Acceleration sensed safe upper rate envelope for calculating the hemodynamic upper rate limit for a rate adaptive cardiac rhythm management device

• US 5,792,195 A
• Filed: 12/16/1996
• Issued: 08/11/1998
• Est. Priority Date: 12/16/1996
• Status: Expired due to Term

• Current Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Original Assignee: Cardiac Pacemakers Incorporated (Boston Scientific Corporation)
• Inventors: Spinelli, Julio C., Carlson, Gerrard M., Jones, Bruce R.
• Primary Examiner(s): Kamm, William E.
• Assistant Examiner(s): LAYNO, CARL HERNANDZ
• Application Number:
  US08/766,339
• Time in Patent Office: 603 Days
• Field of Search: 607/17-19, 128/701, 128/714, 128/715
• US Class Current: 607/17
• CPC Class Codes: A61N 1/36542 controlled by body motion, e.g. acceleration

Abstract

An implantable cardiac rhythm management device includes an accelerometer along with apparatus for processing the analog signal output from the accelerometer for deriving therefrom the time of occurrence of a selected heart sound in relation to a previously occurring ventricular depolarization event. The thus-derived heart sound information can be used to establish a hemodynamic upper rate limit for a rate adaptive pacemaker.

Claims

1. Cardiac stimulating apparatus, comprising:

pulse generator means for producing cardiac stimulating pulses at predetermined times;

means for sensing R-wave events;

accelerometer means for sensing movement of body structures of a patient, said accelerometer means producing an analog electrical output signal proportional to such movement;

signal processing means coupled to receive the analog electrical output signals and signals corresponding to R-wave events for isolating in said analog electrical output signal components corresponding to a predetermined heart sound in relation to a time of occurrence of said R-wave events; and

means responsive to the length of an interval between the occurrence of R-wave events and a next occurring predetermined heart sound for controlling the time that the pulse generator means produces cardiac stimulating pulses.

2. The apparatus as in claim 1 wherein the signal processing means includes:

pre-processing means coupled to receive the analog electrical output signal of the accelerometer means for producing therefrom a log normalized heart sound envelope signal.

3. The apparatus as in claim 2 wherein the pre-processing means includes:

bandpass filter means having an upper and a lower cut-off frequency defining a pass-band for filtering out components in the analog output signal due to at least one of breathing, body motion, muscle twitch and voice.

4. The apparatus as in claim 3 wherein the pre-processing means further includes:

automatic gain control means coupled to said bandpass filter means for removing any transient spikes in the analog electrical output signal whose amplitude exceeds a predetermined threshold and whose time constant is less than a predetermined value.

5. The apparatus as in claim 4 wherein the preprocessing means further includes:

means coupled to the automatic gain control means for low-pass filtering and absolute value of an output from said bandpass filter means for producing a heart sound envelope signal; and

means coupled to receive the heart sound envelope signal for logarhythmically compressing said heart sound envelope signal.

6. The apparatus as in claim 2 and further including:

means for establishing a time window beginning and ending at predetermined times following the given R-wave event where the beginning and ending times are a function of a detected R-to-R interval, the means for establishing a time window capturing only a portion of the log normalized heart sound envelope signal falling within said time window.

7. The apparatus as in claim 6 wherein the window'"'"'s beginning and ending times straddle an expected time of occurrence of the predetermined heart sound.

8. The apparatus as in claim 7 wherein the predetermined heart sound comprises heart sound S₂.

9. The apparatus as in claim 7 wherein the predetermined beginning and ending times are expressed as:

S_b =119.0-0.625(HR-120) in counts at a 500 ms sample rate for all heart rates (HR) and

S_e =198.0-2.0875(HR-120) in counts at 500 ms sample rate where HR<

120 bpm and

S_e =198.0-1.225(HR-120) in counts at 500 ms sample rate where HR>

120 bpm.

10. The apparatus as in claim 6 and further including means for comparing the portion of the log normalized heart sound envelope captured by the means for establishing a time window during a current R-to-R interval with the portion of the log normalized heart sound envelope captured by the means for establishing a time window during a preceding R--R interval for morphologic similarity;

and

means for computing an ensemble average of only the captured portion of the log normalized heart sound envelope captured during the current R-to-R interval that is morphologically similar to the portion of the log normalized heart sound envelope captured during the preceding R-to-R interval; and

means for storing a computed result.

11. The apparatus as in claim 10 and further including means for calculating a time interval between a predetermined peak in the stored computed result and a preceding R-wave event.

12. In an implantable, rate-adaptive cardiac pacer of the type comprising R-wave sensing means, ventricular pacing means, an accelerometer-based activity sensing means for producing analog electrical output signal corresponding to movement of body structures, and a microprocessor-based controller coupled to the R-wave sensing means, the ventricular pacing means and the accelerometer-based activity sensing means for controlling the operation of the ventricular pacing means to issue ventricular pacing pulses at a rate determined by the activity sensing means, apparatus for establishing a hemodynamic upper rate limit for the ventricular pacing means, comprising:

signal processing means including the microprocessor-based controller for deriving from the analog electrical output signals a time interval between a detection of an R-wave event by the R-wave sensing means and an ensuing heart sound;

means for producing a control signal proportional to said time interval; and

means responsive to said control signal for setting said upper rate limit.

13. The cardiac pacer as in claim 12 wherein the microprocessor-based controller includes:

memory means for storing a fixed parameter corresponding to a time interval between heart sound, S₂, and heart sound, S₃, measured when a patient in whom the pacer is implanted is inactive; and

means for adding said fixed parameter to said time interval between a detection of an R-wave event by the R-wave sensing means and the ensuing heart sound, said control signal being proportional to a sum thereof.

14. The cardiac pacer as in claim 13 wherein the signal processing means includes:

pre-processing means coupled to receive the analog electrical output signals from the accelerometer means for producing therefrom a log normalized heart sound envelope signal.

15. The apparatus as in claim 14 wherein the preprocessing means includes:

bandpass filter means having an upper and a lower cut-off frequency defining a pass-band for eliminating components in the analog output signal due to breathing, body motion, muscle twitch and voice.

16. The cardiac pacer as in claim 15 wherein the preprocessing means further includes automatic gain control means coupled to the bandpass filter means for removing any transient spikes in the analog electrical output signal whose amplitude exceeds a predetermined threshold and whose time constraint is less than a predetermined value.

17. The cardiac pacer as in claim 16 wherein the preprocessing means further includes:

means coupled to the automatic gain control means for low-pass filtering an absolute value of an output from the bandpass filter means for producing a heart sound envelope signal; and

means coupled to receive the heart sound envelope signal for logarhythmically compressing said heart sound envelope.

18. The cardiac pacer as in claim 14 and further including means for establishing a time window beginning and ending at predetermined times following a given R-wave signal where the beginning and ending times are a function of a detected R-to-R interval;

the means for establishing a time window capturing only a portion of the log normalized heart sound envelope falling within said time window.

19. A cardiac pacer as in claim 18 and further including:

means for comparing the portion of the log normalized heart sound envelope captured by the means for establishing a time window during a current R-to-R interval with the portion of the log normalized heart sound envelope captured by the means for establishing a time window during a preceding R-to-R interval for morphologic similarity; and

means for computing an ensemble average of only the captured portion of the log normalized heart sound envelope captured during the current R-to-R interval that is morphologically similar to the portion of the log normalized heart sound envelope captured during the preceding R-to-R interval and storing a computed result.

20. The cardiac pacer as in claim 19 and further including:

means for calculating a time interval between a predetermined peak in the stored computed result and a preceding R-wave.