Systems and methods for measuring tissue impedance through an electrosurgical cable

• US 9,636,165 B2
• Filed: 02/14/2014
• Issued: 05/02/2017
• Est. Priority Date: 07/29/2013
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Covidien LP (Medtronic Plc)
• Inventors: Waskiewicz, Alexander M., Larson, Eric J., Ford, Carolyn G.
• Primary Examiner(s): Stoklosa, Joseph
• Assistant Examiner(s): Kim, Eunhwa
• Application Number:
  US14/181,114
• Publication Number:
  US 20150032099A1
• Time in Patent Office: 1,173 Days
• CPC Class Codes: A61B 18/1233 with circuits for assuring patient safety
• CPC Class Codes: A61B 18/1442 Probes having pivoting end effectors, e.g. forceps
• CPC Class Codes: A61B 18/16 Indifferent or passive electrodes for grounding
• CPC Class Codes: A61B 2018/00577 Ablation
• CPC Class Codes: A61B 2018/00607 Coagulation and cutting with the same instrument
• CPC Class Codes: A61B 2018/00875 Resistance or impedance

Abstract

The electrosurgical systems and methods of the present disclosure include a tissue resistance measurement system that compensates for capacitive parasitics in a cable connecting an electrosurgical generator to and electrosurgical cable to estimate the real resistance of a tissue load. The electrosurgical generator includes an output stage coupled to an electrical energy source and generates electrosurgical energy. The electrosurgical generator includes a plurality of sensors sensing a voltage and current of the electrosurgical energy and a controller controlling the output stage. The controller includes a calculator that calculates a real part of an impedance based on the sensed voltage and current, an estimator that estimates a resistance of the tissue using a solution to a quadratic equation that is a function of the real part of the impedance, and a control signal generator configured to generate a control signal for the output stage based on the resistance of the tissue.

Claims

1. An electrosurgical generator that delivers electrosurgical energy to tissue via a cable and an instrument coupled to the cable, the electrosurgical generator comprising:

an output stage coupled to an electrical energy source and configured to generate electrosurgical energy;

a plurality of sensors configured to sense a voltage waveform and a current waveform of the generated electrosurgical energy; and

a controller configured to control the output stage, the controller comprising;

a calculator configured to calculate a real part of an impedance based on the sensed voltage and current waveforms;

an estimator configured to estimate a resistance of the tissue using a solution to a quadratic equation that is a function of a real part of the impedance; and

a control signal generator configured to generate a control signal based on the estimated resistance of the tissue,

wherein the solution to the quadratic equation is

2. The electrosurgical generator according to claim 1, wherein the estimator uses the calculated real part of the impedance as the estimated resistance of the tissue when the calculated real part of the impedance is less than a predetermined value.

3. The electrosurgical generator according to claim 1, wherein the quadratic equation is derived from a model of the cable having a series inductor and a shunt capacitor.

4. The electrosurgical generator according to claim 3, wherein the estimated resistance of the tissue is a larger solution to the quadratic equation when a phase difference between the voltage waveform and the current waveform is less than or equal to −

45 degrees, or a smaller solution when the phase difference is greater than −

45 degrees.

5. The electrosurgical generator according to claim 1, further comprising a memory for storing a lookup table that maps a plurality of values of the real part of the impedance to a plurality of corresponding values of the estimated resistances of the tissue that are calculated according to a solution to the quadratic equation that is a function of the real part of the impedance,

wherein the estimator estimates the resistance of the tissue by accessing the lookup table to determine the estimated resistance of the tissue corresponding to the calculated real part of the impedance.

6. The electrosurgical generator according to claim 5, wherein, when the calculated real part of the impedance is between two real parts of the impedance in the lookup table, the estimator selects the estimated resistance of the tissue corresponding the real part of the impedance in the lookup table that is nearest to the calculated real part of the impedance or interpolates between the two real parts of the impedance in the lookup table to determine the estimated resistance of the tissue.

7. The electrosurgical generator according to claim 1, further comprising a memory for storing a lookup table that maps a plurality of values of the real part of the impedance to a plurality of corresponding correction factors that are calculated based on a solution to the quadratic equation that is a function of the real part of the impedance,

wherein the estimator estimates the resistance of the tissue by accessing the lookup table to determine a desired correction factor corresponding to the calculated real part of the impedance and multiplying the desired correction factor by the calculated real part of the impedance to obtain the estimated resistance of the tissue.

8. The electrosurgical generator according to claim 1, wherein the estimator estimates the resistance of the tissue using a polynomial equation that is determined by performing a polynomial curve fit to the solution to the quadratic equation that is a function of the real part of the impedance.

9. The electrosurgical generator according to claim 1, further comprising an inductor coupled to the output stage and tuned to a shunt capacitance and a series inductance of the cable so that the calculated real part of the impedance is sufficiently resistive.

10. The electrosurgical generator according to claim 9, wherein the shunt capacitance of the cable is a capacitance value measured when electrodes of the instrument are not in contact with tissue.

11. The electrosurgical generator according to claim 9, wherein an inductance value of the inductor is equal to

12. A method of controlling an electrosurgical generator that delivers electrosurgical energy to tissue via a cable and an instrument coupled to the cable, the method comprising:

sensing a voltage waveform and a current waveform of the generated electrosurgical energy;

calculating a real part of an impedance based on the sensed voltage and current waveforms;

estimating a resistance of the tissue using a solution to a quadratic equation that is a function of a real part of the impedance; and

generating a control signal to control the output from an output stage of the electrosurgical generator based on the estimated resistance of the tissue,

wherein the solution to the quadratic equation is

13. The method according to claim 12, further comprising:

determining whether the calculated real part of the impedance is less than a predetermined value; and

using the calculated real part of the impedance as the estimated resistance of the tissue when the calculated real part of the impedance is less than the predetermined value.

14. The method according to claim 12, further comprising using a larger solution to the quadratic equation as the estimated resistance of the tissue when a phase difference between the voltage waveform and the current waveform is smaller than or equal to −

45 degrees, or using a smaller solution to the quadratic equation as the estimated resistance of the tissue when the phase difference is greater than −

45 degrees.

15. The method according to claim 12, wherein the quadratic equation is derived from a model of the cable having a series inductor and a shunt capacitor.

16. The method according to claim 12, further comprising storing a lookup table that maps a plurality of values of the real part of the impedance to a plurality of corresponding values of the estimated resistances of the tissue that are calculated according to a solution to the quadratic equation that is a function of the real part of the impedance,

wherein estimating the resistance of the tissue includes accessing the lookup table to determine the estimated resistance of the tissue corresponding to the calculated real part of the impedance.

17. The method according to claim 16, wherein, when the calculated real part of the impedance is between two real parts of the impedance in the lookup table, estimating the resistance of the tissue includes selecting the estimated resistance of the tissue corresponding the real part of the impedance in the lookup table that is nearest to the calculated real part of the impedance or interpolating between the two real part of the impedance in the lookup table to determine the estimated resistance of the tissue.

18. The method according to claim 12, further comprising storing a lookup table that maps a plurality of values of the real part of the impedance to a plurality of corresponding correction factors that are calculated based on a solution to the quadratic equation that is a function of the real part of the impedance,

wherein the estimating the resistance of the tissue includes accessing the lookup table to determine a desired correction factor corresponding to the calculated real part of the impedance and multiplying the desired correction factor by the calculated real part of the impedance to obtain the estimated resistance of the tissue.

19. The method according to claim 12, wherein estimating the resistance of the tissue includes using a polynomial equation that is determined by performing a polynomial curve fit to the solution to the quadratic equation that is a function of the real part of the impedance.

20. A non-transitory computer-readable storage medium storing a program that, when executed a processor, performs a method of controlling an electrosurgical generator that delivers electrosurgical energy to tissue via a cable and an instrument coupled to the cable, the method comprising:

sensing a voltage waveform and a current waveform of the generated electrosurgical energy;

calculating a real part of an impedance based on the sensed voltage and current waveforms;

estimating a resistance of the tissue using a solution to a quadratic equation that is a function of a real part of the impedance; and

generating a control signal to control the output from an output stage of the electrosurgical generator based on the estimated resistance of the tissue,

wherein the solution to the quadratic equation is