Techniques for accurately deriving physiologic parameters of a subject from photoplethysmographic measurements

• US 20080167564A1
• Filed: 01/10/2008
• Published: 07/10/2008
• Est. Priority Date: 01/10/2007
• Status: Active Grant

• Current Assignee: STARR Life Sciences Corp.
• Original Assignee: STARR Life Sciences Corp.
• Inventors: Hete, Benard F., Ayers, Eric J.
• Granted Patent: US 8,298,154 B2
• Time in Patent Office: Days
• US Class Current: 600/508
• CPC Class Codes: A61B 5/02405 Determining heart rate variability
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/0816 Measuring devices for examining respiratory frequency measuring frequency of electric signals G01R23/00
• CPC Class Codes: A61B 5/0823 Detecting or evaluating cough events
• CPC Class Codes: A61B 5/1455 using optical sensors, e.g. spectral photometrical oximeters
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7253 characterised by using transforms
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

Several techniques are disclosed for isolating either heart or breath rate data from a photoplethysmograph, which is a time domain signal such as from a pulse oximeter. The techniques involve the use of filtering in the frequency domain, after a Fast Fourier Transform (FFT) has been conducted on a given photoplethysmograph also references as a given set of discrete time-domain data. The filtering may be applied to an identified fundamental frequency and one or more harmonics for heart related parameters. The filter may be truncated to the frequency data set and further applied multiple times to improve roll off. After filtering, an Inverse FFT (IFFT) is used to reconstruct the time-domain signal, except with undesirable frequency content eliminated or reduced. Calculation or measurement of parameters is then conducted on this reconstructed time-domain signal.

Claims

1. A method for deriving physiologic parameters of a subject from photoplethysmographic measurements comprising the steps of:

Obtaining a frequency spectrum of a time-based photoplethysmograph of the subject;

Detecting a series of peaks of the frequency spectrum;

Calculating a change of frequency between adjacent detected peaks of the series of detected peaks of the frequency data set; and

Identifying a repeating change of frequency between adjacent detected peaks of the series of detected peaks of the frequency data set; and

Utilizing the identified repeating change of frequency to derive physiologic parameters of the subject.

2. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 1 further comprising the steps of:

Filtering the frequency spectrum based upon the identified repeating change of frequency between adjacent detected peaks of the series of detected peaks of the frequency spectrum;

Performing an IFFT on the filtered frequency spectrum to form a filtered time-based physiologic signal; and

Calculating physiologic parameters from the filtered time-based physiologic signal.

3. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 2 wherein the physiologic parameters include breathing parameters of the subject.

4. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 2 wherein the step of obtaining a frequency spectrum of a time-based photoplethysmograph of the subject is by applying an FFT to the time-based photoplethysmograph of the subject, and wherein the subject is a rodent.

5. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 2 wherein the step of filtering the frequency spectrum based upon the identified repeating change of frequency between adjacent detected peaks of the series of detected peaks of the frequency spectrum includes applying a windowed-sinc filter to the frequency data set.

6. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 5 wherein the windowed-sinc filter kernel has no greater number of points than are in the time-domain data set.

7. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 5 wherein the windowed-sinc filter is applied multiple times to the frequency spectrum.

8. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 1 wherein the step of detecting a series of peaks of the frequency spectrum comprises the steps of:

A) providing a preset original threshold amplitude;

B) reviewing the frequency spectrum from high frequency to low frequency and identifying a first peak that crosses the preset original threshold amplitude value;

C) increasing a subsequent threshold amplitude;

D) reviewing the frequency spectrum from high frequency to low frequency from the last identified peak and identifying a subsequent peak that crosses the subsequent threshold amplitude value.

9. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 8 wherein the step of detecting a series of peaks of the frequency spectrum further comprises the steps of repeating steps C) and D) to detect additional peaks.

10. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 1 wherein the physiologic parameters include heart rate and the detected peaks include a fundamental heart rate frequency and at least one harmonic of the fundamental heart rate frequency, and further including the steps of:

Filtering the frequency data set to isolate the fundamental heart rate frequency and at least one harmonic of the heart rate frequency;

Performing an IFFT on the filtered frequency data set to form a filtered time-based physiologic signal; and

Calculating physiologic parameters from the filtered time-based physiologic signal.

11. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 10 wherein the filtering of the frequency data set includes the application of a windowed-sinc filter to the frequency data set.

12. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 10 wherein the filtering of the frequency data set includes the application of a square window filter to the frequency spectrum.

13. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 1 further comprising the steps of filtering the frequency spectrum to compensate the frequency spectrum for an earlier applied filter.

14. A method for deriving physiologic parameters of a subject from photoplethysmographic measurements comprising the steps of:

Obtaining a frequency spectrum of a time-based photoplethysmograph of the subject;

Detecting a series of peaks of the frequency spectrum;

Filtering the frequency spectrum by applying a filter to the frequency spectrum in multiple passes;

Obtaining a filtered time-based physiologic signal from the filtered frequency spectrum; and

Calculating physiologic parameters from the filtered time-based physiologic signal.

15. A method for deriving physiologic heart related parameters of a subject from photoplethysmographic measurements comprising the steps of:

Obtaining a frequency spectrum of a time-based photoplethysmograph of the subject;

Detecting a series of peaks of the frequency spectrum, wherein the physiologic parameters include heart rate and the detected peaks include a fundamental heart rate frequency and at least one harmonic of the fundamental heart rate frequency;

Filtering the frequency spectrum to isolate the fundamental heart rate frequency and at least one harmonic of the heart rate frequency;

Obtaining a filtered time-based physiologic heart signal from the filtered frequency spectrum; and

Calculating physiologic parameters from the filtered time-based physiologic heart signal.

16. A method for deriving physiologic parameters of a subject from photoplethysmographic measurements comprising the steps of:

Obtaining a frequency spectrum of a time-based photoplethysmograph of the subject;

Detecting at least one peak of the frequency spectrum;

Determining a fundamental frequency whereby

i) if more than one peak is detected then calculating a change of frequency between adjacent detected peaks of the series of detected peaks of the frequency spectrum and attempting to identify a repeating change of frequency between adjacent detected peaks of the series of detected peaks of the frequency spectrum, wherein the fundamental frequency is the repeating change of frequency, and

ii) if only one peak is detected or if a change of frequency cannot be identified then identifying a maximum peak, wherein the maximum peak is the fundamental frequency; and

Utilizing the fundamental frequency to derive physiologic parameters of the subject.

17. A method for deriving physiologic parameters of a subject from photoplethysmographic measurements comprising the steps of:

Obtaining a frequency spectrum of a time-based photoplethysmograph of the subject;

Detecting a series of peaks of the frequency spectrum;

Filtering the frequency spectrum by applying a windowed filter on the frequency data set;

Obtaining a filtered time-based physiologic signal from the filtered frequency spectrum; and

Calculating physiologic parameters from the filtered time based physiologic signal.

18. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 17 wherein the windowed filter is a windowed-sinc filter and wherein the windowed-sinc filter kernel has no greater number of points than are in the time-domain data set.

19. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 17 wherein the windowed filter is a windowed-sinc filter and wherein the windowed-sinc filter is applied multiple times to the frequency data set.

20. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 17 wherein the windowed filter is one of a square windowed filter, a Bartlett windowed filter, a Blackman windowed filtered, a Hanning windowed filter, a Hamming window filter, and a trapezoid window filter.

21. A method for deriving physiologic parameters of a subject from photoplethysmographic measurements comprising the steps of:

A) Obtaining a frequency spectrum of the time-based photoplethysmograph;

B) Detecting a series of peaks of the frequency spectrum wherein the step of detecting a series of peaks of the frequency spectrum comprises the steps of

i) providing a preset original threshold amplitude;

ii) reviewing the frequency spectrum from high frequency to low frequency and identifying a first peak that crosses the preset original threshold amplitude value;

iii) increasing the subsequent threshold amplitude;

iv) reviewing the frequency data set from high frequency to low frequency from the last identified peak and identifying a second peak that crosses the subsequent threshold amplitude value; and

C) Utilizing the identified peaks to derive physiologic parameters of the subject.

22. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 21 wherein the step of detecting a series of peaks of the frequency spectrum further comprises the steps of repeating steps i) and iv) to detect additional peaks.

23. The method for deriving physiologic parameters of a subject from photoplethysmographic measurements according to claim 21 wherein the increasing of the subsequent threshold includes setting the subsequent threshold amplitude equal to at least fifty percent of the amplitude of the last identified peak.