Rate adaptive cardiac rhythm management device using transthoracic impedance

• US 6,161,042 A
• Filed: 05/21/1999
• Issued: 12/12/2000
• Est. Priority Date: 02/27/1998
• Status: Expired due to Term

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Hartley, Jesse W., Stessman, Nicholas J., Reedstrom, Scott A., Cohen, Marc H., Nelson, James P., Check, Steven D.
• Primary Examiner(s): Schaetzle, Kennedy
• Application Number:
  US09/316,690
• Time in Patent Office: 571 Days
• Field of Search: 607/9, 607/17, 607/20, 607/28, 600/547
• US Class Current: 607/20
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance

Abstract

A cardiac rhythm management (CRM) device detects transthoracic impedance, extracts ventilation or other information, and adjusts a delivery rate of the CRM therapy accordingly. A four-phase sequence of alternating direction current pulse stimuli is periodically delivered to a patient'"'"'s thorax. A transthoracic impedance signal is extracted using a weighted demodulation. Signal processing extracts ventilation information and removes cardiac stroke information using an adaptive lowpass filter. The adaptive filter cutoff frequency is based on the patient'"'"'s heart rate; a higher cutoff frequency is provided for higher heart rates. Peak/valley detection indicates tidal volume, which is integrated to extract minute ventilation (MV). Short and long term averages are formed and compared to establish a MV indicated rate. Rate adjustment ignores MV information when a noise-measurement exceeds a threshold. An interference avoidance circuit delays delivery of the stimuli when telemetry pulses or other interfering signals are detected.

Claims

1. A cardiac rhythm management device, comprising:

an exciter, adapted to be coupled to a thorax of a patient for repeatedly delivering a multiphase stimulus thereto;

a signal processor, including a receiver for obtaining transthoracic impedance information responsive to the stimuli;

a demodulator, included in the signal processor, the demodulator including sampling elements for demodulating the transthoracic impedance in response to different phases of the multiphase stimulus, where the demodulator combines weighted samples of the transthoracic impedance information to attenuate unwanted signals;

a therapy circuit, adapted to bc coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the transthoracic impedance.

2. The device of claim 1, in which the exciter includes a bridge switcher providing the multiphase stimulus.

3. The device of claim 2, in which the exciter includes:

first and second capacitors, adapted for coupling the bridge switcher to the thorax of the patient; and

a bias circuit for precharging the first and second capacitors.

4. The device of claim 1, in which the demodulator includes a switched-capacitor sampler and integrator.

5. The device of claim 4, in which the switched capacitor sampler includes capacitors that weight pulses corresponding to the phases in the multiphase stimulus.

6. The device of claim 1, further comprising:

a noise-reversion circuit coupled to the demodulator for receiving a noise response signal in the absence of stimuli;

a noise comparator, included in the noise-reversion circuit, for comparing the noise response signal to a threshold value, and providing a resulting noise comparator output signal indicating whether the noise response signal exceeds the threshold value; and

wherein the controller adjusts the rate of delivery of the cardiac rhythm management therapy independent of the transthoracic impedance if the comparator output indicates that the noise response signal exceeds the threshold value.

7. A cardiac rhythm management system, including the cardiac rhythm management device of claim 1, the cardiac rhythm management system further comprising:

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

8. The cardiac rhythm management system of claim 7, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

9. The cardiac rhythm management system of claim 8, wherein the indifferent electrode is carried by a header portion of the housing.

10. A cardiac rhythm management device comprising:

an exciter, adapted to be coupled to a thorax of a patient for delivering stimuli thereto;

a signal processor, including a receiver for obtaining a transthoracic impedance responsive to the stimuli, the signal processor extracting ventilation information from the transthoracic impedance, wherein the signal processor includes an adaptive lowpass filter for removing a cardiac stroke component of the transthoracic impedance signal, a cutoff frequency of the adaptive lowpass filter being adaptively based on a heart rate of the patient, the cutoff frequency of the adaptive lowpass filter being independent of a breathing rate signal from the patient;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the ventilation information.

11. The device of claim 10, in which the signal processor includes:

an analog signal processing circuit, receiving the transthoracic impedance signal and including an analog-to-digital (AID) converter providing a responsive digital transthoracic impedance signal; and

wherein the adaptive lowpass filter includes a digital 4-pole Chebyshev filter including a state-space structure.

12. The device of claim 10, wherein the controller selects the cutoff frequency of the adaptive lowpass filter to be higher when the heart activity signal indicates a higher heart rate and lower when the heart activity signal indicates a lower heart rate.

13. The device of claim 10, wherein the controller selects between a number of discrete lowpass cutoff frequencies, each lowpass cutoff frequency corresponding to a particular range of values of the heart rate.

14. The device of claim 13, wherein the controller selects the cutoff frequency of the adaptive lowpass filter at approximately 0.5 Hz, 0.75 Hz, and 1.0 Hz when the heart rate is respectively approximately less than 68 beats per minute, between 68 and 88 beats per minute, and greater than 88 beats per minute.

15. A cardiac rhythm management system, including the cardiac rhythm management device of claim 10, the cardiac rhythm management system further comprising:

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

16. The cardiac rhythm management system of claim 15, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

17. The cardiac rhythm management system of claim 16, wherein the indifferent electrode is carried by a header portion of the housing.

18. A cardiac rhythm management device, comprising:

an exciter, adapted to be coupled to a thorax of a patient for repeatedly delivering a multiphase stimulus thereto;

a signal processor, including a receiver for obtaining transthoracic impedance information responsive to the stimuli;

a demodulator, included in the signal processor, the demodulator including sampling elements for demodulating the transthoracic impedance in response to different phases of the multiphase stimulus and a switched-capacitor sampler and integrator, where the switched capacitor sampler includes capacitors that weight pulses corresponding to the phases in the multiphase stimulus;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the transthoracic impedance.

19. The device of claim 18, in which the exciter includes a bridge switcher providing the multiphase stimulus.

20. The device of claim 19, in which the exciter includes:

first and second capacitors, adapted for coupling the bridge switcher to the thorax of the patient; and

a bias circuit for precharging the first and second capacitors.

21. The device of claim 18, further comprising:

a noise-reversion circuit coupled to the demodulator for receiving a noise response signal in the absence of stimuli;

a noise comparator, included in the noise-reversion circuit, for comparing the noise response signal to a threshold value, and providing a resulting noise comparator output signal indicating whether the noise response signal exceeds the threshold value; and

wherein the controller adjusts the rate of delivery of the cardiac rhythm management therapy independent of the transthoracic impedance if the comparator output indicates that the noise response signal exceeds the threshold value.

22. A cardiac rhythm management system, including the cardiac rhythm management device of claim 18, the cardiac rhythm management system further comprising:

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

23. The cardiac rhythm management system of claim 22, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

24. The cardiac rhythm management system of claim 23, wherein the indifferent electrode is carried by a header portion of the housing.

25. A cardiac rhythm management device, comprising:

an exciter, adapted to be coupled to a thorax of a patient for repeatedly delivering a multiphase stimulus thereto;

a signal processor, including a receiver for obtaining transthoracic impedance information responsive to the stimuli;

a demodulator, included in the signal processor, the demodulator including sampling elements for demodulating the transthoracic impedance in response to different phases of the multiphase stimulus;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto;

a noise-reversion circuit coupled to the demodulator for receiving a noise response signal in the absence of stimuli;

a noise comparator, included in the noise-reversion circuit, for comparing the noise response signal to a threshold value, and providing a resulting noise comparator output signal indicating whether the noise response signal exceeds the threshold value; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the transthoracic impedance, wherein the controller adjusts the rate of delivery of the cardiac rhythm management therapy independent of the transthoracic impedance if the comparator output indicates that the noise response signal exceeds the threshold value.

26. The device of claim 25, in which the exciter includes a bridge switcher providing the multiphase stimulus.

27. The device of claim 26, in which the exciter includes:

first and second capacitors, adapted for coupling the bridge switcher to the thorax of the patient; and

a bias circuit for precharging the first and second capacitors.

28. The device of claim 25, in which the demodulator includes a switched-capacitor sampler and integrator.

29. The device of claim 28, in which the switched capacitor sampler includes capacitors that weight pulses corresponding to the phases in the multiphase stimulus.

30. A cardiac rhythm management system, including the cardiac rhythm management device of claim 25, the cardiac rhythm management system further comprising:

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

31. The cardiac rhythm management system of claim 30, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

32. The cardiac rhythm management system of claim 31, wherein the indifferent electrode is carried by a header portion of the housing.

33. A cardiac rhythm management device comprising:

an exciter, adapted to be coupled to a thorax of a patient for repeatedly delivering a multiphase stimulus thereto;

a signal processor, including a receiver for obtaining transthoracic impedance information responsive to the stimuli;

a demodulator, included in the signal processor, the demodulator including sampling elements for demodulating the transthoracic impedance in response to different phases of the multiphase stimulus;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto;

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the transthoracic impedance;

an endocardial lead, carrying first and second electrodes adapted to be implanted in the heart, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

34. The cardiac rhythm management system of claim 33, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

35. The cardiac rhythm management system of claim 34, wherein the indifferent electrode is carried by a header portion of the housing.

36. A cardiac rhythm management device comprising:

an exciter, adapted to be coupled to a thorax of a patient for delivering stimuli thereto;

a signal processor, including a receiver for obtaining a transthoracic impedance responsive to the stimuli, the signal processor extracting ventilation information from the transthoracic impedance, wherein the signal processor includes an adaptive lowpass filter for removing a cardiac stroke component of the transthoracic impedance signal, a cutoff frequency of the adaptive lowpass filter being adaptively based on a heart rate of the patient, the cutoff frequency of the adaptive lowpass filter being independent of a breathing rate signal from the patient;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the ventilation information, where the controller selects between a number of discrete lowpass cutoff frequencies, each lowpass cutoff frequency corresponding to a particular range of values of the heart rate.

37. The device of claim 36, wherein the controller selects the cutoff frequency of the adaptive lowpass filter at approximately 0.5 Hz, 0.75 Hz, and 1.0 Hz when the heart rate is respectively approximately less than 29 beats per minute, between 68 and 88 beats per minute, and greater than 88 beats per minute.

38. The device of claim 36, in which the signal processor includes:

an analog signal processing circuit, receiving the transthoracic impedance signal and including an analog-to-digital (A/D) converter providing a responsive digital transthoracic impedance signal; and

wherein the adaptive lowpass filter includes a digital 4-pole Chebyshev filter including a state-space structure.

39. The device of claim 36, wherein the controller selects the cutoff frequency of the adaptive lowpass filter to be higher when the heart activity signal indicates a higher heart rate and lower when the heart activity signal indicates a lower heart rate.

40. A cardiac rhythm management system, including the cardiac rhythm management device of claim 36, the cardiac rhythm management system further comprising:

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

41. The cardiac rhythm management system of claim 40, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

42. The cardiac rhythm management system of claim 41, wherein the indifferent electrode is carried by a header portion of the housing.

43. A cardiac rhythm management device comprising:

an exciter, adapted to be coupled to a thorax of a patient for delivering stimuli thereto;

a signal processor, including a receiver for obtaining a transthoracic impedance responsive to the stimuli, the signal processor extracting ventilation information from the transthoracic impedance, wherein the signal processor includes an adaptive lowpass filter for removing a cardiac stroke component of the transthoracic impedance signal, a cutoff frequency of the adaptive lowpass filter being adaptively based on a heart rate of the patient, the cutoff frequency of the adaptive lowpass filter being independent of a breathing rate signal from the patient;

a therapy circuit, adapted to be coupled to a heart of the patient for delivering cardiac rhythm management therapy thereto; and

a controller, coupled to the therapy circuit for adjusting a rate of delivery of the cardiac rhythm management therapy based on the ventilation information;

an endocardial lead, carrying first and second electrodes, the lead being coupled to the cardiac rhythm management device; and

a housing, carrying the cardiac rhythm management device, the housing including third and fourth electrodes.

44. The cardiac rhythm management system of claim 43, wherein the first electrode is a tip electrode, the second electrode is a ring electrode, the third electrode is a case electrode, and the fourth electrode is an indifferent electrode.

45. The cardiac rhythm management system of claim 44, wherein the indifferent electrode is carried by a header portion of the housing.