Locating features in a photoplethysmograph signal

• US 20050004479A1
• Filed: 09/24/2002
• Published: 01/06/2005
• Est. Priority Date: 09/28/2001
• Status: Abandoned Application

• Current Assignee: Isis Innovation Limited, University of Oxford
• Original Assignee: Isis Innovation Limited
• Inventors: Townsend, Neil William, Germuska, Richard Bartholomew
• Application Number:
  US10/490,545
• Publication Number:
  US 20050004479A1
• Time in Patent Office: Days
• US Class Current: 600/500
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

A method and apparatus for locating a feature in a photoplethysmograph or blood pressure signal, comprising a series of signal complexes each having a principal peak (or equivalent trough), is disclosed. The signal is processed to identify a reference point on the upslope of a principal peak. The signal is then searched for the feature in the vicinity of the reference point.

Claims

1. A method of locating a feature in a digitised photoplethysmograph signal comprising a series of signal complexes each having a principal peak, the method comprising the steps of:

processing the signal to identify a reference point on the upslope of a principal peak; and

searching for the feature in the vicinity of the reference point;

2. The method of claim 1 wherein the step of processing includes the step of applying a gradient function to the signal to determine a gradient waveform.

3. The method of claim 2 wherein the step of processing further includes the step of detecting a reference peak in the gradient waveform.

4. The method of claim 2 wherein the step of processing further includes the steps of:

applying a peak enhancement function to the gradient waveform; and

detecting a reference peak in the peak enhanced gradient waveform.

5. The method of claim 4 wherein the peak enhancement function comprises a cube function.

6. The method of claim 3 wherein the step of processing further includes the step of discarding the reference peak if it fails to meet a threshold criterion.

7. The method of claim 6 wherein the threshold criterion is calculated using the size of one or more of the preceding reference peaks.

8. The method of claim 6 wherein the threshold criterion is modified if no reference peak meeting the threshold criterion is detected within a predetermined interval.

9. The method of claim 4 wherein the reference point on the upslope of a principal peak is determined from the location of the reference peak.

10. The method of claim 1 wherein the step of processing includes a preliminary step of applying a band-pass filter to the signal.

11. The method of claim 1 wherein the step of searching for the feature comprises the step of scanning the signal in the vicinity of the reference point by applying a predetermined scan criterion to a plurality of points of the signal in the vicinity of the reference point.

12. The method of claim 1 wherein the step of searching for the feature comprises the step of fitting a curve to the signal in the vicinity of the reference point and identifying the feature from a corresponding feature in the fitted curve.

13. The method of claim 11 wherein the step of searching for the signal peak is carried out on the signal following a step of band pass filtering of the signal.

14. The method of claim 1 wherein the feature is the principal peak of a signal complex.

15. The method of claim 1 further—

20 comprising the step of determining a pulse rate from the timings within said signal of a plurality of said features.

34. A photoplethysmograph adapted to carry out the method steps of claim 1.

35. A computer readable data carrier comprising computer program instructions for carrying out the method steps of claim 1 when executed on suitable computer apparatus.

16. Apparatus for locating multiple instances of a feature in a digitised photoplethysmograph signal which comprises a series of signal complexes each having a principal peak, the apparatus comprising:

a signal processing unit adapted to receive said signal and to identify reference points on the upslopes of said principal peaks; and

a search unit adapted to receive said reference points and to search said signal for said feature in the vicinity of each reference point.

17. The apparatus of claim 16 wherein the signal processing unit is adapted to apply a gradient function to said signal to determine a gradient waveform.

18. The apparatus of claim 17 wherein the signal processing unit is further adapted to detect reference peaks in the gradient waveform.

19. The apparatus of claim 17 wherein the signal processing unit is further adapted to apply a peak enhancement function to the gradient waveform and to detect reference peaks in the peak enhanced gradient waveform.

20. The apparatus of claim 19 wherein the peak enhancement function comprises a cube function.

21. The apparatus of claim 18 wherein the signal processing unit is further adapted to discard a reference peak if it fails to meet a threshold criterion.

22. The apparatus of claim 21 wherein the signal processing unit is adapted to calculate a threshold criterion for a particular reference peak using the magnitude of one or more of the preceding reference peaks.

23. The apparatus of claim 21 wherein the signal processing unit is adapted to modify the threshold criterion if no reference peak meeting the criterion is detected within a predetermined interval of the signal.

24. The apparatus of claim 19 wherein the signal processing unit is adapted to determine the reference point on the upslope of each principal peak from the location of a corresponding reference peak.

25. The apparatus of claim 16 further comprising at least one band pass filter arranged to filter the signal either before or after the application of a gradient function to the signal.

26. The apparatus of claim 16 wherein the search unit is adapted to scan the signal in the vicinity of each said reference point by applying a predetermined scan criterion to a plurality of signal points in the vicinity of each said reference point.

27. The apparatus of claim 16 wherein the search unit is adapted to fit a curve to the signal in the vicinity of each said reference point and to identify an instance of the feature from a corresponding feature in the fitted curve.

28. The apparatus of claim 26 further adapted to band pass filter the signal before carrying out the step of scanning or fitting.

29. The apparatus of claim 16 wherein the features for location are the principal peaks of signal complexes.

30. The apparatus of claim 16 further comprising a pulse rate unit adapted to calculate a pulse rate from the locations within said signal of a plurality of said features.

31. A method for locating a feature in a digitised blood pressure signal comprising a series of signal complexes each having a principal peak, the method comprising the steps of:

processing the signal to identify a reference point on the upslope of a principal peak; and

searching for the feature in the vicinity of the reference point.

32. Apparatus for locating multiple instances of a feature in a digitised blood pressure signal which comprises a series of signal complexes each having a principal peak, the apparatus comprising:

a signal processing unit adapted to receive said signal and to identify reference points on the upslopes of said principal peaks; and

a search unit adapted to receive said reference points and to search said signal for said feature in the vicinity of each reference point.

33. A computer program product for locating multiple instances of a feature in a digitised photoplethysmograph or blood pressure signal having a series of signal complexes each having a principal peak, the product comprising a computer readable storage medium carrying computer program instructions providing:

a signal processing element adapted to receive said signal and to identify reference points on the upslopes of said principal peaks; and

a search element adapted to receive said reference points and to search said signal for instances of said feature in the vicinity of each reference point.

36. A computer readable data carrier comprising computer program code, for locating a feature in a digised blood pressure signal comprising a series of signal complexes each having a principal peak, when executed on a computer, the program code including elements adapted to:

process the signal to identify a reference point on the upslope of a principal peak; and

search for the feature in the vicinity of the reference point.