Method and system for extracting cardiac parameters from plethysmographic signals

• US 6,783,498 B2
• Filed: 03/27/2002
• Issued: 08/31/2004
• Est. Priority Date: 03/26/2002
• Status: Expired due to Term

• Current Assignee: Adidas AG
• Original Assignee: VivoMetrics, Inc.
• Inventors: Sackner, Marvin A., Inman, Dana Michael
• Primary Examiner(s): Jones, Mary Beth
• Assistant Examiner(s): Natnithithadha, Navin
• Application Number:
  US10/107,078
• Publication Number:
  US 20030187341A1
• Time in Patent Office: 888 Days
• Field of Search: 600/481, 600/483, 600/504, 600/506, 600/507, 600/508, 600/509, 600/513, 600/519, 600/521, 600/485, 600/301, 600/300, 600/500, 128/925, 128/923, 128/920
• US Class Current: 600/481
• 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 5/02438 with portable devices, e.g. worn by the patient
• CPC Class Codes: A61B 5/0245 by using sensing means generating electric signals, i.e. ECG signals
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295
• CPC Class Codes: A61B 5/0816 Measuring devices for examining respiratory frequency measuring frequency of electric signals G01R23/00
• CPC Class Codes: A61B 5/1135 by monitoring thoracic expansion
• CPC Class Codes: A61B 5/6804 Garments; Clothes
• CPC Class Codes: A61B 5/721 using a separate sensor to detect motion or using motion information derived from signals other than the physiological signal to be measured
• CPC Class Codes: A61B 5/725 using specific filters therefor, e.g. Kalman or adaptive filters A61B5/316 take precedence
• CPC Class Codes: A61B 5/7278 Artificial waveform generation or derivation, e.g. synthesising signals from measured signals A61B5/327 takes precedence

Abstract

A method and system for extracting cardiac parameters from a plethysmographic signal is described wherein the plethysmographic signal is passed through a first filter to remove non-cardiac components of the signal. A second filter averages a plurality of cardiac cycles and cardiac parameters are extracted from the averaged cardiac signal.

Claims

1. A method for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter, the method comprising the steps of:

(a) performing a frequency domain filtering operation on the plethysmographic signal producing a first filtered signal;

(b) performing a time domain filtering operation on the first filtered signal, producing a second filtered signal; and

(c) extracting the cardiac parameter from the second filtered signal.

2. The method of claim 1 wherein the frequency domain filtering operation includes a band-pass filter.

3. The method of claim 2 wherein the band-pass filter is characterized by a lower corner frequency that is determined by a heart rate.

4. The method of claim 1 wherein the time domain filtering operation is an ensemble averaging operation.

5. The method of claim 4 wherein the ensemble averaging operation further comprises the steps of:

(i) associating a plurality of segments of the plethysmographic signal with events characteristic of a cardiac cycle;

(ii) shifting a plurality of segments to align the events associated with each of the plurality of events characteristic of the cardiac cycle;

(iii) constructing an ensemble averaged cardiac cycle signal from the average of the plurality of aligned segments.

6. The method of claim 5 where the event characteristic of a cardiac cycle comprises an indicia derived from the electrocardiographic R-wave.

7. The method of claim 5 wherein the ensemble averaging operation further includes the step of reconstructing a thoracocardiograph signal from the ensemble averaged cardiac cycle signal.

8. The method of claim 1 wherein the cardiac parameter is a stroke volume.

9. The method of claim 1 wherein the cardiac parameter is a cardiac output.

10. The method of claim 1 wherein the cardiac parameter is a pre-ejection period.

11. The method of claim 1 wherein the cardiac parameter is a peak ejection rate.

12. The method of claim 1 wherein the cardiac parameter is the time to peak ejection rate.

13. A system for extracting cardiac parameters from a plethysmographic signal, the plethysmographic signal being responsive to at least one cardiac parameter, the system comprising:

(a) a first frequency-domain filter receiving the plethysmographic signal, having a dynamic lower cutoff frequency, and producing a first-filtered signal,

(b) a second time-domain filter receiving the first filtered signal and producing a second filtered plethysmographic signal; and

(c) a processor for extracting the cardiac parameter from the second filtered signal.

14. The system of claim 13 wherein the lower cutoff frequency is greater than a respiratory rate and less than a cardiac rate.

15. The system of claim 13 wherein the lower corner frequency is determined by a cardiac rate.

16. The system of claim 15 wherein the lower corner frequency is between 0.6*HR and 0.8*HR where HR is the cardiac rate.

17. The system of claim 15 wherein the lower corner frequency is approximately 0.7*HR where HR is the cardiac rate.

18. The system of claim 13 wherein the second filter is an ensemble averaging filter characterized by the equation $\hat{f}$

(n,t)=∑

i=-Wi=+W

 



wi

f

(t+(Rn+i-Rn))

where {circumflex over (f)}(n,t) is an ensemble averaged signal for the n-th cardiac cycle as a function of time, t, f(t) is a filtered TCG signal, R_n is the time of the n-th cardiac cycle R-wave, w_i are the cycle weights, and 2W+1 is the ensemble size.

19. The system of claim 18 wherein the ensemble size is between 25 and 500.

20. The system of claim 18 wherein the ensemble size is between 50 and 250.

21. The system of claim 18 wherein the ensemble size is approximately 150.

22. A system for generating a thoracocardiograph signal comprising:

(a) a first digitizer for converting a first signal generated by an inductive plethysmographic sensor to a digitized first signal;

(b) a first digital filter for transforming the digitized first signal into a first filtered signal, the first filter characterized by a frequency pass-band based on a heart rate; and

(c) a second digital filter for transforming the first filtered signal into a thoracocardiograph signal, the second filter characterized by averaging segments of the first filtered signal based on events characteristic of the cardiac cycles.

23. The thoracocardiograph of claim 22 further comprising a plethysmographic inductive sensor for generating the first signal.

24. The thoracocardiograph of claim 22 further comprising a electrocardiograph for determining the heart rate and for providing indicia of events characteristic of the cardiac cycles.

25. A computer-readable medium comprising instructions for controlling a computer to generate a thoracocardiograph signal from a plethysmographic signal responsive to cardiac activity by:

(a) frequency domain filtering the plethysmographic signal producing a first filtered signal; and

(b) time domain filtering the first filtered signal producing thoracocardiograph signal.

26. The computer-readable medium of claim 25 further comprising instructions for controlling the computer to extract at least one of the cardiac parameter from the thoracocardiograph signal.

27. The computer-readable medium of claim 25 further comprising instructions for controlling the computer to receive the plethysmographic signal from an inductive plethysmographic sensor.

28. A method for extracting cardiac parameters from a plethysmographic signal characterized by a heart rate, the method comprising the steps of:

(a) performing a first band-pass filtering operation on the plethysmographic signal producing a first filtered signal, the first filtering operation characterized by a lower corner frequency less than the heart rate;

(b) performing a second band-pass filtering operation on the plethysmographic signal producing a second filtered signal, the second filtering operation characterized by a lower corner frequency greater than the lower corner frequency of the first filtering operation;

(c) interpolating the first filtered signal and the second filtered signal based on the heart rate to produce a filtered plethysmographic signal; and

(d) extracting cardiac parameters from the filtered plethysmographic signal.

29. The method of claim 28 further including the step of averaging the filtered plethysmographic signal prior to extracting the cardiac parameters.

30. A system for extracting cardiac parameters from a plethysmographic signal comprising:

(a) means for receiving a heart rate;

(b) a first filter characterized by a first lower corner frequency, the first lower corner frequency not greater than the heart rate, the first filter capable of receiving the plethysmographic signal and generating a first filtered signal;

(c) a second filter characterized by a second lower corner frequency, the second lower corner frequency greater than the first lower corner frequency, the second filter capable of receiving the plethysmographic signal and generating a second filtered signal; and

(d) a processor for generating a filtered plethysmographic signal by interpolating the first filtered signal and the second filtered signal based on the heart rate and extracting a cardiac parameter from the filtered plethysmographic signal.