PATENT ABSTRACT
An electrosurgical system is disclosed. The electrosurgical system includes an electrosurgical generator adapted to supply electrosurgical energy to tissue. The electrosurgical generator includes impedance sensing circuitry which measures impedance of tissue, a processor configured to determine whether a tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid, and an electrosurgical instrument including at least one active electrode adapted to apply electrosurgical energy to tissue. A tissue cooling period is provided to enhance operative outcomes.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of commonly-owned U.S. patent application Ser. No. 11/657,170, filed Jan. 24, 2007, now U.S. Pat. No. 7,972,328 which claims priority to U.S. Provisional Application Ser. No. 60/761,443, filed Jan. 24, 2006, the entirety of each being hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an electrosurgical system and method for performing electrosurgical procedures. More particularly, the present disclosure relates to sealing tissue, wherein energy is administered to match measured impedance to a desired impedance, and a tissue cooling time is observed prior to the completion of the seal. 
     2. Background of Related Art 
     Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, or coagulate tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode (e.g., a return pad) carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. The patient return electrode is placed remotely from the active electrode to carry the current back to the generator. 
     In bipolar electrosurgery, one of the electrodes of the hand-held instrument functions as the active electrode and the other as the return electrode. The return electrode is placed in close proximity to the active electrode such that an electrical circuit is formed between the two electrodes (e.g., electrosurgical forceps). In this manner, the applied electrical current is limited to the body tissue positioned between the electrodes. When the electrodes are sufficiently separated from one another, the electrical circuit is open and thus inadvertent contact of body tissue with either of the separated electrodes does not cause current to flow. 
     Bipolar electrosurgery generally involves the use of forceps. A forceps is a pliers-like instrument which relies on mechanical action between its jaws to grasp, clamp and constrict vessels or tissue. So-called “open forceps” are commonly used in open surgical procedures whereas “endoscopic forceps” or “laparoscopic forceps” are, as the name implies, used for less invasive endoscopic surgical procedures. Electrosurgical forceps (open or endoscopic) utilize mechanical clamping action and electrical energy to effect hemostasis on the clamped tissue. The forceps include electrosurgical conductive plates which apply the electrosurgical energy to the clamped tissue. By controlling the intensity, frequency and duration of the electrosurgical energy applied through the conductive plates to the tissue, the surgeon can coagulate, cauterize and/or seal tissue. 
     Tissue or vessel sealing is a process of liquefying the collagen, elastin and ground substances in the tissue so that they reform into a fused mass with significantly-reduced demarcation between the opposing tissue structures. Cauterization involves the use of heat to destroy tissue and coagulation is a process of desiccating tissue wherein the tissue cells are ruptured and dried. 
     Tissue sealing procedures involve more than simply cauterizing or coagulating tissue to create an effective seal; the procedures involve precise control of a variety of factors. For example, in order to affect a proper seal in vessels or tissue, it has been determined that two predominant mechanical parameters must be accurately controlled: the pressure applied to the tissue; and the gap distance between the electrodes (i.e., distance between opposing jaw members or opposing sealing plates). In addition, electrosurgical energy must be applied to the tissue under controlled conditions to ensure creation of an effective vessel seal. Techniques have been developed whereby the energy applied to the tissue is varied during the tissue sealing process to achieve a desired tissue impedance trajectory. When a target tissue impedance threshold is reached, the tissue seal is deemed completed and the delivery of electrosurgical energy is halted. 
     SUMMARY 
     The present disclosure relates to a vessel or tissue sealing system and method. In particular, the system discloses an electrosurgical instrument, which may be a bipolar forceps having two jaw members configured for grasping tissue. Each of the jaw members may include a sealing plate which communicates electrosurgical energy to the tissue. At the start of the procedure, the system may transmit an initial interrogatory pulse for determining initial tissue impedance. Additionally or alternatively at the start of the procedure, the system may identify characteristics of the electrosurgical instrument. The system determines whether tissue reaction has occurred and calculates the desired impedance trajectory. The system calculates a target impedance value at each time step based on a predefined desired rate of change of impedance. The system then controls measured tissue impedance to match target impedance. The system may sense parameters related to the sealing process. For example without limitation, the system may sense a temperature, a tissue type, and/or a fluid type. Additionally or alternatively, the system may determine an aggregate amount of energy delivered during the sealing process. The delivery of energy may be halted when the measured impedance is above threshold for a predetermined period of time. The threshold is defined as a specified impedance level above the initial measured impedance value. 
     After the delivery of energy is halted, the system may provide a tissue cooling time. The cooling time may allow reformed collagen within the fused tissue to solidify, or set in place, between the jaw members. The cooling time may promote denaturation of collagen. The cooling time may be any duration of time, such as a fixed period of time, or an adaptive time, which is dependent upon parameters relating to the tissue fusion (sealing) process, for example without limitation, tissue temperature, tissue impedance, tissue mass, energy delivery, and/or instrument characteristics. Upon expiration of a cooling period the sealing process is completed. The system may provide an indication that the end of the sealing process is completed, such as an audible sound (i.e., “endtone”), whereupon the user may release the jaws. 
     During the cooling time, cooling of tissue may be effectuated by conduction, i.e., residual heat from fused tissue is drawn away from the tissue by, for example without limitation, the instrument jaws, surrounding tissue, or surrounding fluids such as blood or saline. In embodiments, a coolant, such as saline, may be introduced to the surgical site to promote cooling. It is further envisioned that active cooling elements may be included in the disclosed system, for example without limitation, heat pipes, cooling jackets, and thermoelectric (Peltier effect) devices. 
     In embodiments, it is envisioned that an initial “baseline” cooling time is established. The baseline “cool-down” time may be dependent upon a sealing process parameter that is determined during sealing process initialization, for example without limitation, an initial tissue impedance measurement, an initial temperature, an initial fluid measurement, and/or a property of the forceps or instrument (i.e., jaw size, jaw angle, instrument type, thermal coefficients, and the like). 
     According to one aspect of the present disclosure, an electrosurgical system is disclosed. The electrosurgical system includes an electrosurgical generator adapted to supply electrosurgical energy to tissue. The electrosurgical generator may include impedance sensing circuitry which measures impedance of tissue, a processor configured to determine whether a tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid, and an electrosurgical instrument including at least one active electrode adapted to apply electrosurgical energy to tissue. The electrosurgical generator may include temperature sensing circuitry and/or fluid sensing circuitry. Additionally or alternatively, the electrosurgical generator may include circuitry for identifying characteristics of an electrosurgical instrument coupled thereto. The electrosurgical instrument may include an identification module to enable the electrosurgical generator to identify the instrument. For example without limitation, the identification module may include at least one resistive element have a resistance value corresponding to a characteristic of the instrument, such as the instrument configuration (i.e., model number), a unique instrument identifier (i.e., serial number) and/or a thermal property of the jaws. In embodiments, the identification module may include computer memory (i.e., read-only memory or flash memory), RFID tag, optical tag (i.e., barcode), or other encoding as will be familiar to the skilled artisan. In embodiments, the instrument includes a sensor in operable communication with the generator that is configured to sense the included angle between the jaws, which angle may be indicative of the size and/or mass of tissue held therebetween. The generator may use an algorithm or a lookup table to determine a desired cool-down time based upon the identification module. 
     According to another aspect of the present disclosure, an electrosurgical generator is disclosed. The electrosurgical generator includes all RF output stage adapted to supply electrosurgical energy to tissue and impedance sensing circuitry which measures impedance of tissue. The generator also includes a processor configured to determine whether tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid. The processor may be configured to generate a target impedance trajectory as a function of measured impedance and desired rate of change based on the tissue reaction determination, wherein the target impedance trajectory includes a plurality of target impedance values. The generator may include an electrosurgical instrument including at least one active electrode adapted to apply electrosurgical energy to tissue. The processor may be configured to determine the duration of a cooling time in accordance with, for example without limitation, characteristics of the electrosurgical instrument, tissue properties (i.e., impedance, temperature), surgical site properties (i.e., presence of fluid at the site), an amount of energy delivered to tissue (i.e., net energy delivery), jaw angle (i.e., the included angle between the opposing jaw members), and/or operator-entered parameters. 
     A method for performing an electrosurgical procedure is also contemplated according to the present disclosure. The method includes the steps of grasping tissue between the jaws of an electrosurgical instrument, applying electrosurgical energy at an output level to tissue from an electrosurgical generator, determining whether tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid, generating a target impedance trajectory as a function of measured impedance and desired rate of change based on the tissue reaction determination, the target impedance trajectory including a plurality of target impedance values, discontinuing the application of electrosurgical energy to tissue, allowing tissue to cool down during a cooling period, and releasing tissue from the jaws of the electrosurgical instrument. In embodiments, the method includes the steps of sensing the included angle formed by the jaw members and adjusting energy delivery and/or cooling period time in accordance therewith. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a perspective view of one embodiment of an electrosurgical system according to the present disclosure; 
         FIG. 2  is a schematic block diagram of a generator algorithm according to the present disclosure; 
         FIG. 3  is a rear, perspective view of the end effector of  FIG. 1  shown with tissue grasped therein; 
         FIG. 4  is a side, partial internal view of an endoscopic forceps according to the present disclosure; 
         FIG. 5  is a perspective view of an open bipolar forceps according to the present disclosure; 
         FIGS. 6A and 6B  shows a flow chart showing a sealing method using the endoscopic bipolar forceps according to the present disclosure; 
         FIG. 7  shows a graph illustrating the changes occurring in tissue impedance during sealing utilizing the method shown in  FIGS. 6A and 6B ; and 
         FIG. 8  shows a current v. impedance control curve according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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, laparoscopic instrument, or an open instrument. It should also be appreciated that different electrical and mechanical connections and other considerations may apply to each particular type of instrument, however, the novel aspects with respect to vessel and tissue scaling are generally consistent with respect to both the open or endoscopic designs. 
     In the drawings and in the description which follows, the term “proximal” refers to the end of the forceps  10  which is closer to the user, while the term “distal” refers to the end of the forceps which is further from the user. 
       FIG. 1  is a schematic illustration of an electrosurgical system  1 . The system  1  includes an electrosurgical forceps  10  for treating patient tissue. Electrosurgical RF energy is supplied to the forceps  10  by a generator  2  via a cable  18  thus allowing the user to selectively coagulate and/or seal tissue. 
     As shown in  FIG. 1 , the forceps  10  is an endoscopic version of a vessel sealing bipolar forceps. The forceps  10  is configured to support an effector assembly  100  and generally includes a housing  20 , a handle assembly  30 , a rotating assembly  80 , and a trigger assembly  70  which mutually cooperate with the end effector assembly  100  to grasp, seal and, if required, divide tissue. Forceps  10  also includes a shaft  12  which has a distal end  14  which mechanically engages the end effector assembly  100  and a proximal end  16  which mechanically engages the housing  20  proximate the rotating assembly  80 . 
     The forceps  10  also includes a plug (not shown) which connects the forceps  10  to a source of electrosurgical energy, e.g., generator  2 , via cable  18 . Handle assembly  30  includes a fixed handle  50  and a movable handle  40 . Handle  40  moves relative to the fixed handle  50  to actuate the end effector assembly  100  and enable a user to selectively grasp and manipulate tissue  400  as shown in  FIG. 3 . Forceps  10  may also include an identification module (not explicitly shown) such as a resistor or computer memory readable by the generator  2  to identify the forceps. 
     Referring to  FIGS. 1 ,  3  and  4 , end effector assembly  100  includes a pair of opposing jaw members  110  and  120  each having an electrically conductive sealing plate  112  and  122 , respectively, attached thereto for conducting electrosurgical energy through tissue  400  held therebetween. More particularly, the jaw members  110  and  120  move in response to movement of handle  40  from an open position to a closed position. In open position the sealing plates  112  and  122  are disposed in spaced relation relative to one another. In a clamping or closed position the sealing plates  112  and  122  cooperate to grasp tissue and apply electrosurgical energy thereto. In embodiments, end effector assembly  100  includes a jaw angle sensor (now explicitly shown) that is adapted to sense the included angle  114  between opposing jaw members  110  and  120  and is configured to operably couple to generator  2 . 
     Jaw members  110  and  120  are activated using a drive assembly (not shown) enclosed within the housing  20 . The drive assembly cooperates with the movable handle  40  to impart movement of the jaw members  110  and  120  from the open position to the clamping or closed position. Examples of a handle assemblies are shown and described in commonly-owned U.S. application Ser. No. 10/369,894 entitled “VESSEL SEALER AND DIVIDER AND METHOD MANUFACTURING SAME” and commonly owned U.S. application Ser. No. 10/460,926 entitled “VESSEL SEALER AND DIVIDER FOR USE WITH SMALL TROCARS AND CANNULAS” which are both hereby incorporated by reference herein in their entirety. 
     Jaw members  110  and  120  also include outer housings on insulators  116  and  126  which together with the dimension of the conductive plates of the jaw members  110  and  120  are configured to limit and/or reduce many of the known undesirable effects related to tissue sealing, e.g., flashover, thermal spread and stray current dissipation. 
     In addition, the handle assembly  30  of the present disclosure may include a four-bar mechanical linkage which provides a unique mechanical advantage when sealing tissue between the jaw members  110  and  120 . For example, once the desired position for the sealing site is determined and the jaw members  110  and  120  are properly positioned, handle  40  may be compressed fully to lock the electrically conductive sealing plates  112  and  122  in a closed position against the tissue. The details relating to the inter-cooperative relationships of the inner-working components of forceps  10  are disclosed in the above-cited commonly-owned U.S. patent application Ser. No. 10/369,894. Another example of an endoscopic handle assembly which discloses an off-axis, lever-like handle assembly, is disclosed in the above-cited U.S. patent application Ser. No. 10/460,926. 
     The forceps  10  also includes a rotating assembly  80  mechanically associated with the shaft  12  and the drive assembly (not shown). Movement of the rotating assembly  80  imparts similar rotational movement to the shaft  12  which, in turn, rotates the end effector assembly  100 . Various features along with various electrical configurations for the transference of electrosurgical energy through the handle assembly  20  and the rotating assembly  80  are described in more detail in the above-mentioned commonly-owned U.S. patent application Ser. Nos. 10/369,894 and 10/460,926. 
     As best seen with respect to  FIGS. 1 and 4 , end effector assembly  100  attaches to the distal end  14  of shaft  12 . The jaw members  110  and  120  are pivotable about a pivot  160  from the open to closed positions upon relative reciprocation, i.e., longitudinal movement, of the drive assembly (not shown). Again, mechanical and cooperative relationships with respect to the various moving elements of the end effector assembly  100  are further described by example with respect to the above-mentioned commonly-owned U.S. patent application Ser. Nos. 10/369,894 and 10/460,926. 
     It is envisioned that the forceps  10  may be designed such that it is fully or partially disposable depending upon a particular purpose or to achieve a particular result. For example, end effector assembly  100  may be selectively and releasably engageable with the distal end  14  of the shaft  12  and/or the proximal end  16  of the shaft  12  may be selectively and releasably engageable with the housing  20  and handle assembly  30 . In either of these two instances, the forceps  10  may be either partially disposable or replaceable, such as where a new or different end effector assembly  100  or end effector assembly  100  and shaft  12  are used to selectively replace the old end effector assembly  100  as needed. 
     The generator  2  includes input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator  2 . In addition, the generator  2  includes 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., coagulating, tissue sealing, division with hemostatis, etc.). It is also envisioned that the forceps  10  may include a plurality of input controls which may be redundant with certain input controls of the generator  2 . Placing the input controls at the forceps  10  allows for easier and faster modification of RF energy parameters during the surgical procedure without requiring interaction with the generator  2 . 
       FIG. 2  shows a schematic block diagram of the generator  2  having a controller  4 , a high voltage DC power supply  7  (“HVPS”), an RF output stage  8 , and a sensor circuitry  11 . The DC power supply  7  provides DC power to an RF output stage  8  which then converts DC power into RF energy and delivers the RF energy to the forceps  10 . The controller  4  includes a processor  5  operably connected to a memory  6  which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The processor  5  includes an output port which is operably connected to the HVPS  7  and/or RF output stage  8  allowing the processor  5  to control the output of the generator  2  according to either open and/or closed control loop schemes. A closed loop control scheme may be a feedback control loop wherein the sensor circuitry  11  provides feedback to the controller  4  (i.e., information obtained from one or more of sensing mechanisms for sensing various tissue parameters such as tissue impedance, tissue temperature, fluid presence, output current and/or voltage, etc.). The controller  4  then signals the HVPS  7  and/or RF output stage  8  which then adjusts DC and/or RF power supply, respectively. The controller  4  also receives input signals from the input controls of the generator  2  and/or forceps  10 . The controller  4  utilizes the input signals to adjust the power output of the generator  2  and/or instructs the generator  2  to perform other control functions. 
     It is known that sealing of the tissue  400  is accomplished by virtue of a unique combination of gap control, pressure and electrical control. In other words, controlling the intensity, frequency and duration of the electrosurgical energy applied to the tissue through the sealing plate  112  and  122  are important electrical considerations for sealing tissue. In addition, two mechanical factors play an important role in determining the resulting thickness of the sealed tissue and the effectiveness of the seal, i.e., the pressure applied between the opposing jaw members  110  and  120  (between about 3 kg/cm2 to about 16 kg/cm2) and the gap distance “G” between the opposing sealing plates  112  and  122  of the jaw members  110  and  120 , respectively, during the sealing process (between about 0.001 inches to about 0.006 inches). One or more stop members  90  may be employed on one or both sealing plates to control the gap distance. A third mechanical factor has recently been determined to contribute to the quality and consistency of a tissue seal, namely the closure rate of the electrically conductive surfaces or sealing plates during electrical activation. 
     Since the forceps  10  applies energy through electrodes, each of the jaw members  110  and  120  includes a pair of electrically sealing plates  112 ,  122  respectively, disposed on an inner-facing surface thereof. Thus, once the jaw members  110  and  120  are fully compressed about the tissue  400 , the forceps  10  is now ready for selective application of electrosurgical energy as shown in  FIG. 4 . At that point, the electrically sealing plates  112  and  122  cooperate to seal tissue  400  held therebetween upon the application of electrosurgical energy. 
     The system  1  according to the present disclosure regulates application of energy and pressure to achieve an effective seal capable of withstanding high burst pressures. The generator  2  applies energy to tissue at constant current based on the current control curve of  FIG. 8  which is discussed in more detail below. Energy application is regulated by the controller  4  pursuant to an algorithm stored within the memory  6 . The algorithm maintains energy supplied to the tissue at constant voltage. The algorithm varies output based on the type of tissue being sealed. For instance, thicker tissue typically requires more power, whereas thinner tissue requires less power. Therefore, the algorithm adjusts the output based on tissue type by modifying specific variables (e.g., voltage being maintained, duration of power application etc.). In embodiments, the algorithm adjusts the output based on jaw angle. 
     As mentioned above, various methods and devices are contemplated to automatically regulate the closure of the jaw members  110  and  120  about tissue to keep the pressure constant during the sealing process. For example, the forceps  10  may be configured to include a ratchet mechanism (not explicitly shown) which initially locks the jaw members  110  and  120  against the tissue under a desired tissue pressure and then increases the pressure according to the command from the processor  5  to an optimum tissue pressure. The ratchet mechanism (not explicitly shown) is configured to adjust the pressure based on electrical activation and/or the tissue reaction. It is also envisioned that the pressure may be controlled in a similar manner towards the end of the seal cycle, i.e., release pressure. The pressure may be held constant or varied during a cooling period. A similar or the same ratchet mechanism (not explicitly shown) may be employed for this purpose as well. The ratchet mechanism (not explicitly shown) may be configured to automatically release or unlock at the end of a cooling period. Other controllable closure mechanisms or pressure-applying mechanism are also envisioned which may be associated with the handle assembly  30 , the housing  20  and/or the jaw members  110  and  120 . Any of these mechanisms may be housed in the housing  20  or form a part of each particular structure. The ratchet, closure, and/or pressure-applying mechanism may include any suitable actuating device, for example without limitation, a solenoid, stepper motor, vacuum actuator, and/or a pressure actuator. 
     It is also envisioned that one or more stop members  90  may be selectively controllable to regulate the closure pressure and gap distance to affect the seal. Commonly-owned U.S. application Ser. No. 10/846,262 describes one such variable stop system which may be used for this purpose, the entire contents being incorporated by reference herein. 
     From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. For example and as mentioned above, it is contemplated that any of the various jaw arrangements disclosed herein may be employed on an open forceps such as the open forceps  700  shown in  FIG. 5 . The forceps  700  includes an end effector assembly  600  which is attached to the distal ends  516   a  and  516   b  of shafts  512   a  and  512   b , respectively. The end effector assembly  600  includes a pair of opposing jaw members  610  and  620  which are pivotally connected about a pivot pin  665  and which are movable relative to one another to grasp vessels and/or tissue. Each of the opposing jaw members  610 ,  620  includes electrically sealing plates  112 ,  122  which allow the open forceps  700  to be used for clamping tissue for sealing. 
     Each shaft  512   a  and  512   b  includes a handle  515  and  517 , respectively, disposed at the proximal end  514   a  and  514   b  thereof which each define a finger hole  515   a  and  517   a , respectively, therethrough for receiving a finger of the user. Finger holes  515   a  and  517   a  facilitate movement of the shafts  512   a  and  512   b  relative to one another which, in turn, pivot the jaw members  610  and  620  from an open position wherein the jaw members  610  and  620  are disposed in spaced relation relative to one another to a clamping or closed position wherein the jaw members  610  and  620  cooperate to grasp tissue or vessels therebetween. Further details relating to one particular open forceps are disclosed in commonly-owned U.S. application Ser. No. 10/962,116 filed Oct. 8, 2004 entitled “OPEN VESSEL SEALING INSTRUMENT WITH CUTTING MECHANISM AND DISTAL LOCKOUT”, the entire contents of which being incorporated by reference herein. 
     The method of scaling tissue according to the present disclosure is discussed below with reference to  FIGS. 6A-B . In addition,  FIG. 7  shows a graph illustrating the changes to tissue impedance when tissue is seated utilizing the method of  FIGS. 6A-B . The method is embodied in a software-based algorithm which is stored in memory  6  and is executed by processor  5 . 
     In step  302 , the vessel sealing procedure is activated (e.g., by pressing of a foot pedal or handswitch) and a host processor (e.g., processor  5 ) activates a vessel sealing algorithm and loads a configuration file. The configuration file may include a variety of variables which control the algorithm, e.g., end impedance threshold (EndZ), baseline cooling time (Base_Cool_T), and forceps/instrument identification (ForcepsID). Certain variables of the configuration file may be adjusted based on the instrument being used and the bar settings selected by a surgeon. A configuration file may be loaded from a data store included within controller  4 . Additionally or alternatively, a configuration file may be loaded from a data store included within forceps  10 . In embodiments a plurality of configuration files may be included within controller  4 . A configuration file may be selected and loaded by the algorithm in accordance with the type of forceps being utilized, e.g., the ForcepsID. In embodiments, forceps  10  are interrogated by controller  4  to ascertain ForcepsID, whereupon a configuration file corresponding to ForcepsID is loaded. Base_Cool_T may be determined in accordance with ForcepsID. 
     In step  304 , the algorithm begins with an impedance sense phase, shown as phase I in  FIG. 7 , during which the algorithm senses the tissue impedance with an interrogatory impedance sensing pulse of approximately 100 ms duration. The measured value of tissue impedance is stored as a variable DZDT_Start_Z. Tissue impedance is determined without appreciably changing the tissue. An adaptive cooling time (Adaptive_Cool_T) may be determined by adjusting the value indicated by Base_Cool_T in accordance with tissue impedance (DZDT_Start_Z). The cool-down time may be adjusted in accordance with additional or alternative factors as will be further described herein. During this interrogation or error-checking phase the generator  2  provides constant power to check for a short or an open circuit, in order to determine if tissue is being grasped. The cumulative (i.e., net amount) of energy delivered to the tissue during the sealing procedure may be stored in a variable (E_Total). E_Total may be determined in any suitable manner, for example without limitation, by integrating the output power over the power delivery time. In embodiments, the output power is sampled and totalized on a periodic basis to yield an approximation of total energy delivery. Processor  5  may be configured to execute an interrupt service routine (ISR) that is programmed to periodically sense and totalize cumulative output power (E_Total). Variables corresponding to the maximum energy delivery rate (E_Max), minimum energy delivery rate (E_Min), and an average energy delivery rate (E_Avg) may additionally or alternatively sensed and/or computed and stored. 
     Thermal properties related to the tissue may be sensed, recorded and/or computed during the sealing process. Such properties may include, without limitation, total thermal energy sensed, which may be expressed as the sensed temperature integrated over the time of the procedure (T_total), maximum tissue temperature (T_Max), minimum tissue temperature (T_Min), and average tissue temperature (T_Avg). Fluid properties, i.e., a total quantity of fluid, which may be expressed as the sensed quantity of fluid integrated over the time of the procedure (F_Total), a maximum fluid quantity (F_Max), a minimum fluid quantity (F_Min), and an average fluid quantity of fluid (F_Avg), may additionally or alternatively be sensed, recorded and/or computed. 
     In step  306 , a determination is made whether the measured impedance is greater than a pre-programmed high impedance threshold, represented by the variable ImpSense_HiLimit, or less than a pre-programmed low impedance threshold, represented by the variable ImpSense_LowLimit. If in step  306  a short circuit is detected, e.g., impedance is below a low impedance threshold or if a an open circuit is detected, e.g., impedance is above a high impedance threshold, the algorithm in step  364  issues a regrasp alarm, and the algorithm exits in step  308 . If, otherwise, no fault is detected in step  306  (i.e., no short and no open circuit detected), the algorithm starts the cook phase in step  310 . The generator  2  then generates the pre-programmed ramping of current in its outer-loop and constant current per current curve within its inner-loop according to the current control curve shown in  FIG. 8 . 
     The curve of  FIG. 8  may be modified by intensity settings. In particular, selecting a specific intensity setting (e.g., low, medium, high, etc.) selects a corresponding value, represented by a variable, Cook_AmpMult, which then multiplies the curve. The Cook_AmpMult variable is specified in the configuration file and may range from about 2 Amps to about 5.5 Amps in some embodiments. In other embodiments, the Cook_AmpMult variable may range from about 2 Amps to about 8 Amps. 
     The control curve for this algorithm is designed as a current curve which decreases rapidly from low impedances to high, although it could also be represented as a power or voltage curve. The control curve is designed ideally to reduce power with increasing impedances higher than approximately 24 ohms. This shape provides several advantages: 1) this curve allows high power with low impedance tissues, which allows the tissue to heat rapidly at the start of the seal cycle; 2) this shape tames the positive feedback caused by increase in delivered power as a result of increasing impedance 3) the curve allows a slower control system for Z control as the output power is reduced as the impedance rises, thus keeping the tissue impedance from rising too quickly. 
     After the error checking phase, in step  310  the algorithm initiates application of the RF energy by delivering current linearly over time to heat the tissue. It is envisioned RE energy may be delivered in a non-linear or in a time-independent step manner from zero to an “on” state. Delivery may be controlled through other parameters such as voltage and/or power and/or energy. Once initiated, the ramping of energy continues until one of two events occurs: 1) the maximum allowable value is reached or 2) the tissue “reacts.” The term “tissue reaction” is a point at which intracellular and/or extra-cellular fluid begins to boil and/or vaporize, resulting in an increase in tissue impedance. In the case when the maximum allowable value is reached, the maximum value is maintained until the tissue “reacts.” In the event that the tissue reacts prior to reaching the maximum value, the energy required to initiate a tissue “reaction” has been attained and the algorithm moves to an impedance control state. 
     To identify that a tissue reaction has occurred, there are two elements which are considered. The first consideration is the minimum tissue impedance obtained during the heating period. In step  312 , the algorithm continuously monitors the tissue impedance after the onset of energy to identify the lowest value reached and then in step  314  stores this value as the variable ZLow. As time progresses throughout the entire energy activation cycle, the stored value is updated anytime a new value is read that is lower than the previous Zlow, represented by phase II in  FIG. 7 . In other words, during steps  312 ,  314  and  316 , the generator  2  waits for the tissue impedance to drop. The generator  2  also captures EndZ_Offset impedance, which corresponds to the initial measured tissue impedance. The EndZ_Offset impedance is used to determine the threshold for terminating the procedure. In step  314 , EndZ_Offset impedance is measured approximately 100 ms after initial application of electrosurgical energy, which occurs approximately during phase I. 
     The second consideration in identifying tissue reaction is a predetermined rise in impedance. This is represented by the variable Z_Rise, which is loaded from the configuration file and can range from about 1 ohm to about 750 ohms. In step  316  the algorithm waits for a predetermined period of time to identify whether a rise in impedance has occurred, represented by phases IIIa and IIIb in  FIG. 7 . In step  318 , the algorithm repeatedly attempts to identify a tissue reaction by determining if Z(t)&gt;ZLow+Z_Rise where Z(t) is the impedance at any time during sampling. In step  320 , the algorithm verifies whether the timer for waiting for impedance to rise has expired. 
     If the tissue does not rise within the predetermined period of time (e.g., in step  320  the timer has expired) then, the generator  2  issues a regrasp alarm due to the tissue not responding. In particular, in step  324  the generator  2  verifies whether the procedure is complete by comparing measured impedance to the impedance threshold. If the measured impedance is greater than the impedance threshold, the tissue is sealed and the electrosurgical energy (e.g., RF power) is shut off and the algorithm proceeds to step  360  wherein the cooling timer is activated. 
     In the step  360  the actual cooling time (Adaptive_Cool_T) is determined in accordance with the initial impedance (DZDT_Start_Z), final impdance (DZDT_End_Z), the instrument type (Forceps_ID), energy delivered to the tissue (i.e., E_total, E_Max, E_Min and/or E_Avg), thermal properties (i.e., T_total, T_Max, T_Min and/or T_Avg), and/or fluid properties (i.e., F_total, F_Max, F_Min and/or F_Avg). It is envisioned the actual cooling time may range from about zero seconds to about ten seconds. In embodiments, the actual cooling time may range from about a half a second to about two seconds. In embodiments, Adaptive_Cool_T is initially set to Base_Cool_T. Adaptive_Cool_T may then be increased or decreased in accordance with biologic or operational parameters. For example without limitation, Adaptive_Cool_T may be increased by an amount correlated to the extent by which a parameter exceeds a parameter threshold, and, conversely, Adaptive_Cool_T may be decreased by an amount correlated to the extent by which a parameter falls short of a parameter threshold. In embodiments, Adaptive_Cool_T may only be increased, or only decreased. In yet other embodiments, a parameter may cause an increase in Adaptive_Cool_T, a parameters may cause a decrease in Adaptive_Cool_T, and a parameter may cause both and increase and a decrease in Adaptive_Cool_T. 
     After the cooling period has expired and the endtone signaled, the sealing procedure ends with step  328 , which prevents sealing tissue that has already been sealed. 
     If the tissue is not sealed, then in step  326  the generator determines whether the measured impedance is below the impedance threshold, and if so then the generator  2  issues a regrasp alarm in step  364  and exits in step  308 . 
     To check for the reaction stability, the algorithm has a hysteresis identifier (Z_HIST) defined by a specified drop in impedance occurring in under a specified duration in time. This is used to filter out the noise which may be mistaken by the algorithm for the actual rise in impedance. In step  325 , the algorithm determines whether the measured impedance is less than the rise in impedance above the lowest impedance minus the hysteresis identifier (i.e., Z(t)&lt;Zlow+Z_Rise−Z_Hist). Step  325  is repeated for a specified period of time by determining whether a timer has expired in step  322  (Z_Hist tmr), the repetition of the loop is determined in step  327 . 
     After the tissue reacts and tissue impedance begins to rise, if the impedance drops below a hysteresis value within an allotted time, the system identifies the event “not stable” as shown in phase IIIa. The algorithm also begins looking for the next rise in impedance by determining if the measured impedance is greater than the specified level of impedance, defined by the equation Z(t)&lt;Zlow+Z_Rise−Z_Hist. If the timer expires and the impedance has not dropped below the hysteresis value, the reaction is considered stable and the impedance control state is implemented. 
     Once it is established that the tissue has reacted as shown in phase IIIb, the algorithm calculates the desired impedance trajectory based on the actual impedance and the desired rate of change in step  330 . In step  332 , the algorithm calculates a target impedance value for the control system at each time-step, based on a predefined desired rate of change of impedance (dZ/dt), represented as phase IV in  FIG. 7 . The desired rate of change may be stored as a variable and be loaded during the step  302 . The control system then attempts to adjust the tissue impedance to match the target impedance. The target impedance takes the form of a target trajectory with the initial impedance value and time taken when the tissue reaction is considered real and stable. It is envisioned that the trajectory could take a non-linear and/or quasi-linear form. Thus, when the measured impedance is greater than the rise in impedance above lowest impedance (i.e., Z(t)&gt;ZLow+ZRise), the algorithm calculates a Z trajectory based on the actual impedance and desired dZ/dt, i.e., a rate of rise of impedance over time, selected manually or automatically based on tissue type determined by the selected instrument. 
     The target impedance trajectory includes a plurality of a target impedance values at each time step. The algorithm drives tissue impedance along the target impedance trajectory by adjusting the power output level to substantially match tissue impedance to a corresponding target impedance value. While the algorithm continues to direct the RF energy to drive the tissue impedance to match the specified trajectory, the algorithm monitors the impedance to make the appropriate corrections. The algorithm determines whether tissue fusion is complete and the system should cease RF energy in phase V as shown in  FIG. 7 . This is determined by monitoring the actual measured impedance rising above a predetermined threshold and staying above the threshold for a predetermined period of time. The threshold is defined as a specified level, EndZ, above the initial impedance value, EndZ_Offset. This determination minimizes the likelihood of terminating electrosurgical energy early when the tissue is not properly or completely sealed. 
     In step  334 , it is determined if the measured impedance is greater than as the specified level of impedance above the initial impedance value (i.e., Z(t)&gt;EndZ+EndZ_Offset), if yes, the algorithm verifies whether this state is maintained for the given time. In step  336 , the algorithm initializes the timer, DZDT_ENDZ_TIMER. In step  338 , the algorithm performs the determination of step  334  for the duration of the timer DZDT_ENDZ_TIMER, which may be about 400 ms, the expiration of which is verified in step  340 . If the seating portion of the vessel seating process (i.e., not including cool-down time) has exceeded a predetermined time period (e.g., maximum seal timer) which may be about 12 seconds, the algorithm exits with an alarm. This alerts the user to a possible unfused tissue condition. 
     It is envisioned that the EndZ value ranges from about 10 ohms to about 1000 ohms above the minimum impedance reached and EndZ_Offset is the tissue impedance approximately about 100 ms after the onset of RF energy. Further, the time duration for a cycle shut-off condition to verify tissue fusion has occurred, (i.e., the value of DZDT_ENDZ_TIMER) may range from 0 seconds to 2 seconds. It is also envisioned that the value of the EndZ_Offset could be calculated from a variety of different methods and utilizing a variety of different parameters such as the starting tissue impedance, the minimum impedance, the impedance at maximum current or minimum voltage, the impedance at either a positive or negative slope change of impedance, and/or a constant value specified within the programming or by the end user. 
     Once the timer expires and if the measured impedance is still above EndZ+EndZ_Offset the RF is shut off. However, it must be verified whether tissue reaction has not occurred too quickly (e.g., the control system failed to maintain control). This event is identified if the final measured impedance value deviated from the end target value by greater than a predetermined value, ENDZ_TRAJ_LIMIT. The ENDZ_TRAJ_LIMIT ranges from about 1 ohm to about 500 ohms. In step  342 , the algorithm determines whether the measured impedance is below ENDZ_TRAJ_LIMIT. This event aids in mitigating the occurrences of the algorithm exiting while the tissue is not fused. If in step  342 , the measured impedance is determined to be below ENDZ_TRAJ_LIMIT, then in step  360  the algorithm goes into a wait state having a duration in accordance with Adaptive_Cool_T to enable the fused tissue to set. After the expiration of the wait state, the algorithm in step  362  issues a seal complete signal, which may be an audio indication (i.e., an “endtone”) and/or a visual indication, and in the step  328  the algorithm exits. 
     Prior to proceeding to step  334  to determine if the seal process is complete, the algorithm performs a plurality of error checks. In particular, the algorithm determines whether excessive fluid has entered the field or an object has been encountered that causes the impedance to drop unexpectedly to affect the ongoing tissue reaction. This event is identified by a negative deviation between the target impedance and tissue impedance (i.e. tissue impedance is less than target impedance) as represented by phase VI in  FIG. 7 . Therefore, to identify that this event has occurred and is real (e.g., not an arcing event) several conditions are verified. In step  344 , the algorithm determines whether the impedance dropped below a reset threshold value, RstLim, above the lowest impedance reached, ZLow and whether the impedance deviated sufficiently from the target request. Therefore, this event is identified as: Z(t)&lt;=RstLim+ZLow &amp; Z(t)&lt;target−RstLim. It is recognized that the RstLim ranges from about 1 ohm to about 750 ohms. If no drop in impedance or deviation has occurred then the sealing process was successful and the algorithm proceeds to step  334  as discussed above. If a deviation has been detected, then in step  346  the algorithm performs a subsequent verification. 
     In step  346 , at the onset of successfully meeting both of these conditions, the algorithm begins a timer, DZDT_ZTRAJ_RST_TMR, to define if the deviation event is true and stable or false and transient. In step  348 , the algorithm determines whether the measured impedance is above the reset threshold value, RstLim, above the lowest impedance reached, ZLow plus a hysteresis value, ZIlist. If this condition is satisfied before the timer DZDT_ZTRAJ_RST_TMR expires in step  350 , the event is considered transient and the algorithm continues to direct the electrosurgical energy to cause the tissue impedance to follow the previous trajectory by returning to step  332 . 
     If the condition described above in step  348  occurs and the timer expires in step  350 , the event is deemed real and the algorithm proceeds to step  352  where the algorithm adjusts to look for tissue reaction as described earlier with respect to step  318 . Specifically, in step  354 , the impedance is monitored to identify a rise above the minimum value, Zlow, and once this occurs as represented by phase VII in  FIG. 7 , the trajectory is recalculated to begin at the new reaction impedance and the trajectory time is reset by returning to step  332  as represented by phase VIII in  FIG. 7 . The algorithm then continues with the same series of events described previously until tissue fusion is identified. If a rise in impedance is not detected in step  354  within a predetermined period of time then the algorithm proceeds to step  364  in which the algorithm issues a regrasp alarm, and in the step  308  the process concludes. 
     In normal operation, the algorithm directs the RF energy to maintain a match between the tissue impedance and the target value throughout time. Independent of the actual tissue impedance the target trajectory is incremented in a normal fashion during all events unless a reset trajectory is requested. However, it is also envisioned that the trajectory could enter a holding pattern with respect to the last value at any event when the actual tissue impedance deviates significantly from the target impedance until either a reset condition is requested or the tissue impedance realigns with the target value. 
     It is recognized that a number of methods not described here are possible to identify the condition described. The logic intent is to identify an event that results in notable and significant deviation from the impedance target by the tissue and thereby justifying a new target trajectory. Initializing a new trajectory results in mitigating excessive energy delivery to the tissue as the impedance deviates from the target and therefore prevents an uncontrollable tissue effect once the tissue re-reacts. 
     If during the initial RF energy ramp or during a negative deviation of tissue impedance from the target impedance, the tissue does not rise above the lowest measured impedance by a pre-defined amount within a pre-defined time then the algorithm will exit with an alarm. This alerts the user to a possible attempt to seal tissue which is already desiccated or sealed, an attempt to seal tissue which is so large that the tissue is not sufficiently affected by the RF energy delivered, an attempt to seal non-tissue, or a persistent short circuit during the sealing process. 
     The algorithm according to the present disclosure allows for the slow desiccation of tissue and for collagen to denature in a slow controllable fashion. As desiccation progresses, the resulting seal gains plastic-like qualities, becoming hard and clear, which makes the seal capable of withstanding higher burst pressures. 
     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.