Method and Apparatus to Measure Bioelectric Impedance of Patient Tissue

• US 20090264792A1
• Filed: 04/20/2009
• Published: 10/22/2009
• Est. Priority Date: 04/18/2008
• Status: Active Grant

• Current Assignee: Medtronic Monitoring, Inc. (Medtronic Plc)
• Original Assignee: Corventis
• Inventors: Mazar, Scott T.
• Granted Patent: US 8,412,317 B2
• Time in Patent Office: Days
• US Class Current: 600/547
• CPC Class Codes: A61B 2560/0223 of calibration, e.g. protocols for calibrating sensors
• CPC Class Codes: A61B 5/0002 Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network A61B5/07 takes precedence; transmission systems for measured values G08C; transmission H04B; transmission of digital information H04L; wireless communication networks H04W
• CPC Class Codes: A61B 5/0006 ECG or EEG signals
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/0531 Measuring skin impedance
• CPC Class Codes: A61B 5/0809 by impedance pneumography
• CPC Class Codes: A61B 5/259 using conductive adhesive means, e.g. gels
• CPC Class Codes: A61B 5/282 Holders for multiple electrodes for introduction into the body A61B5/287
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/318 Heart-related electrical modalities, e.g. electrocardiography [ECG]
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: A61B 7/00 Instruments for auscultation

Abstract

A device to measure tissue impedance comprises drive circuitry coupled to calibration circuitry, such that a calibration signal from the calibration circuitry corresponds to the current delivered through the tissue. Measurement circuitry can be coupled to measurement electrodes and the calibration circuitry, such that the tissue impedance can be determined in response to the measured calibration signal from the calibration circuitry and the measured tissue impedance signal from the measurement electrodes. Processor circuitry comprising a tangible medium can be configured to determine a complex tissue impedance in response to the calibration signal and the tissue impedance signal. The processor can be configured to select a frequency for the drive current, and the amount of drive current at increased frequencies may exceed a safety threshold for the drive current at lower frequencies.

Claims

1. A device for measuring an impedance of a tissue of a patient, the device comprising:

calibration circuitry comprising an impedance;

at least four electrodes configured to couple to the tissue of the patient, the at least four electrodes comprising at least two measurement electrodes and at least two drive electrodes;

drive circuitry coupled to the at least two drive electrodes and the calibration circuitry to pass a current through the at least two drive electrodes and the calibration circuitry;

measurement circuitry configured to couple to the at least two measurement electrodes and the calibration circuitry, the measurement circuitry configured to measure a calibration signal from the calibration circuitry and a tissue impedance signal from the at least two measurement electrodes; and

processor circuitry comprising a tangible medium configured to determine the impedance of the tissue in response to the calibration signal and the tissue impedance signal.

2. The device of claim 1 wherein the processor circuitry comprises as least one of an impedance converter or a microcontroller.

3. The device of claim 1 wherein the processor circuitry is configured to determine the impedance of the tissue with a discrete Fourier transform of at least one of measurement signal or the current signal.

4. The device of claim 1 wherein the calibration circuitry is connected in series between the drive circuitry and the at least two measurement electrodes to calibrate the tissue impedance measurement when the at least two electrodes are connected to the patient.

5. The device of claim 1 wherein the drive circuitry is configured to pass the current through the tissue and the calibration circuitry to generate the tissue measurement signal and the calibration signal when the at least four electrodes are connected to the tissue.

6. The device of claim 5 wherein the calibration circuitry comprises a calibration resistor, and the measurement circuitry is configured to measure the calibration signal in response to the current through the calibration resistor and the tissue.

7. The device of claim 6 wherein the measurement circuitry is configured to measure the tissue measurement signal in response to the current through the tissue and the calibration resistor.

8. The device of claim 7 wherein the processor is configured to determine the tissue impedance in response to the calibration signal and the tissue measurement signal.

9. The device of claim 1 further comprising at least one switch coupled to the drive circuitry, the measurement circuitry, the calibration circuitry and the at least four electrodes, the at least one switch comprising a first configuration and a second configuration, wherein in the first configuration the at least one switch couples the measurement circuitry to the calibration circuitry to measure the calibration signal and wherein in the second configuration the at least one switch couples the measurement circuitry to the at least two measurement electrodes to measure the tissue impedance signal.

10. The device of claim 1, wherein the processor circuitry is coupled to the at least one switch to select the first configuration or the second configuration.

11. The device of claim 1, wherein the measurement circuitry comprises a first measurement circuit configured to measure the calibration signal and a second measurement circuit configured to measure the tissue impedance signal.

12. The device of claim 1, wherein the calibration circuitry comprises at least one resistor connected in series to the drive circuitry and the at least two drive electrodes, such that a resistance of the resistor corresponds to at least 90% the impedance of the calibration circuitry.

13. The device of claim 1, wherein the calibration circuitry comprises a resistance, wherein the calibration signal comprises a complex calibration signal, wherein the tissue impedance signal comprises a complex tissue impedance signal, and wherein the processor is configured to determine a complex impedance of the tissue in response to the complex calibration signal and the complex tissue impedance signal.

14. The device of claim 1, wherein the processor is configured to store a calibration value comprising a resistance of the calibration circuitry that corresponds to a real number, and wherein the calibration signal corresponds to the resistance of the calibration circuitry, delays of the drive circuitry and delays of the measurement circuitry.

15. The device of claim 14, wherein the processor is configured to determine a complex calibration coefficient in response to the calibration value and the calibration signal.

16. The device of claim 15, wherein the tissue impedance comprises a complex tissue impedance and processor is configured to determine the complex tissue impedance in response to the complex calibration coefficient and the tissue impedance signal.

17. The device of claim 16, wherein the processor is configured to determine a complex tissue parameter from the tissue impedance signal and wherein the processor is configured to determine the complex tissue impedance with at least one of a complex multiplication or a complex division of the complex calibration coefficient and the complex tissue parameter.

18. The device of claim 17, wherein the processor is configured to determine the complex tissue parameter with a discrete Fourier transform of the tissue impedance signal and determine the complex calibration coefficient with a discrete Fourier transform of the calibration signal.

19. The device of claim 14, wherein the delays of the drive circuitry and the measurement circuitry correspond to a phase angle of the calibration signal of at least about 90 degrees.

20. The device of claim 1, wherein the processor is configured to select a first frequency and a second frequency to measure impedance signals of the calibration circuitry at each of the first frequency and the second frequency, and configured to measure impedance signals of the tissue at each of the first frequency and the second frequency.

21. The device of claim 20, wherein the processor is configured to determine impedance of the tissue at the each of the first frequency and the second frequency in response to impedance signals of the calibration circuitry measured at each of the first frequency and the second frequency and impedance signals of the tissue measured at each of the first frequency and the second frequency.

22. The device of claim 1, wherein the processor is configured to store a tolerance range and measure the calibration circuitry in response to the impedance signal of the tissue and the tolerance range.

23. The device of claim 22, wherein the tolerance range comprises plus or minus twenty percent of a baseline tissue impedance measurement and the processor is configured to measure the calibration circuitry in response to the tissue impedance outside the tolerance range.

24. A device for measuring an impedance of a tissue of a patient, the device comprising:

at least four electrodes configured to couple to the tissue of the patient, the at least four electrodes comprising at least two drive electrodes and at least two measurement electrodes;

drive circuitry coupled to the at least two drive electrodes to pass a variable current through the tissue to generate a tissue measurement signal, the drive circuitry configured to increase the current from a first current amount at a first frequency to a second current amount at a second frequency, the second frequency greater than the first frequency; and

measurement circuitry coupled to the at least two measurement electrodes to determine the impedance of the tissue in response to the tissue measurement signal, the measurement circuitry comprising a variable gain of the measurement signal configured to decrease from a first gain at the first frequency to a second gain at the second frequency.

25. The device of claim 24, wherein the variable current of the drive circuitry comprises a drive current frequency response and the variable gain of the measurement circuitry comprises variable gain frequency response and wherein the variable gain frequency response comprises an inverse of the drive current frequency response.

26. The device of claim 24, wherein the drive circuitry is configured to increase the second current amount to at least four times the first current amount and wherein the measurement circuitry is configured to decrease the second gain to no more than about one half of the first gain.

27. The device of claim 24, wherein the drive circuitry is configured to increase the second current amount to at least ten times the first current amount and wherein the measurement circuitry is configured to decrease the second gain to no more than about one third of the first gain.

28. The device of claim 24, wherein the second frequency is at least 1 kHz and the second current amount is at least 10 μ

A and no more than 1000 μ

A and wherein the first frequency corresponds to a first safety threshold of the first current and the second frequency corresponds to a second safety threshold of the second current and wherein the drive circuitry is configured to exceed the first safety threshold with the second current amount and not to exceed the second safety threshold with the second current amount.

29. The device of claim 28 wherein the drive circuitry is configured to exceed the first safety threshold with the second current by at least a factor of two.

30. The device of claim 28 wherein the safety threshold of the first current corresponds to 10 μ

A or a product of the first current in μ

A times the first frequency in kHz, whichever is greater.

31. A method of measuring patient impedance, the method comprising:

providing at least four electrodes comprising at least two drive electrodes and at least two measurement electrodes, the at least two drive electrodes connected in series to a calibration resistor;

providing measurement circuitry to measure a tissue impedance signal from the measurement electrodes;

passing a drive current through the at least two drive electrodes and the calibration resistor with drive circuitry,

measuring a current signal from the calibration resistor in response to the current through the calibration resistor;

measuring the tissue impedance signal from the measurement electrodes; and

determining the tissue impedance in response to the current signal and the tissue impedance signal.

32. The method of claim 31 wherein the current signal from the calibration resistor is measured with the measurement circuitry.

33. The method of claim 31 wherein the tissue impedance is determined with an impedance converter.

34. The method of claim 33 wherein the current signal from the calibration resistor comprises a first voltage that is converted into a first current and the first current is fed into the impedance converter and wherein the tissue impedance signal from the measurement electrodes comprises a second voltage that is converted to a second current and the second current fed into the impedance converter.

35. The method of claim 33 wherein the drive circuitry comprising a network to limit the drive current through the patient, such that the network increases the drive current through the patient as a frequency of the drive current increases.

36. The method of claim 35 wherein the measurement circuitry comprises a variable gain that decreases when the frequency is increases and the drive current increases.