Implantable cardiac stimulator with impedance based autothreshold

• US 5,735,883 A
• Filed: 12/13/1996
• Issued: 04/07/1998
• Est. Priority Date: 12/13/1996
• Status: Expired due to Term

• Current Assignee: Intermedics Incorporated
• Original Assignee: Sulzer Intermedics, Inc. (Sulzer AG)
• Inventors: Paul, Patrick J., Prutchi, David
• Primary Examiner(s): Getzow, Scott M.
• Application Number:
  US08/766,870
• Time in Patent Office: 480 Days
• Field of Search: 607/5, 607/28, 607/17
• US Class Current: 607/28
• CPC Class Codes: A61N 1/3712 Auto-capture, i.e. automatic adjustment of the stimulation threshold

Abstract

An implantable pacemaker with apparatus for detecting capture or adjusting the strength or duration of pacing pulses by assessing the mechanical evoked response that may be distinctly sensed through impedance sensing, pressure sensing, plethysmography or other suitable methods. When capture is to be detected or the strength or duration of the pacing pulses is to be adjusted, two pacing pulses are delivered to the heart in each cycle of a series of cardiac cycles. The first pulse is varied in strength or duration or both. The second pulse is maintained at a consistently high strength or duration to assure capture. The impedance of the heart is measured during a time window following the first pulse which is predicted to include a recognizable feature of the impedance waveform of the heart following a stimulating pulse. The magnitude of the first pulse is gradually changed until capture is lost. When the stimulation effect of the first pulse on the heart changes, the impedance measured during the window will change distinctly, indicating that a stimulus threshold has been detected.

Claims

1. A cardiac stimulator, comprising:

at least one stimulus generator,

a control circuit coupled to said generator for causing said generator to generate a sequence of paired stimuli at a selected pacing rate, a leading stimulus of each pair of the paired stimuli having an adjustable energy content beginning with an energy content on one side of the capture threshold, and a trailing stimulus of each pair of the paired stimuli always having an energy content above the capture threshold;

a circuit for adjusting the energy content of the leading stimulus toward the capture threshold;

a sensor for detecting a mechanical property of the heart during a time window at a selected time after application of at least one of said stimuli; and

a circuit coupled to said sensor detecting a substantial change in magnitude in the mechanical property thereby identifying the capture threshold from the energy content of the leading stimulus.

2. The cardiac stimulator according to claim 1, wherein the sensor is an impedance sensor.

3. The cardiac stimulator according to claim 1 further comprising

a circuit coupled to said controlling circuit adjusting a duration of said leading stimulus to an adjusted duration;

memory circuits coupled to said control circuitry for storing values of said leading stimulus whenever a substantial change in said mechanical property is sensed, and

a microprocessor coupled to said memory circuits for calculating a chronaxie from said capture thresholds and setting a stimulus level as a function of said chronaxie.

4. The cardiac stimulator according to claim 1 wherein said sensor detects said mechanical property of the heart at a calibration time after application of at least one of said stimuli, said calibration time being shorter than said selected time, and further comprising means for eliminating a value of said mechanical property detected at said calibration time from a value of said mechanical property detected at said selected time thereby reducing measurement error.

5. A method for automatically assessing a capture threshold of an implantable pacemaker electrically coupled to cardiac tissue, comprising:

(a) generating a sequence of paired stimuli at a prescribed pacing rate, a leading stimulus of each pair of the paired stimuli having an adjustable energy content beginning with an energy content on one side of the capture threshold, and a trailing stimulus of each pair of the paired stimuli always having an energy content above the capture threshold;

(b) applying the sequence of paired stimuli to the cardiac tissues while adjusting the energy content of the leading stimulus toward the capture threshold;

(c) detecting a mechanical property of the heart during a time window at a selected time after application of at least one of said stimuli; and

(d) defining the capture threshold to be approximately equal to the energy content of the leading stimulus of the leading stimulus of the paired stimuli that immediately precedes a substantial change in magnitude in the detected mechanical property.

6. The method according to claim 5, wherein the detected mechanical property is the impedance of the heart.

7. The method according to claim 6 wherein the energy content of the leading stimulus is decreased.

8. The method according to claim 7 wherein the voltage of the leading stimulus is decreased.

9. The method according to claim 5 wherein the energy content of the leading stimulus is decreased.

10. The method according to claim 9 wherein the voltage of the leading stimulus is decreased.

11. The method according to claim 5 further comprising

(e) adjusting a duration of said leading stimulus to an adjusted duration;

(f) detecting at least one additional capture threshold associated with said adjusted duration;

(g) calculating a chronaxie from said capture thresholds; and

(h) setting a stimulus level as a function of said chronaxie.

12. The method according to claim 5 further comprising

detecting said mechanical property of the heart at a calibration time after application of at least one of said stimuli, said calibration time being shorter than said selected time, and

eliminating a value of said mechanical property detected at said calibration time from a value of said mechanical property detected at said selected time thereby reducing measurement error.

13. The method according to claim 5 further comprising measuring a period of time from a stimulus to a selected feature of said mechanical property and setting said selected time as a function of said period of time.

14. The method according to claim 13 wherein the step of measuring a period of time comprises measuring a plurality of periods over a plurality of cardiac cycles and combining said plurality of periods to determine said period of time.

15. An implantable cardiac pacemaker comprising:

(a) means for generating a sequence of paired stimuli at a prescribed pacing rate, a leading stimulus of each pair of the paired stimuli having an adjustable energy content beginning with an energy content on one side of the capture threshold, and a trailing stimulus of each pair of the paired stimuli always having an energy content above the capture threshold;

(b) means for applying the sequence of paired stimuli to the cardiac tissues and for adjusting the energy content of the leading stimulus toward the capture threshold;

(c) means for detecting a mechanical property of the heart during a time window at a selected time after application of at least one of said stimuli; and

(d) means for defining the capture threshold to be approximately equal to the energy content of the leading stimulus of the leading stimulus of the paired stimuli that immediately precedes a substantial change in magnitude in the detected mechanical property.

16. The cardiac pacemaker according to claim 15, wherein the means for detecting a mechanical property is means for detecting the impedance of the heart.

17. The cardiac pacemaker according to claim 16 wherein the means for adjusting the energy content of the leading stimulus is means for decreasing the energy content of said leading stimulus.

18. The cardiac pacemaker according to claim 17 wherein the means for decreasing the energy content is means for decreasing the voltage of the first stimulus.

19. The cardiac pacemaker according to claim 15 wherein the means for adjusting the energy content of the leading stimulus is means for decreasing the energy content of the leading stimulus.

20. The cardiac pacemaker according to claim 19 wherein means for decreasing the energy content is means for decreasing the voltage of the leading stimulus.

21. The cardiac pacemaker according to claim 15 further comprising

(e) means for adjusting a duration of said leading stimulus to an adjusted duration;

(f) means for detecting at least one additional capture threshold associated with said adjusted duration;

(g) means for calculating a chronaxie from said capture thresholds; and

(h) means for setting a stimulus level as a function of said chronaxie.

22. The cardiac pacemaker according to claim 15 further comprising

means for detecting said mechanical property of the heart at a calibration time after application of at least one of said stimuli, said calibration time being shorter than said selected time, and

means for eliminating a value of said mechanical property detected at said calibration time from a value of said mechanical property detected at said selected time thereby reducing measurement error.