Calibration of impedance monitoring of respiratory volumes using thoracic D.C. impedance

• US 20060241513A1
• Filed: 04/26/2005
• Published: 10/26/2006
• Est. Priority Date: 04/26/2005
• Status: Active Grant

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Stahmann, Jeffrey E., Hartley, Jesse W., Moon, Boyce, Hatlestad, John
• Granted Patent: US 7,603,170 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 5/0809 by impedance pneumography
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance
• CPC Class Codes: A61N 1/3756 Casings with electrodes thereon, e.g. leadless stimulators

Abstract

A system includes an implantable medical device that includes a trans-thoracic impedance measurement circuit providing a trans-thoracic impedance signal of a subject. A controller is coupled to the trans-thoracic impedance circuit. The controller extracts a respiration signal from the trans-thoracic impedance signal, measures a breathing volume of the subject using the amplitude of the respiration signal and a breathing volume calibration factor, computes an adjusted breathing volume calibration factor using a reference baseline value of the trans-thoracic impedance and a measured baseline value of the trans-thoracic impedance, and computes a calibrated breathing volume using the adjusted breathing volume calibration factor.

Claims

1. A system comprising:

a medical device, including;

an implantable trans-thoracic impedance measurement circuit to provide a trans-thoracic impedance signal of a subject; and

a controller coupled to the trans-thoracic impedance circuit, the controller operable to;

store a reference baseline value of a DC component of a trans-thoracic impedance;

measure a more recent value of the DC component of a trans-thoracic impedance different from the reference baseline value;

adjust a breathing volume calibration factor using the reference baseline value and the more recent measured value of a DC component of a trans-thoracic impedance;

extract a respiration signal from the trans-thoracic impedance signal, the respiration signal including a respiration amplitude; and

compute a breathing volume of the subject using the respiration amplitude scaled by the adjusted breathing volume calibration factor.

2. The system of claim 1, wherein the adjusted breathing volume calibration factor includes a ratio of a reference DC trans-thoracic impedance to a more recent measured value of the DC trans-thoracic impedance.

3. The system of claim 1, wherein the medical device is an implantable medical device and further includes a posture detecting circuit coupled to the controller, and wherein the controller is further operable to detect a first patient posture and to adjust a breathing volume calibration factor if the more recent value of a DC component of a trans-thoracic impedance is measured while a patient is in the first posture.

4. The system of claim 1, wherein the medical device is an implantable medical device and further includes a posture detecting circuit coupled to the controller, and wherein the controller is further operable to store a first reference baseline impedance value associated with a first posture and at least one other reference baseline impedance value associated with another posture and to adjust the calibration factor when at least one of:

(a) a measured change from the first reference baseline value is detected while a patient is in the first posture;

(b) a measured change from the at least one other reference baseline value is detected while the patient is in the other posture.

5. The system of claim 4, wherein the controller is further operable to compute the breathing volume using a first calibration factor associated with a first posture and at least one other calibration factor associated with another posture, and to adjust the first calibration factor when a measured change from the first reference baseline value is detected while a patient is in the first posture and to adjust the other calibration factor when a measured change from the other reference baseline value is detected while the patient is in the other posture.

6. The system of claim 1, wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in measured DC trans-thoracic impedance values.

7. The system of claim 1, wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in breathing volumes.

8. The system of claim 1, wherein a breathing volume calibration factor is adjusted using the measured reference baseline value of a DC component of a trans-thoracic impedance and the measured more recent value of the DC trans-thoracic impedance and further using a measure of a change in hematocrit in blood of the subject.

9. The system of claim 1, wherein the system further includes a second device operable to communicate with the medical device, wherein the second device includes a display, and wherein the second device is further operable to display breathing volume information obtained from the trans-thoracic impedance signal.

10. The system of claim 9, wherein the second device is further operable to display a histogram of DC trans-thoracic impedance of the patient.

11. The system of claim 9, wherein the second device is further operable to display trending information of DC trans-thoracic impedance.

12. The system of claim 9, wherein the second device is further operable to display an indication of fluid accumulation in a patient'"'"'s lungs.

13. The system of claim 1, wherein the system further includes a second device operable to communicate with the medical device, wherein the second device is in communication with a computer network.

14. A method comprising:

detecting a change in DC trans-thoracic impedance;

adjusting a breathing volume calibration factor in accordance with the change in DC trans-thoracic impedance; and

measuring breathing volume of a patient using an implantable medical device, the medical device adjusting the measured breathing volume using the breathing volume calibration factor.

15. The method of claim 14, wherein adjusting the breathing volume calibration factor includes adjusting using a ratio of a reference DC impedance and a measured DC impedance, and wherein measuring breathing volume includes calibrating a raw measurement of breathing volume using the breathing volume calibration factor.

16. The method of claim 14, wherein measuring breathing volume includes measuring trans-thoracic impedance.

17. The method of claim 14, wherein detecting a change in DC trans-thoracic impedance includes monitoring DC trans-thoracic impedance over time.

18. The method of claim 14, wherein using a breathing volume calibration factor includes using the breathing volume calibration factor to associate a measurement of ohms-per-breath with respiratory volume-per-breath.

19. The method of claim 14, wherein detecting a change in DC trans-thoracic impedance includes determining a baseline DC trans-thoracic impedance and detecting changes in impedance from the baseline DC trans-thoracic impedance.

20. The method of claim 14, wherein adjusting a breathing volume calibration factor includes using a change in DC trans-thoracic impedance measured while a patient is in a first patient posture.

21. The method of claim 14, wherein detecting a change in DC trans-thoracic impedance includes:

determining a first baseline DC trans-thoracic impedance for a first patient posture and at least one other baseline DC trans-thoracic impedance for at least one other patient posture; and

detecting changes in impedance from the first baseline impedance when the patient is in the first posture and detecting changes in impedance from the other baseline impedance when the patient is in the other posture.

22. The method of claim 14, wherein measuring breathing volume using an implantable medical device includes monitoring breathing volume with a cardiac rhythm management device.

23. The method of claim 22, wherein the method further includes changing a pacing rate of the cardiac rhythm management device in response to the measured breathing volume.

24. The method of claim 14, wherein the method further includes storing trending information related to DC trans-thoracic impedance.

25. The method of claim 14, wherein the method further includes storing trending information related to breathing volumes.

26. The method of claim 14, wherein the method further includes communicating the trending information to an external device for display.

27. The method of claim 14, wherein the method further includes communicating DC trans-thoracic impedance information to an external device connected to a computer network.

28. A method comprising:

measuring a trans-thoracic impedance signal of a subject using an implantable medical device;

adjusting the breathing volume calibration factor using a reference baseline value of the trans-thoracic impedance and a measured change from the reference baseline value of the trans-thoracic impedance;

extracting a respiration signal from the trans-thoracic impedance signal, the respiration signal including a component of the trans-thoracic impedance that is modulated by a breathing of the subject, the respiration signal including an amplitude; and

computing a breathing volume of the subject using the amplitude of the respiration signal and a breathing volume calibration factor.

29. The method of claim 28, wherein extracting a respiration signal includes filtering the trans-thoracic impedance signal to attenuate frequencies higher than 0.5 Hz.

30. The method of claim 28, wherein measuring a breathing volume of the subject using the amplitude of the respiration signal includes measuring breathing volume using a peak-to-peak amplitude.

31. The method of claim 28, wherein adjusting the breathing volume calibration factor includes using a reference baseline value of the DC trans-thoracic impedance and a measured change from the reference baseline value of the DC trans-thoracic impedance wherein the reference baseline value and the change from the reference baseline value are measured for a first patient posture.

32. The method of claim 28, wherein computing a breathing volume calibration factor includes computing the calibration factor using a measure of hematocrit in blood of the subject.

33. The method of claim 28, wherein computing a breathing volume calibration factor includes computing the calibration factor using a measure of resistivity of blood within a heart.

34. The method of claim 28, wherein the reference baseline value of the trans-thoracic impedance includes a DC trans-thoracic impedance value measured over a period of time.

35. The method of claim 28, wherein the calibration factor includes a ratio of a reference baseline DC trans-thoracic impedance to a measured DC trans-thoracic impedance, and wherein the calibrated breathing volume is computed by multiplying the measured breathing volume by the calibration factor.

36. A system comprising:

a medical device, including;

an implantable trans-thoracic impedance measurement circuit to provide a trans-thoracic impedance signal of a subject, the measurement circuit including an adjustable test current circuit; and

a controller coupled to the trans-thoracic impedance circuit, the controller operable to;

store a reference baseline value of a DC component of a trans-thoracic impedance;

measure a more recent value of the DC component of a trans-thoracic impedance different from the reference baseline value;

adjust a test current value until the measured value of the DC component of a trans-thoracic impedance is substantially the same as the reference baseline DC voltage value;

extract a respiration signal from the trans-thoracic impedance signal, the respiration signal including a respiration amplitude; and

compute a breathing volume of the subject.

37. The system of claim 36, wherein the controller is further operable to adjust the test current value by the ratio of the reference baseline DC trans-thoracic impedance value to the more recent measured value of DC voltage value.

38. The system of claim 36, wherein the medical device includes a storage circuit coupled to the controller, the storage circuit for storing at least one trend in measured DC trans-thoracic impedance values.