# Patent

## Title
Imaginary impedance process monitoring and intelligent shut-off

## Abstract

An electrosurgical generator for supplying electrosurgical energy to tissue is disclosed. The generator includes sensor circuitry configured to measure an imaginary impedance and/or a rate of change of the imaginary impedance of tissue. The generator also includes a controller configured to regulate output of the electrosurgical generator based on the measured imaginary impedance and/or the rate of change of the imaginary impedance.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
The present application is a divisional of U.S. patent application Ser. No. 12/477,245 entitled “IMAGINARY IMPEDANCE PROCESS MONITORING AND INTELLIGENT SHUT-OFF” filed Jun. 3, 2009, which is a continuation-in-part of U.S. Pat. No. 8,167,875 entitled “ENERGY DELIVERY ALGORITHM FOR MEDICAL DEVICES,” U.S. Pat. No. 8,152,802 entitled “ENERGY DELIVERY ALGORITHM FILTER PRE-LOADING,” and U.S. Pat. No. 8,162,932 entitled “ENERGY DELIVERY ALGORITHM IMPEDANCE TREND ADAPTATION,” all of which were filed on Jan. 12, 2009, the disclosures of each of which are hereby incorporated by reference herein in their entirety.


BACKGROUND
1. Technical Field
The present disclosure relates to electrosurgical apparatuses, systems and methods. More particularly, the present disclosure is directed to electrosurgical systems and methods for monitoring electrosurgical procedures and intelligent termination thereof based on imaginary impedance.
2. Background of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ohmic, resistive, ultrasonic, microwave, cryogenic, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon that is applied to the tissue. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator.
Ablation is most commonly a monopolar procedure that is particularly useful in the field of cancer treatment, where one or more RF ablation needle electrodes that (usually of elongated cylindrical geometry) are inserted into a living body. A typical form of such needle electrodes incorporates an insulated sheath disposed over an exposed (uninsulated) tip. When the RF energy is provided between the return electrode and the inserted ablation electrode, RF current flows from the needle electrode through the body. Typically, the current density is very high near the tip of the needle electrode, which tends to heat and destroy surrounding issue.
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 with body tissue with either of the separated electrodes prevents the flow of current.
Bipolar electrosurgical techniques and instruments can be used to coagulate blood vessels or tissue, e.g., soft tissue structures, such as lung, brain and intestine. A surgeon can either cauterize, coagulate/desiccate and/or simply reduce or slow bleeding, by controlling the intensity, frequency and duration of the electrosurgical energy applied between the electrodes and through the tissue. In order to achieve one of these desired surgical effects without causing unwanted charring of tissue at the surgical site or causing collateral damage to adjacent tissue, e.g., thermal spread, it is necessary to control the output from the electrosurgical generator, e.g., power, waveform, voltage, current, pulse rate, etc.
It is known that measuring the electrical impedance and changes thereof across the tissue at the surgical site provides a good indication of the state of desiccation or drying of the tissue, e.g., as the tissue dries or loses moisture, the impedance across the tissue rises. This observation has been utilized in some electrosurgical generators to regulate the electrosurgical power based on measured tissue impedance.
SUMMARY
According to one embodiment of the present disclosure, an electrosurgical generator for supplying electrosurgical energy to tissue is disclosed. The generator includes sensor circuitry configured to measure an imaginary impedance and/or a rate of change of the imaginary impedance of tissue. The generator also includes a controller configured to regulate output of the electrosurgical generator based on the measured imaginary impedance and/or the rate of change of the imaginary impedance.
A method for supplying electrosurgical energy to tissue is also contemplated by the present disclosure. The method includes the steps of: measuring an imaginary impedance and/or a rate of change of the imaginary impedance of tissue and regulating output of the electrosurgical generator based on the measured imaginary impedance and/or the rate of change of the imaginary impedance.
Another method for supplying electrosurgical energy to tissue is also contemplated by the present disclosure. The method includes the steps of: measuring an imaginary impedance and/or a rate of change of the imaginary impedance of tissue and regulating output of the electrosurgical generator based on the measured imaginary impedance and/or the rate of change of the imaginary impedance and during the regulating step comparing the rate of change of the imaginary impedance with a first predetermined threshold and a second predetermined threshold.



BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the present disclosure are described herein with reference to the drawings wherein:
FIG. 1A is a schematic block diagram of a monopolar electrosurgical system according to one embodiment of the present disclosure;
FIG. 1B is a schematic block diagram of a bipolar electrosurgical system according to one embodiment of the present disclosure;
FIG. 2 is a schematic block diagram of a generator according to an embodiment of the present disclosure;
FIG. 3 is a flow chart of a method according to one embodiment of the present disclosure;
FIG. 4 is a plot of imaginary impedance of tissue during application of electrosurgical energy according to one embodiment of the present disclosure;
FIG. 5 is a plot of imaginary impedance of tissue during application of electrosurgical energy according to one embodiment of the present disclosure;
FIG. 6 is a plot of imaginary impedance of tissue during application of electrosurgical energy according to one embodiment of the present disclosure; and
FIG. 7 is a plot of imaginary impedance of tissue during application of electrosurgical energy according to one embodiment of the present disclosure.



DETAILED DESCRIPTION
Particular embodiments of the present disclosure are 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.
The generator according to the present disclosure can perform monopolar and bipolar electrosurgical procedures as well as microwave ablation procedures, including vessel sealing procedures. The generator may include a plurality of outputs for interfacing with various electrosurgical instruments (e.g., a monopolar active electrode, return electrode, bipolar electrosurgical forceps, footswitch, etc.). Further, the generator includes electronic circuitry configured for generating radio frequency power specifically suited for various electrosurgical modes (e.g., cutting, blending, division, etc.) and procedures (e.g., monopolar, bipolar, vessel sealing).
FIG. 1A is a schematic illustration of a monopolar electrosurgical system according to one embodiment of the present disclosure. The system includes an electrosurgical instrument 2 having one or more electrodes for treating tissue of a patient P. The instrument 2 is a monopolar type instrument including one or more active electrodes (e.g., electrosurgical cutting probe, ablation electrode(s), etc.). Electrosurgical RF energy is supplied to the instrument 2 by a generator 20 via an supply line 4, which is connected to an active terminal 30 (FIG. 2) of the generator 20, allowing the instrument 2 to coagulate, seal, ablate and/or otherwise treat tissue. The energy is returned to the generator 20 through a return electrode 6 via a return line 8 at a return terminal 32 (FIG. 2) of the generator 20. The active terminal 30 and the return terminal 32 are connectors configured to interface with plugs (not explicitly shown) of the instrument 2 and the return electrode 6, which are disposed at the ends of the supply line 4 and the return line 8, respectively.
The system may include a plurality of return electrodes 6 that are arranged to minimize the chances of tissue damage by maximizing the overall contact area with the patient P. In addition, the generator 20 and the return electrode 6 may be configured for monitoring so-called “tissue-to-patient” contact to insure that sufficient contact exists therebetween to further minimize chances of tissue damage.
FIG. 1B is a schematic illustration of a bipolar electrosurgical system according to the present disclosure. The system includes a bipolar electrosurgical forceps 10 having one or more electrodes for treating tissue of a patient P. The electrosurgical forceps 10 includes opposing jaw members having an active electrode 14 and a return electrode 16 disposed therein. The active electrode 14 and the return electrode 16 are connected to the generator 20 through cable 18, which includes the supply and return lines 4, 8 coupled to the active and return terminals 30, 32, respectively (FIG. 2). The electrosurgical forceps 10 is coupled to the generator 20 at a connector 21 having connections to the active and return terminals 30 and 32 (e.g., pins) via a plug disposed at the end of the cable 18, wherein the plug includes contacts from the supply and return lines 4, 8.
The generator 20 includes suitable input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator 20. In addition, the generator 20 may include one or more display screens for providing the user with variety of output information (e.g., intensity settings, treatment complete indicators, etc.). The controls allow the user to adjust power of the RF energy, waveform parameters (e.g., crest factor, duty cycle, etc.), and other parameters to achieve the desired waveform suitable for a particular task (e.g., coagulating, tissue sealing, intensity setting, etc.). The instrument 2 may also include a plurality of input controls that may be redundant with certain input controls of the generator 20. Placing the input controls at the instrument 2 allows for easier and faster modification of RF energy parameters during the surgical procedure without requiring interaction with the generator 20.
FIG. 2 shows a schematic block diagram of the generator 20 having a controller 24, a high voltage DC power supply 27 (“HVPS”) and an RF output stage 28. The HVPS 27 is connected to a conventional AC source (e.g., electrical wall outlet) and provides high voltage DC power to an RF output stage 28, which then converts high voltage DC power into RF energy and delivers the RF energy to the active terminal 30. The energy is returned thereto via the return terminal 32.
In particular, the RF output stage 28 generates sinusoidal waveforms of high RF energy. The RF output stage 28 is configured to generate a plurality of waveforms having various duty cycles, peak voltages, crest factors, and other suitable parameters. Certain types of waveforms are suitable for specific electrosurgical modes. For instance, the RF output stage 28 generates a 100% duty cycle sinusoidal waveform in cut mode, which is best suited for ablating, fusing and dissecting tissue and a 1-25% duty cycle waveform in coagulation mode, which is best used for cauterizing tissue to stop bleeding.
The generator 20 may include a plurality of connectors to accommodate various types of electrosurgical instruments (e.g., instrument 2, electrosurgical forceps 10, etc.). Further, the generator 20 is configured to operate in a variety of modes such as ablation, monopolar and bipolar cutting coagulation, etc. It is envisioned that the generator 20 may include a switching mechanism (e.g., relays) to switch the supply of RF energy between the connectors, such that, for instance, when the instrument 2 is connected to the generator 20, only the monopolar plug receives RF energy.
The controller 24 includes a microprocessor 25 operably connected to a memory 26, which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microprocessor 25 includes an output port that is operably connected to the HVPS 27 and/or RF output stage 28 allowing the microprocessor 25 to control the output of the generator 20 according to either open and/or closed control loop schemes. Those skilled in the art will appreciate that the microprocessor 25 may be substituted by any logic processor (e.g., control circuit) adapted to perform the calculations discussed herein.
A closed loop control scheme is a feedback control loop wherein sensor circuitry 22, which may include a plurality of sensors measuring a variety of tissue and energy properties (e.g., tissue impedance, tissue temperature, output current and/or voltage, voltage and current passing through the tissue, etc.), provides feedback to the controller 24. Such sensors are within the purview of those skilled in the art. The controller 24 then signals the HVPS 27 and/or RF output stage 28, which then adjust DC and/or RF power supply, respectively. The controller 24 also receives input signals from the input controls of the generator 20 or the instrument 2. The controller 24 utilizes the input signals to adjust power outputted by the generator 20 and/or performs other control functions thereon.
The present disclosure provides for a system and method for monitoring electrosurgical procedures using imaginary impedance. The use of imaginary impedance to control delivery of electrosurgical energy is discussed with respect to performing ablation procedures. Those skilled in the art will appreciate that the illustrated embodiments may be utilized with other electrosurgical procedures and/or modes.
FIG. 3 shows a method for controlling output of the generator 20 based on imaginary impedance. The method may be embodied as a software application embedded in the memory 26 and executed by the microprocessor 25 to control generator 20 output based on measured imaginary impedance or changes in imaginary impedance as a function of time. Complex impedance consists of real and imaginary impedance. Real impedance is identified with resistance and imaginary impedance is identified with reactance. In addition, reactive impedance may be either inductive or capacitive. Purely resistive impedance exhibits no phase shift between the voltage and current, whereas reactance induces a phase shift θ between the voltage and the current passing through the tissue, thus imaginary impedance may be calculated based on the phase angle or phase shift between the voltage and current waveforms.
Changes in the imaginary impedance during energy delivery may be used as an indication of changes in tissue properties due to energy application. More specifically, imaginary impedance may be used to detect the formation of microbubbles, bubble fields and tissue desiccation that impart an electrical reactivity to the tissue that corresponds to sensed imaginary impedance. The tissue reactivity is reflective of the energy that is being delivered into the tissue. Thus, the measured change in imaginary impedance may be used as an indication of the amount of energy resident in the tissue. Monitoring of the resident energy in combination with monitoring of the energy being supplied by the generator allows for calculation of energy escaping the tissue during treatment, thereby allowing for determination of efficiency of the treatment process as well as any inadvertent energy drains.
With reference to FIG. 3, in step 200, ablation energy is delivered into tissue and the imaginary impedance is measured by the sensor circuitry 22. The sensor circuitry 22 measures voltage and current waveforms passing through the tissue and determines the imaginary impedance (e.g., the imaginary component of the complex impedance) based on the phase angle between the waveforms. FIG. 4 illustrates a graph 100 of imaginary impedance vs. time (10 samples per second). The graph 100 includes detected impedance curve 110 having multiple pulses reflective of pulsatile application of electrosurgical energy. The impedance curve 110 is represented using negative numbers (e.g., −3Ω to −6Ω), such that higher negative numbers indicate greater reactive inductance and smaller negative numbers indicate more reactive capacitance. In step 202, the impedance curve 110 is pre-filtered to allow for faster processing to generate a pre-filtered curve 120. Various filters may be utilized to achieve the pre-filtered curve 120, such as Kalman Filter and the like.
With respect to FIG. 4, at the start of application of electrosurgical energy, imaginary impedance starts at a negative value (e.g. from about −5Ω to about −7Ω) and rapidly increases by about 2, this trend is also illustrated in FIGS. 5-7 as discussed in more detail below. The rapid increase in imaginary impedance slows after about 2 to 3 minutes. The trend in imaginary impedance (e.g., rapid increase then slowing after a few minutes) also correlates to the temperature trends observed during ablation. Without being constrained to a particular theory, it is believed that as energy is applied to the tissue, microbubbles form in the intracellular and intercellular space, resulting in low starting imaginary impedance (e.g., more negative, associated with more inductive). As the temperature of the tissue increases, liquid water is driven away from the tissue regions close to phase transition temperature (e.g., 80° C. and above), more microbubbles form, steam bubbles increase in size and these regions become desiccated. The desiccated regions of tissue have higher impedance (e.g., more positive, associated with more positive) and therefore contribute to capacitive impedance. These phenomena are mostly reversible because as the temperature increases and drives the water out, osmotic pressures generate a reverse flow of water. As a result, the tissue seeks a new equilibrium or steady state condition between a desiccated state and a hydrated state to reestablish energy balance.
Once the equilibrium is achieved, the thermal kill zone (e.g., ablation size of the lesion) does not grow significantly. Thus, establishment of equilibrium correlates to the maximum thermal kill zone and may be used to determine whether termination of energy application is appropriate. In other words, monitoring of imaginary impedance allows for determination of the equilibrium, which correlates with maximum thermal kill zone and may therefore, serve as a suitable threshold of intelligent shut-off.
Determination of the equilibrium may be determined by analyzing the slope of the pre-filtered curve 120 or the rate of change of the imaginary impedance. The determination of the slope may be performed at the sensor circuitry 22 and/or the controller 24. A slope of about 0 is believed to be reflective of the establishment of equilibrium, whereas a negative slope corresponds to reduction in energy accumulation within the tissue. Prior to slope analysis, the pre-filtered curve 120 is filtered using single pole recursive filter. Thus, the first filter smoothes out the impedance curve 110 and the recursive filtering detects direction and magnitude of slope changes as described below.
In step 204, the slope of the pre-filtered curve 120 (e.g., rate of change of the imaginary impedance) is determined. According to one embodiment of the present disclosure, the determination of the rate of change may be achieved via single pole recursive filtering that averages a predetermined number of imaginary impedance values to achieve the rate of change value. Any number of impedance filters may be used and are based on the following formula (1):

ZfXn=Zin*A+ZfXn-1*B  (1)

A and B are dependent on a time constant and may be specified by the user, via the input controls of the generator 20, for each particular impedance filter ZfX. When calculating A and B, the following formulas may be used:

B=e^(−1/number of samples);

A=1−B.

The sample rate may also be specified by the user for calculating the number of samples. In formula (1), Zin is the new root mean square imaginary impedance value (e.g., ZiRMS) just calculated, and ZfXn-1 is the filtered imaginary impedance, for the filter number specified by X, from the previous iteration through the loop, and ZfXn is the new filtered impedance value for the filter number specified by X. In one embodiment, the sample rate for calculating the number of samples may be synchronized with the loop time of the microprocessor 25. Accordingly, within about 5 time constants, the final output of the imaginary impedance filter may be provided that corresponds to the slope of the pre-filtered curve 120. In another embodiment, an initial base imaginary impedance may be used to preload the imaginary impedance filters.
In step 206, the slope of the pre-filtered curve 120 is analyzed. In one embodiment, the slope is analyzed using three regions (e.g., two thresholds). In step 208, it is determined whether the slope is above a first predetermined threshold (e.g., a positive threshold number). In step 210 it is determined whether the slope is between the first threshold and a second predetermined threshold (e.g., a negative number). In step 212, it is determined if the slope is below the second threshold. In another embodiments, a plurality of regions may be utilized based on multiple actions that need to be performed in response to varying slope values. Based on the analysis of the rate of change of the imaginary impedance (e.g., slope) and/or the imaginary impedance, the controller 24 adjusts the output of the generator 20 as discussed in more detail below.
When the slope is above the first threshold, this indicates that the thermal profile is growing and that energy application may continue in step 208. The process then reverts to step 206 to continue slope monitoring and energy application. When the slope is between the first and second thresholds, the thermal profile is in equilibrium which denotes that equilibrium has been reached and an intelligent shut-off process is commenced as shown in step 214. Once it is determined that equilibrium has been reached, a verification is made if a predetermined time delay has expired. This provides a second verification to determine that a substantial portion of the tissue has been treated. The time delay may be user-selectable either by entering a predetermined time value or by selecting one of proposed delay periods. In one embodiment, one of the options may be a time delay corresponding to the shortest time for establishing termination of the procedure and another option may be a time delay corresponding to a conservative treatment regimen that assurance 100% cell kill ratio.
Effects of choosing different time delays is illustrated in FIG. 5, which shows graph 100 of imaginary impedance vs. time (10 samples per second). Imaginary impedance increases initially and then reaches equilibrium as illustrated by the plateau and then decreases significantly. This directs the generator 20 to commence the intelligent shut-off process. If a first time delay “A” (e.g., 5.0 minutes) is selected, the treatment is terminated at the arrow “A.” If a second time delay “B” (e.g., 10.8 minutes) is chosen, the treatment is terminated at the arrow “B.” The time delay “A” represents a very aggressive time delay, while the second time delay “B” illustrates a very conservative time delay. While there is a 5.8 minute difference between the time delays, the resulting ablation volumes only differed by 0.05 cm in diameter. This illustrates that a longer time delay may not necessarily produce larger ablation volumes and a selection of an appropriate time delay period in certain situations may be user-selectable.
In one embodiment, an intermediate time delay may also be utilized. An intermediate time delay is triggered in step 216 once an equilibrium is reached and the slope detection still continues to make sure that the slope trends do not change. If the slope increases above the first threshold, then energy application resumes. As shown in FIG. 6, which shows graph 100 of imaginary impedance vs. time (10 samples per second), the pre-filtered curve initially reaches equilibrium and then has a slightly negative slope. At this point, the intermediate time delay is triggered and slope interrogation continues. In other words, the process then reverts to step 206 to continue slope monitoring and energy application.
When the slope is less than the second threshold, this denotes that energy application efficiency is decreasing and the procedure should be terminated. This may be caused by proximity to a blood vessel and other obstructions. FIG. 7 shows graph 100 of imaginary impedance vs. time (10 samples per second) having the pre-filtered curve 120 with substantial undulations. Upon encountering negative slopes that are below the second threshold, the process in step 218 terminates the application of energy and/or alerts the user of the decrease in energy application.
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.

## Claims

1. A method for supplying electrosurgical energy to tissue, comprising the steps of:
measuring at least one of an imaginary impedance and a rate of change of the imaginary impedance of tissue; and
regulating output of the electrosurgical energy from an electrosurgical generator based on the at least one of the imaginary impedance and rate of change of the imaginary impedance:
to continue outputting the electrosurgical energy when the rate of change of the imaginary impedance is above a first predetermined threshold;
to discontinue outputting of the electrosurgical energy when the rate of change of the imaginary impedance is below a second predetermined threshold; and
to commence termination of the electrosurgical energy when the rate of change of the imaginary impedance is between the first and second predetermined thresholds for a duration of a predetermined time delay.

2. The method according to claim 1, further comprising the step of:
recursively processing the imaginary impedance to obtain an averaged value of the imaginary impedance.

3. The method according to claim 2, wherein the recursively processing step further includes the step of preloading at least two recursive filters with an initial value of the imaginary impedance.

4. The method according to claim 1, wherein the regulating step further includes the step of:
restarting application of the electrosurgical energy when the rate of change of the imaginary impedance is above the first predetermined threshold during the predetermined time delay.

5. A method for supplying electrosurgical energy to tissue, comprising the steps of:
measuring an imaginary impedance and a rate of change of the imaginary impedance of tissue; and
regulating the electrosurgical energy based on at least one of the imaginary impedance and rate of change of the imaginary impedance, wherein the regulating step further includes the step of:
comparing the rate of change of the imaginary impedance with a first predetermined threshold and a second predetermined threshold;
continuing application of the electrosurgical energy to tissue when the rate of change of the imaginary impedance is above the first predetermined threshold;
discontinuing application of the electrosurgical energy to tissue when the rate of change of the imaginary impedance is below the second predetermined threshold; and
terminating the electrosurgical energy when the rate of change of the imaginary impedance is between the first and second predetermined thresholds for a duration of a predetermined time delay.

6. The method according to claim 5, further comprising the step of:
recursively processing the imaginary impedance to obtain an averaged value of the imaginary impedance.

7. The method according to claim 5, wherein the regulating step further includes the step of:
restarting application of the electrosurgical energy when the rate of change of the imaginary impedance is above the first predetermined threshold during the predetermined time delay.