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 sealing 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 . fig1 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 fig1 , 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 fig3 . 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 fig1 , 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 fig1 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 . fig2 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 fig4 . 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 fig8 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 fig5 . 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 sealing tissue according to the present disclosure is discussed below with reference to fig6 a - b . in addition , fig7 shows a graph illustrating the changes to tissue impedance when tissue is sealed utilizing the method of fig6 a - 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 fig7 , 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_eotal ). 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 fig8 . the curve of fig8 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 rf 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 fig7 . 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 ins 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 fig7 . 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 impedance ( 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 fig7 . 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 fig7 . 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 thau 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 sealing portion of the vessel scaling 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 re 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 fig7 . 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 , zhist . 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 fig7 , 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 fig7 . 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 .
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 .