or proximate the output stage  2 S. In embodiments of the present disclosure and in particular reference to  FIG. 5 , the controller  24  may be provided with and/or in operative communication with an inner loop control module  402  and an outer loop control module  404  through which various priority tasks (e.g., loops) may be executed. The inner and outer loop control modules  402 ,  404  may be software modules executable by the microprocessor  25  of the controller  24  ( FIG. 2 ) and both may receive signals generated by the sensor module  22 . 
         [0057]    The inner and outer loop control modules  402 ,  404  continually receive real-time sensed values, such as current I and voltage V, from the sensor module  22  as well as a time t. The modules  402 ,  404  perform calculations on the sensed values to derive additional real-time values, such as power P and impedance Z. For example, the value for change in impedance (dz/dt) is obtained in accordance with: 
         [0000]        dz/dt =( Z−Z _OLD)/( t−t _OLD); and 
         [0000]      Z_OLD=Z; 
         [0058]    where Z is the impedance in accordance with values measured at time t; and 
         [0059]    Z_OLD is the stored impedance in accordance with values measured at a previous time interval at time t_OLD. The inner and outer loop control modules  402 ,  404  process the real-time sensed values and output an RF command to the generator  20  which controls the output power needed for achieving a desired tissue effect. 
         [0060]      FIG. 6A  shows a normal priority task  410  controlled by the inner loop control module  402  for automatic power adjustment to achieve peak conductance. The normal priority task  410  adjusts the power output by the generator  20  to continuously achieve peak tissue conductance (i.e., lowest tissue impedance) irrespective of changes to tissue (e.g., thickness, bubble formation, temperature, etc.), variations in generator output (e.g., manual adjustment of generator output power), and device and/or accessory selection (e.g., monopolar device, bipolar forceps, etc.). The inner loop control module  402  utilizes the normal priority task  410  to continuously monitor average tissue impedance over a period of time (e.g., a dZ AVE /dt waveform) as an indicator of tissue conductance since tissue conductance is inversely proportional to tissue impedance. The module  402  then automatically adjusts the output power of the generator  20  to provide the lowest possible tissue impedance and, thus, the highest possible tissue conductance. The normal priority task  410  is characterized by a dual-control loop that continuously interrogates (e.g., via the sensor module  22 ) the slope of the average impedance waveform over a particular window of time and adjusts the output power of the generator  20  in response to the direction of the slope (e.g., m=0, m&lt;0, or m&gt;0) detected over the duration of that particular window of time. 
         [0061]    During operation of the normal priority task  410 , an initial increase of the output power of the generator  20  is made over the duration of a first sample window of time (e.g., a user-defined time delay). During the first sample window of time, the sensor module  22  determines a first slope of the average impedance waveform in response to the initial increase in output power of the generator  20 . During a second sample window of time, a second adjustment is made to the power output by the generator  20  and a second slope of the average impedance waveform is determined. If the second slope of the average impedance waveform is substantially the same as the first slope of the average impedance waveform, a third adjustment to the output power of the generator  20  is made. In this scenario, the second adjustment made to the power output by the generator  20  is a “reverse” adjustment to that of the first adjustment made to the power output by the generator  20 . 
         [0062]    During a first sampled time delay “t 1 ,” the tissue ablation procedure is activated (e.g., by pressing of a foot pedal or handswitch) and a host processor (e.g., microprocessor  5 ) activates the normal priority task  410  to monitor changes in average impedance as a function of time (e.g., dz/dt). More specifically, an initial increase ΔP i  in output power of the generator  20  is made during time delay t 1  while the sensor module  22  continuously monitors a first sampled average tissue impedance Z 1   AVE  to detect changes therein in response to the initial increase ΔP i  in output power of the generator  20  as a function of time delay t 1 . The change in Z 1   AVE  may be embodied as a waveform interpreted by the inner loop control module  402  which represents the first average impedance Z 1   AVE  as a function of time delay t 1  (e.g., dZ 1   AVE /dt 1 ). In this manner, the normal priority task  410  may monitor the slope of the impedance waveform as an indication of changes in average tissue impedance over a sample window of time. 
         [0063]    In embodiments, time delay  11  may be up to four (4) seconds during which the initial increase ΔP i  in output power of the generator  20  is made at a rate of twenty (20) watts per second. In this configuration, the output power of the generator  20  may be gradually increased to eighty watts (80) over the duration of the time delay  11 . 
         [0064]    If, over the duration of the first time delay t 1 , the first average impedance Z 1   AVE  decreases (e.g., the slope of dZ 1   AVE /dt 1 &lt;0) in response to the initial increase ΔP i  in power output, the controller  24  makes a first adjustment ΔP 1  to increase the power output over the duration of a second time delay “t 2 .” In certain embodiments, the second time delay t 2  may be up to four (4) seconds to allow the sensor module  22  to record a sufficient sample of data related to changes in tissue impedance. 
         [0065]    Conversely, if over the duration of the first time delay t 1 , the first average tissue impedance Z 1   AVE  either increases or is unchanged (e.g., the slope of dZ 1   AVE /dt 1 &gt;0) in response to the initial increase ΔP i  in power output, the controller  24  makes a second adjustment ΔP 2  to decrease the power output over the duration of the second time delay t 2 . 
         [0066]    As the controller  24  makes the second adjustment ΔP 2  to decrease the power output over the duration of the second time delay t 2 , the sensor module  22  continuously monitors for changes in a second sampled average tissue impedance Z 2   AVE . In embodiments, the second adjustment ΔP 2  may be as much as a five (5) watt decrease over the duration of the second time delay t 2 . If, over the duration of the second time delay t 2 , the second average tissue impedance Z 2   AVE  either increases or is unchanged (e.g., the slope of dZ 1   AVE /dt 1 &gt;0) in response to the second adjustment ΔP 2  to decrease the power output, the controller  24  makes a third adjustment ΔP 3  to increase the power output by the generator  20  over the duration of a third time delay “t 3 .” The normal priority task  410  is thereafter repeated. If, over the duration of the second time delay t 2 , the second average tissue impedance Z 2   AVE  decreases (e.g., the slope of dZ 1   AVE /dt 1 &lt;0) in response to the second adjustment ΔP 2  to decrease the power output, the controller  24  makes a fourth adjustment ΔP 4  to decrease the power output over the duration of the third time delay t 3 , and the normal priority task  410  is repeated. 
         [0067]    In this way, the normal priority task  410  performs a reverse adjustment of power output by the generator  20  over the duration of the third time delay t 3  relative to the adjustment made over the duration of the second time delay t 2  in response to the same direction of the change (e.g., same slope direction) in the average tissue impedance as detected by the controller  24  during the first time delay t 1 . That is, over the duration of the first time delay t 1 , if the first average tissue impedance Z 1   AVE  either increases or is unchanged in response to the initial increase ΔP i  in power output the controller  24  makes the second adjustment ΔP 2  to decrease the power output over the duration of the second time delay t 2 . Conversely, if the second average tissue impedance Z 2   AVE  either increases or is unchanged in response to the second adjustment ΔP 2  to decrease the power output the controller  24  makes the third adjustment ΔP 3  to increase the power output over the duration of the third time delay t 3 . It is in this manner that the normal priority task  410  operates to control the power output by the generator  20  to achieve the highest possible tissue conductance and, thus, the lowest possible tissue impedance throughout the duration of a given procedure. 
         [0068]    In embodiments, the duration of the time delays t 1 , t 2 , t 3 , the amount of the power adjustments ΔP i , ΔP 1 , ΔP 2 , ΔP 3 , ΔP 4 , the rate at which the power adjustments ΔP i , ΔP 1 , ΔP 2 , ΔP 3 , ΔP 4  are made, and the maximum level to which the power output may be increased by ΔP i  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0069]    The outer loop control module  404  is layered over the inner loop control module  402  and runs concurrently therewith to provide additional control of the generator  20  to reach a desired output value or effect. The outer loop control module  404  utilizes a high priority task  420  to prevent tissue properties (e.g., impedance) from rising and/or falling outside the peak conductance range or a so-called “run-away state.” Upon activation, the sensor module  22  records a baseline impedance value Z BASE  and transmits this value to the controller  24  for storing in the memory  26 . The high priority task  420  processes the base impedance Z BASE  stored in the memory  26  and continually monitors deviations in average tissue impedance Z AVE  from the base impedance Z BASE  as a function of time (e.g., a dz/dt waveform). The high priority task  420  compares the deviations to a threshold impedance value Z MAX , and automatically adjusts the output power of the generator  20  to counteract increases in the average impedance Z AVE  that breach the threshold value Z MAX . That is, if a rise in average tissue impedance Z AVE  over a sample window of time exceeds the threshold value Z MAX , the output power of the generator  20  is decreased by the controller  24 . In embodiments, the threshold impedance value Z MAX  may be determined by detecting a change in average impedance Z AVE : over some period of time (e.g., an average change of 20 ohms over seven seconds), and comparing this change in average impedance Z AVE  to the base impedance value Z BASE  stored in memory  26 . 
         [0070]      FIG. 6B  shows the high priority task  420  controlled by the outer loop control module  404  for automatic power adjustment based on detected rises in average tissue impedance Z AVE  that exceed the threshold impedance value Z MAX . The high priority task  420  is layered over the normal priority task  410  and runs concurrently therewith. Specifically, the outer loop control module  404  utilizes the high priority task  420  to continuously monitor average tissue impedance as a fund ion of time (e.g., a dz/dt waveform). The average tissue impedance may be an average peak impedance Z PEAK  sampled over a substantially short period of time, e.g., 0.05 seconds. If the rise in peak impedance Z PEAK  exceeds or is equal to a predetermined impedance value ΔP (e.g., 20 ohms nominal) above the base impedance Z BASE ; the controller  24  makes a fifth adjustment ΔP 5  to decrease the power output. 
         [0071]    In embodiments, the output level of the generator  20  at which the base impedance Z BASE  will be determined, the impedance value ΔP, and the fifth adjustment ΔP 5  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0072]    In embodiments, the outer loop control module  404  may utilize a low priority task  430  to determine when to terminate the power output by the generator  20  to end a given tissue treatment. The low priority task  430  is based on a determination that an equilibrium exists between the energy applied by the generator  20  (e.g., via the forceps  10 ) and the energy that is dissipated from the tissue site. The low priority task  430  may determine whether average tissue impedance has achieved its lowest sustainable level and remains at that level without substantial change for a substantial period of time. 
         [0073]    Discussed below with reference to  FIG. 6C , the low priority task  430  is layered over the normal priority task  410  and runs concurrently therewith. The outer loop control module  404  utilizes the low priority task  430  to continuously monitor average tissue impedance as a function of time (e.g., over the duration of a given procedure). Specifically, the controller  24  continually receives a current average tissue impedance value Z AVEn  from the sensor module  22 . Upon processing the current average tissue impedance value Z AVEn , the low priority task  430  compares the current tissue impedance value Z AVEn  to a historical tissue impedance value Z AVEn-1  stored in the memory  26  from the previous iteration through the low priority task  430 . Thereafter, the current average tissue impedance value Z AVEn  is stored in the memory  26  as the historical tissue impedance value Z AVEn-1 . In operation, after the expiration of a fourth time delay “t 4 ” following the activation of the generator  20 , the low priority task  430  monitors average tissue impedance for pertain criteria that may indicate that a given treatment is complete and, thus, the power output may be terminated, in certain embodiments of the present disclosure, this criteria may include determining that the current tissue impedance value Z AVEn  is substantially equivalent to the historical tissue impedance value Z AVEn-1  stored in the memory  26  over the duration of a fifth time delay “t 5 ” after the generator  20  is activated. In response, the generator  20  may continue to output power over the duration of a sixth time delay “t 6 .” Following the time delay t 6 , the generator  20  is turned “off” and the procedure is terminated. In certain embodiments of the low priority task  430 , the power output may be terminated immediately by the controller  24  upon the expiration of the fifth time delay t 5 . Alternatively, output may be adjusted by the controller  24  to a predetermined level by the user (e.g., via the user inputs of the generator  20  and/or a software-based user interface). 
         [0074]    In embodiments, the duration of the time delays t 4 , t 5 , t 6  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0075]    According to embodiments of the present disclosure, the interrogation of impedance may be achieved via single pole recursive filtering.  FIG. 7A  illustrates a software system  500  embedded in the memory  26  and executed by the microprocessor  25  which utilizes a normal priority task  510 , a high priority task  520 , and low priority task  530  to control generator  20  output based on changes in average tissue impedance as a function of time. Each task  510 ,  520 ,  530  processes averaged impedance data received from a plurality of single pole recursive impedance filters that continuously filter and/or average tissue impedance data sensed by the sensor module  22 . 
         [0076]    In the illustrated embodiment, eight impedance filters Zf 1 -Zf 8  are used in conjunction with the software system  500 . Each of the impedance filters Zf 1 -Zf 8  may be formatted for use with the following data averaging formula (I): 
         [0000]        ZfX   n   =Z in* A+Zf   n-1   *B    
         [0077]    A and B are dependent on a time constant and may be specified by the user, via the input controls of the generator  20 , for each particular impedance filter ZfX. When calculating A and B, the following formulas may be used: 
         [0000]        B=c ̂(−1/number of samples); 
         [0000]        A= 1− B.    
         [0078]    The sample rate may also be specified by the user for calculating the number of samples. In formula (1), Zin is the new impedance value (e.g., Z RMS ) just calculated, and ZfX n-1  is the filtered impedance, for the filter number specified by X, from the previous iteration through the loop, and ZfX is the new filtered impedance value for the filter number specified by X. 
         [0079]    Referring now to  FIG. 7B , the normal priority task  510  is made up of three states, namely, an initializing state  550 , a run state  560 , and a peak state  570 . During the initializing state  550 , the ablation procedure is activated (e.g., by pressing of a foot pedal or handswitch) and a host processor (e.g., microprocessor  5 ) activates the software system  500  for monitoring the plurality of impedance filters Zf 1 -Zf 8 . Upon activation of the state  550 , a first timer “T 1 ” is initialized to run concurrently with the initializing state  550 . The first timer T 1  may be set by the user (e.g. via the user inputs of the generator  20  and/or a software-based user interface) as the amount of time the software system  500  waits during the state  550 , after initial activation, before interrogating the plurality of impedance fillers, as will be discussed in further detail below. 
         [0080]    Once turned on, the generator  20  operates at a baseline level of power P BASE . It is at this baseline level of power Phase that the sensor module  22  records a baseline impedance Z BASE  and transmits this value to the controller  24  for storing in the memory  26 . Once the baseline impedance Z BASE  has been recorded, the power output by the generator  20  is ramped by the controller  24  to an initial level P INT . The user may be able to specify (e.g., via the user inputs of the generator  20  and/or a software-based user interface) the rate at which the power output by the generator  20  is ramped as well as a maximum level of power P MAX  to which the generator  20  may be ramped. The power output by the generator  20  is ramped by the controller  24  until either the first timer T 1  expires or P MAX  is reached. 
         [0081]    Upon expiration of the first timer T 1 , the normal priority task  510  stores the baseline impedance Z BASE  into the memory  26  as impedance value Zf 1   n-1  and enters the run state  560 . Once the run state  560  is initialized, the normal priority task  510  starts a second timer “T 2 ” which runs concurrently with the run state  560 . The second timer T 2  may be predetermined by the user (e.g., via the user inputs of the generator  20  and/or a software-based user interface) as the amount of lime the normal priority task  510  operates in the run state  560  prior to interrogating the plurality of impedance filters for average impedance data. 
         [0082]    If the run state  560  is entered from the initializing state  550 , the software system  500  immediately calculates the difference between the current filtered impedance Zf 2   n  and the previous filtered impedance Zf 1   n-1  and compares this difference to a first impedance reference Zdelta 1 . The first impedance reference, Zdelta 1 , is the amount of change from the previous filtered impedance Zf 1   n-1  to the current filtered impedance Zf 2   n  which is a threshold for triggering an increase or decrease in power output by the generator  20 . The impedance reference Zdelta 1  may be predetermined (e.g., via the user inputs of the generator  20  and/or a software-based user interface) by the user. 
         [0083]    If the difference between the current filtered impedance Zf 2   n  and the previous filtered impedance Zf 1   n-1  is less than or equal to Zdelta 1 , the controller  24  makes a first adjustment P 1  to increase the power output by the generator  20  and the normal priority task  510  reenters the run state  560 . Upon reentering the run state  560 , the software system  500  restarts the second timer T 2  and waits for the second timer to expire before interrogating impedance filters Zf 1  and Zf 2  for filtered impedance data. 
         [0084]    If the difference between the current filtered impedance Zf 2   n  and the previous filtered impedance Zf 1   n-1  is greater than Zdelta 1 , the controller  24  makes a second adjustment P 2  to decrease the power output by the generator  20  and the normal priority task  510  enters the peak state  570 . Upon entering the peak state  570 , the software system  500  starts a third timer “T 3 ”, as will be discussed in further detail below. 
         [0085]    In embodiments, the duration of the third timer T 3 , the amount of the first and second power adjustments P 1 , P 2  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0086]    Upon exiting the run state  560 , the software system  500  stores the current filtered impedance value Zf 1   n  into the memory  26  as the previous filtered impedance Zf 1   n-1  and stores the filtered impedance Zf 3   n  into the memory  26  as the previous filtered impedance Zf 3   n-1 . That is, prior to the exiting of the run state  560 , the current filtered impedances Zf 1   n  and Zf 3   n  determined during the present iteration through the run state  560  become the respective previous filtered impedances Zf 1   n-1  and Zf 3   n-1  once the run state  560  is reentered (i.e. once the present iteration through the run state  560  becomes the previous iteration through the run state). 
         [0087]    The third timer T 3  is initialized by the software system  500  to coincide with the initialization of the peak state  570  and to run concurrently therewith. Once the third timer T 3  expires, the software system  500  calculates the difference between the current filtered impedance Zf 4   n  and the previous filtered impedance Zf 3   n-1  and compares this difference to a second impedance reference Zdelta 2 . The second impedance reference. Zdelta 2 , is the amount of change from the previous filtered impedance Zf 3   n-1  to the current filtered impedance Zf 4   n  which is a threshold for triggering an increase or decrease in power output. 
         [0088]    If the difference between the current filtered impedance Zf 4   n  and the previous filtered impedance Zf 3   n-1  is less than the second impedance reference Zdelta 2 , the controller  24  makes a third adjustment P 3  to decrease the power output and the normal priority task  510  reenters the run state  560  and the software system  500  restarts the second timer T 2 . 
         [0089]    If the difference between the current filtered impedance Zf 4   n  and the previous littered impedance Zf 3   n-1  is greater than or equal to the second impedance reference Zdelta 2 , the controller  24  makes a fourth adjustment P 4  to increase the power output and the normal priority task  510  reenters the run state  560  and the software system  500  restarts the second timer T 2 . 
         [0090]    In embodiments, the second impedance reference Zdelta 2  and the third and fourth power adjustments P 3 , P 4  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0091]    Upon exiting the peak state  570 , the software system  500  stores the current filtered impedance Zf 1   n  into the memory  26  as the previous filtered impedance Zf 1   n-1  and stores the current filtered impedance Zf 3   n  into the memory  26  as the previous filtered impedance Zf 3   n-1 . 
         [0092]    In embodiments, the duration of the first, second, and third timers T 1 , T 2 , T 3  and the amount of the first and second power adjustments P 1 , P 2  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0093]    Referring now to  FIG. 7C , the high priority task  520  is layered over the normal priority task  510  and runs concurrently therewith to provide additional control of the generator  20  to reach a desired output value or effect. A fourth timer “T 4 ” is initialized by the software system  500  to coincide with the initialization of the high priority task  520  and to run concurrently therewith. Once the fourth timer T 4  expires, the software system  500  calculates the difference between the current filtered impedance Zf 6   n  and the current filtered impedance Zf 5   n  and compares this difference to a third impedance reference Zdelta 3 . The third impedance reference. Zdelta 3 , is the amount of change from the current filtered impedance Zf 5   n  to the current filtered impedance Zf 6   n  which is a threshold for triggering a decrease in power output. In this manner, the third impedance reference Zdelta 3  operates in a threshold capacity to prevent dangerous conditions such as a run-away state that may arise and lead to continued power increases and impedance rises. 
         [0094]    If the difference between the current filtered impedance Zf 6   n  and the current filtered impedance Zf 5   n  is greater than or equal to the third impedance reference Zdelta 3 , the controller  24  makes a fifth adjustment P 5  to decrease the power output and the software system  500  reenters the high priority task  520  and restarts the fourth timer T 4 . 
         [0095]    If the difference between the current filtered impedance Zf 6   n  and the current filtered impedance Zf 5   n  is less than the third impedance reference Zdelta 3 , the software system  500  enters the normal priority task  510 . Hence, the normal priority task  510  is only entered from the high priority task  520  if the third reference impedance Zdelta 3  threshold is not equaled or exceeded. 
         [0096]    In embodiments, the duration of the fourth timer T 4 , the third impedance reference Zdelta 3 , and the amount of the fifth power adjustment P 5  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0097]    The low priority task  530  is layered over the normal priority task  510  and runs concurrently with both the high priority task  520  and the normal priority task  510  to provide additional control of the generator  20  to terminate the procedure once a desired output value or effect has been achieved. A fifth timer “T 5 ” is initialized to coincide with the initialization of the vessel sealing procedure (e.g., by pressing a foot pedal or handswitch), and to run concurrently therewith. Once the fifth timer T 5  expires, the software system  500  continuously interrogates whether particular impedance conditions indicative of a desired tissue effect exist for the duration of a sixth timer “T 6 ” and, if such criteria is met, accordingly initiates a process to terminate the power output. Specifically, the software system  500  calculates the difference between the current filtered impedance Zf 8   n  and the current filtered impedance Zf 7   n  and compares this difference to a fourth impedance reference Zdelta 4 . The fourth impedance reference Zdelta 4  is the amount of change from the current filtered impedance Zf 7   n  to the current filtered impedance Zf 8  that initiates a seventh timer “T 7 ,” the expiration of which triggers the generator  20  to shut off and the procedure to be terminated. 
         [0098]    If the absolute value of the difference between the current filtered impedance Zf 8   n  and the current filtered impedance Zf 7   n  is less than or equal to the impedance reference Zdelta 3  for the duration of the sixth timer T 6 , the seventh timer T 7  is initialized. In addition, a termination state  535  of the low priority task  530  is triggered to provide for a plurality of options which allow the user to predetermine (e.g., via the user inputs of the generator  20 ) how the generator  20  will behave once the condition discussed above is satisfied for the duration of the sixth timer T 6 . Options available to the user with respect to the termination state  535  may include allowing the generator  20  to operate at it&#39;s current output level for the duration of the seventh timer T 7 , specifying an output level at which generator  20  is to operate for the duration of the seventh timer T 7 , and continuing with the low priority task  530  until the seventh timer T 7  expires. 
         [0099]    If the absolute value of the difference between the current filtered impedance Zf 8   n  and the current filtered impedance Zf 7   n  is not less than or equal to the impedance reference Zdelta 3  for the duration of the sixth timer T 6 , the software system  500  continues to execute the low priority task  530  concurrently with the high priority and normal priority tasks  520  and  530 . 
         [0100]    In embodiments, the duration of the fifth, sixth, and seventh timers T 5 , T 6 , T 7  and the fourth impedance reference Zdelta 4  may be pre-determined by the user via the user inputs of the generator  20  and/or a software-based user interface, as will be discussed in further detail below. 
         [0101]    In embodiments of the present disclosure, an eighth timer T 8  may be specified by the user (e.g. via the user inputs of the generator  20  and/or a software-based user interface) as a “master” timer (i.e., total procedure time) for the operation of the generator  20  in a given procedure. In this configuration, the generator  20  is shut off upon the expiration of the procedure timer T 8  regardless of whether or not the termination state  535  is entered. 
         [0102]    With reference now to  FIG. 8 , a software-based graphical user interface  600  is shown for use with embodiments of the software system  500  of the present disclosure. The interface  600  may include a plurality of editable parameters to allow the user to provide specific values (e.g., via the user inputs of the generator  20 ) for controlling the power output by the generator  20  via the software system  500 . The interface  600  allows the user to test and/or validate the software system  500  of the present disclosure. Specifically, the interface  600  may be organized by priority level and/or task level including a normal priority interface  610 , a high priority interface  620 , and a low priority interface  630 , as shown in  FIG. 8 . Further, a control interface  640  may be provided to allow the user to specify various control parameters such as, for example, the procedure time (e.g. the eighth timer T 8 ) and the file path and/or location of a file to be executed by the software system  500 , etc. 
         [0103]    The normal priority interface  610  is configured to be edited by the user to provide specific parameters for predetermining the behavior of the normal priority task  510  during a given procedure. The normal priority interface  610  may be divided into three sub-interfaces, namely, an initialization state interface  650 , a run state interface  660 , and a peak state interface  670 , to coincide with the three states  550 ,  560 , and  570  of the normal priority task  510 , respectively. The interfaces  650 ,  660 , and  670  may be edited by the user to provide specific parameters for further predetermining the behavior of the normal priority task  510  during a given procedure. 
         [0104]    Referring now to interface  650 , the user may be able to specify parameters related to the initialization state  550  of the normal priority task  510 , such as the duration of the first timer T 1  and power levels of P BASE , P INIT , P RATE , and P MAX . With reference to interface  660 , the user may be able to specify parameters related to the run state  560  of the normal priority task  510 , such as the duration of the second timer T 2 , the first impedance reference Zdelta 1 , and the amount of the first and second power adjustments P 1 . P 2 . With reference to interface  670 , the user may be able to specify parameters related to the peak state  570  of the normal priority task  510 , such as the duration of the third timer T 3 , the second impedance reference Zdelta 2 , and the amount of the third and fourth power adjustments P 3  and P 4 . 
         [0105]    The high priority interface  620  is configured to be edited by the user to provide specific parameters for predetermining the behavior of the high priority task  520  during a given procedure. In particular, the user may be able to specify parameters such as the duration of the fourth timer T 4 , the third impedance reference Zdelta 3 , and the fifth power adjustment P 5 . 
         [0106]    The low priority interface  630  is configured to be edited by the user to provide specific parameters for predetermining the behavior of the low priority task  530  during a given procedure. In particular, the user may be able to specify parameters such as the duration of the fifth, sixth, and seventh timers T 5 , T 6 , T 7  and impedance reference Zdelta 4 . Further, with respect to the termination state  535  of the low priority task  530 , the user may be able to choose from a menu of options (not explicitly shown) to select how the generator  20  will behave over the duration of the seventh timer T 7  once the termination state  535  is entered (e.g. continue current output level, adjust to a predetermined output level, shut off upon the expiration of the seventh timer T 7 , etc.). 
         [0107]    While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.