METHOD AND APPARATUS FOR COMPLEX IMPEDANCE COMPENSATION AND FOR DETERMINING TISSUE MORPHOLOGY BASED ON PHASE ANGLE

• US 20090171235A1
• Filed: 12/28/2007
• Published: 07/02/2009
• Est. Priority Date: 12/28/2007
• Status: Active Grant

• Current Assignee: St. Jude Medical (Abbott Laboratories Incorporated)
• Original Assignee: St. Jude Medical (Abbott Laboratories Incorporated)
• Inventors: Schneider, Clint, Paul, Saurav, Cao, Hong
• Granted Patent: US 8,290,578 B2
• Time in Patent Office: Days
• US Class Current: 600/547
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body

Abstract

A compensation circuit has a predetermined, known complex impedance and is located in a handle of a catheter or in a distal end of a cable that connects to the catheter. The compensation circuit is probed with a pilot signal produced by a compensation control that is external to the catheter, by way of an electrical connection through the connecting cable. The compensation control measures the complex impedance, which is the combination of the circuit'"'"'s known impedance as well as that of the cable. The compensation control then determines the difference between the measured and the known complex impedances. The difference represents that which is attributable to the cable, and is used to compensate or cancel out such cable-related contributions to complex impedance in measurements made over other electrical connections in the same cable. In another aspect, an unknown tissue is identified as one of a plurality of possible tissue types such as regular myocardium, scar and fat based on the measured phase angle of the complex impedance of the unknown tissue.

Claims

1. An apparatus for use with a catheter having an electrode and a cable for making a first electrical connection between the catheter and an electrical device so as to allow the device to electrically interact with the electrode, said apparatus comprising:

a circuit having a predetermined complex impedance responsive to a pilot signal having a predetermined frequency, said circuit being disposed in one of a handle of said catheter and a distal end of said cable;

a compensation control configured to generate said pilot signal, said cable being further configured to make a second electrical connection between the compensation control and said circuit, said compensation control being further configured to measure a complex impedance of said second electrical connection and said circuit responsive to said pilot signal, said compensation control is further configured to produce a difference signal indicative of a difference between said measured complex impedance and said predetermined, known complex impedance.

2. The apparatus of claim 1 wherein said second electrical connection is independent of said first electrical connection.

3. The apparatus of claim 1 wherein said circuit is one of a loop-back circuit, a series-connected resistor circuit, series-connected resistor-inductor (RL) circuit, a series-connected resistor-capacitor (RC) circuit, a series-connected resistor-inductor-capacitor (RLC) circuit, a parallel-connected RL circuit, a parallel-connected RC circuit, a parallel-connected RLC circuit and a combination of one or more of the foregoing.

4. The apparatus of claim 1 wherein said electrical device is a radio frequency (RF) ablation generator, said electrode is an ablation electrode and said first electrical connection is a positive source lead.

5. The apparatus of claim 1 wherein said difference signal is a phase angle offset.

6. The apparatus of claim 5 wherein said electrical device is an ablation generator that includes a complex impedance determining circuit configured to measure a complex impedance using said electrode and based on said phase angle offset.

7. The apparatus of claim 1 wherein said electrical device is an ablation generator having an operating frequency range between 50 kHz and 500 kHz, said predetermined frequency of said pilot signal being no less than about 6 kHz and no greater than about 50 kHz.

8. The apparatus of claim 7 wherein said predetermined frequency is about 20 kHz.

9. The apparatus of claim 1 wherein said compensation control includes a phase detector configured to produce said difference signal.

10. The apparatus of claim 9 wherein said electrical device is an ablation generator that includes a complex impedance determining circuit configured to measure a complex impedance using said electrode and responsive to said difference signal.

11. A method of operating an electrical device with a catheter having an electrode and a cable for making a first electrical connection between the catheter and the electrical device so as to allow the device to electrically interact with the electrode, said method comprising:

providing a circuit with a first predetermined complex impedance when excited by a pilot signal having a predetermined frequency;

exciting the circuit with the pilot signal over a second electrical connection in the cable;

measuring a second complex impedance of a combination of the second electrical connection and the circuit;

determining a difference between the measured complex impedance and the predetermined complex impedance; and

operating the electrical device in accordance with the difference.

12. The method of claim 11 wherein said electrical device comprises an ablation generator and said electrode comprises an ablation electrode, and wherein said operating step includes the substep of:

measuring a third complex impedance over the first electrical connection using the ablation electrode indicative of the combined impedance of tissue in contact with the electrode and the first electrical connection;

determining a fourth complex impedance associated with the tissue based on the measured third complex impedance and the determined difference.

13. The method of claim 11 wherein said step of providing the circuit includes the substep of:

locating the circuit is one of a handle of the catheter and a distal end of the cable.

14. A method of determining tissue type comprising the steps of:

providing a first electrode in contact with tissue where the tissue may be one of a plurality of tissue types;

measuring a complex impedance at a preselected excitation frequency between the first electrode and a second electrode where the measured complex impedance comprises a magnitude and a phase angle; and

identifying one of the tissue types based on at least one of the measured magnitude and the measured phase angle.

15. The method of claim 14 further including the step of:

defining the plurality of tissue types to include regular myocardium, scar and fat.

16. The method of claim 14 wherein said step of providing the first electrode includes the substeps of:

selecting a size between Five French (5 Fr) and Ten French (10 Fr) in diameter and 2 to 8 mm in length for the first electrode.

17. The method of claim 14 wherein said step of providing the first electrode includes the substeps of:

introducing the first electrode using a catheter during a cardiac catheter ablation procedure for contact with the tissue.

18. The method of claim 14 wherein said step of measuring the complex impedance includes the substep of:

selecting the excitation frequency from a range between about 20 kHz and 1000 kHz.

19. The method of claim 18 wherein said range is between about 50 kHz and 500 kHz.

20. The method of claim 14 wherein said second electrode is a dispersive/indifferent reference electrode, said measured complex impedance reflecting properties of tissue proximate said first electrode.

21. The method of claim 14 wherein said first and second electrodes are each electrodes in contact with the tissue, said measured complex impedance reflecting properties of tissue between the first and second electrodes.

22. The method of claim 14 wherein said identifying step includes the substep of:

identifying the tissue type based on the measured phase angle.

23. An apparatus for determining one of a plurality of tissue types, said apparatus comprising;

a source configured to supply an excitation signal at a predetermined frequency to a catheter for application at a first electrode of the catheter;

a measurement circuit configured to measure a complex impedance between said first electrode and a second electrode responsive to said excitation signal being supplied where said measured complex impedance comprises a magnitude and a phase angle; and

a tissue identification logic block configured to identify one of said plurality of tissue types based on at least one of said measured magnitude and phase angle.

24. The apparatus of claim 23 wherein said plurality of tissue types includes regular myocardium, scar and fat.

25. The apparatus of claim 23 wherein a size of said first electrode is between Five French (5 Fr) and Ten French (10 Fr) in diameter and 2 to 8 mm in length.

26. The apparatus of claim 23 wherein said preselected frequency of said excitation signal is between about 20 kHz and 1000 kHz.

27. The apparatus of claim 26 wherein said preselected frequency is between about 50 kHz and 500 kHz.

28. The apparatus of claim 23 wherein said second electrode is a dispersive/indifferent reference electrode, said measured complex impedance reflecting properties of tissue proximate said first electrode.

29. The apparatus of claim 23 wherein said first and second electrodes are in contact with said tissue, said measured complex impedance reflecting properties of tissue between said first and second electrodes.

30. The apparatus of claim 23 wherein said logic block is further configured to identify said one tissue type based on said measured phase angle.

31. A method for determining a type of a tissue that may be one of a plurality of types, comprising the steps of:

(A) measuring a phase angle component of a complex impedance of the tissue; and

(B) identifying one of the plurality of tissue types based on the measured phase angle.

32. The method of claim 31 wherein the measuring step includes the substeps of:

selecting an excitation frequency for an excitation signal;

applying the excitation signal to the tissue and measuring the response signal induced thereby to determine the phase angle.

33. The method of claim 32, wherein the predetermined frequency is between about 20 kHz and 100 kHz.