Apparatus and method for detection and quantification of oscillatory signals

• US 20040097814A1
• Filed: 12/09/2003
• Published: 05/20/2004
• Est. Priority Date: 03/09/2001
• Status: Active Grant

• Current Assignee: Auckland UniServices Limited (Auckland University)
• Original Assignee: Auckland UniServices Limited (Auckland University)
• Inventors: Navakatikyan, Michael Alexander, Malpas, Simon Charles
• Granted Patent: US 7,353,127 B2
• Time in Patent Office: Days
• US Class Current: 600/485
• CPC Class Codes: A61B 5/024 Detecting, measuring or recording pulse rate or heart rate A61B5/0205, A61B5/021 take precedence
• CPC Class Codes: A61B 5/349 Detecting specific parameters of the electrocardiograph cycle
• CPC Class Codes: G06F 2218/10 by analysing the shape of a waveform, e.g. extracting parameters relating to peaks

Abstract

A method of determining the location in time of maxima and/or minima of an oscillatory signal. The method may have application to measurement of biological signals, in particular measurement of heart rate from a pulsatile blood signal. The method includes a first stage including the steps of observation over a measurement period, identifying large local maxima or minima and computing an average interval between the identified local maxima or minima. One or more exclusion periods are located in time in the oscillatory signal, having a duration dependent on the average interval, the exclusion periods used to reject false maxima or minima. Maxima or minima may also be detected as an absolute maximum or minimum between crossing points of a fast and a slow moving average of the oscillatory signal. An exclusion period may also be used to reject false maxima or minima when crossing points are used. Apparatus for performing the method is also claimed.

Claims

1. A method of determining the location in time of maxima and/or minima of an oscillatory signal, the method including the steps of:

i) observing the amplitude of the oscillatory signal over a predefined measurement period and identifying local maxima or minima that have a preselected measure indicative of the size of the maxima or minima, which exceeds a certain threshold;

ii) computing an average interval between local maxima or minima identified in step i);

iii) computing the duration of one or more exclusion periods dependent on said average interval;

iv) determining a location in time of the one or more exclusion periods based on the observed signal;

v) detecting local maxima or minima in the observed signal and rejecting a maximum or minimum if a maximum or minimum is located in time within an exclusion period.

2. The method of claim 1, wherein said certain threshold is calculated as a predetermined proportion of said preselected measure of the local maximum or minimum that has the greatest size according to said preselected measure.

3. The method of claim 1 or claim 2, wherein said preselected measure indicative of the size of the maxima or minima is either the amplitude of the maxima or minima, or the magnitude of the fall or rise to an adjacent minimum or maximum respectively.

4. The method of any one of claims 1 to 3, wherein the duration of the one or more exclusion periods is a predetermined percentage or proportion of said average interval.

5. The method of any one of claims 1 to 4, wherein the location in time of an exclusion period is dependent on the location in time of a crossing point between two moving averages having different averaging intervals.

6. The method of any one of claims 1 to 5, wherein the location in time of an exclusion period is dependent on the location in time of a previously calculated maximum or minimum.

7. The method of claim 5, wherein the averaging interval of each moving average is updat d dependent on said average interval.

28. The method of claim 1 wherein step of identifying local maxima or minima that have a preselected measure indicative of the size of the maxima or minima includes:

determining whether the left or right side of one or more selected maxima or minima has a larger fall or rise respectively and memorising which side has the greater fall and rise and the magnitude of the largest fall or rise within a sample of the oscillatory signal; and

rejecting maxima or minima if the fall or rise respectively is less than a predetermined proportion of the magnitude of the largest fall or rise.

29. The method of any one of the preceding claims when used to detect maxima or minima of a signal representative of pulsatile blood pressure in an animal.

30. The method of claim 29 further including determining a measure of the heart rate from detected maxima or minima of the signal representative of pulsatile blood pressure.

34. A method of determining the frequency or wavelength of an oscillatory signal including determining the location in time of maxima and/or minima of the oscillatory signal by the method according to any one of claims 1 to 30, calculating the difference in time between maxima and/or minima and using said difference in time to calculate the frequency and/or wavelength of the signal.

35. A method of any one of claims 1 to 30 or claim 34 when performed in real-time.

8. A method of determining the location in time of periodic maxima and/or minima of an oscillatory signal, the method including the steps of:

a) observing the oscillatory signal by detecting the amplitude of the oscillatory signal;

b) computing a first and a second moving average of the oscillatory signal, the first moving average having a first averaging time interval and the second moving average having a second averaging time interval longer than the first averaging time interval;

c) determining the location in time of a plurality of crossing points, wherein a crossing point is a point of intersection between the first and second moving averages;

d) defining at least one measurement period as the interval between two crossing points; and

e) determining the location in time of an absolute maximum or minimum of the oscillatory signal within the or each measurement period, thereby determining the location in time of each periodic maxima or minima respectively of the oscillatory signal.

9. The method of claim 8, wherein a crossing point is defined as a first type if the first moving average was less than the second moving average prior to the crossing point and defined as a second type if the first moving average was greater than the second moving average prior to the crossing point and wherein the measurement period is defined as the interval between two consecutive crossing points of the same type.

10. The method of claim 8, wherein a crossing point is defined as a first type if the first moving average was less than the second moving average prior to the crossing point and defined as a second type if the first moving average was greater than the second moving average prior to the crossing point and wherein the measurement period is defined as the interval between two consecutive crossing points of a different type.

11. The method of any one of claims 8 to 10, further including the steps of:

f) defining an exclusion period located in time relative to at least one of each crossing point defining each measurement period;

g) evaluating whether a crossing point defining a first measurement period falls within an exclusion period corresponding to a crossing point defining a second measurement period; and

h) if the crossing point defining the first measurement period falls within the exclusion period corresponding to the crossing point defining the second measurement period, evaluating whether that crossing point or the crossing point corresponding to the exclusion period within which it is located in time satisfies a first predetermined set of criteria and rejecting the crossing point for the purposes of defining a measurement period if said criteria are met.

12. The method of claim 11, wherein if a crossing point is rejected, the method further includes also rejecting the exclusion period associated with the rejected crossing point.

13. The method of claim 11 or claim 12, further including the steps of:

i) detecting the location in time of a plurality of local maxima or minima within the oscillatory signal that exceed a predetermined amplitude threshold; and

j) computing the length of the exclusion period in time dependent on an average value of the separation in time of detected maxima or minima that exceed said predetermined amplitude threshold.

14. The method of claim 13, further including rejecting a local maximum detected in step i) for the purposes of step j) if the local maximum falls within a predetermined time period of a local maximum having a greater value.

15. The method of claim 13, further including rejecting a local minimum detected in step i) for the purposes of step j) if the local minimum falls within a predetermined time period of a local minimum having a lesser value.

16. The method of any one of claims 13 to 15, wherein said average value of the separation in time of detected maxima or minima is the median value.

17. The method of any one of claims 13 to 16, further including band-pass filtering the observed signal prior to performing step i).

18. The method of any one of claims 13 to 17, wherein the predetermined amplitude threshold is computed using the equation:

Thr=(Max_F−

Avg_F)·

ƒ

₁+Avg_F if maxima is determined in step e) and using the equation Thr=(Min_F−

Avg_F)·

ƒ

₁+Avg_F if minima is determined in step e), wherein Thr is the predetermined threshold, Avg_F is an average of the observed signal, Max_F is the maximum of the observed signal, Min_F is the minimum of the observed signal, and f₁, is a preset fraction dependent on the characteristics of the signal and wherein Avg_F, Max_F, and Min_F are determined from a sample of the observed signal.

19. The method of any one of claims 11 to 18, wherein step f) includes locating the exclusion period immediately adjacent to the crossing point, after the crossing point.

20. The method of any one of claims 8 to 19, further including comparing an average value of the observed signal over a sample period with the maximum and minimum values over the same sample period and inverting the signal if required so that the maximum or minimum determined in step e) has a greater variation from the average than the minimum or maximum respectively.

21. The method of claim 20, wherein the reflected or inverted signal is computed as double an average of the observed signal less the value of the observed signal.

22. The method of any one of claims 8 to 21, further including:

defining an exclusion period located in time relative to a detected maximum or minimum within each measurement period; and

evaluating whether a detected maximum or minimum of a first measurement period falls within an exclusion period corresponding to a maximum or minimum of a second measurement period; and

if the maximum or minimum of the first measurement period falls within the exclusion period of the second measurement period, evaluating whether the crossing point corresponding to the maximum or minimum of the first measurement period or the crossing point corresponding to the exclusion period within which the maximum or minimum of the first measurement period falls satisfies a predetermined set of criteria and rejecting a crossing point for the purposes of defining a measurement period if said criteria are met.

23. The method of claim 11 or claim 22, wherein the set of predetermined criteria is based on the relative magnitude of a maximum or minimum in an exclusion period to the magnitude of at least one maximum or minimum respectively in the observed signal.

24. The method of claim 23 when dependent on claim 22, wherein a crossing point is rejected if the amplitude of a maximum within the measurement period defined by that crossing point is less than an adjacent maximum or less than a predetermined percentage of an adjacent maximum.

25. The method of claim 23 when dependent on claim 22, wherein a crossing point is rejected if the amplitude of the corresponding minimum to the crossing point located within the exclusion period is greater than an adjacent minimum or greater than a percentage of an adjacent minimum.

26. The method of any one of claims 22 to 25 when dependent on claim 11, wherein a crossing point is rejected only if it is within an exclusion period corresponding to another crossing point and the maximum or minimum corresponding to the crossing point is within an exclusion period corresponding to another maximum or minimum.

27. The method of any one of claims 8 to 26, further including the step of recording the maximum or minimum value of the oscillatory signal in each measurement period.

31. Apparatus for determining the location in time of maxima and/or minima of an oscillatory waveform including sensing means to sense an oscillatory waveform and generate an oscillatory signal representative of an oscillatory waveform and a computer processing means in communication with the sensing means and having an instruction set to cause said computer processing means to perform the method as defined in any one of claims 2 to 7 or 9 to 31.

32. Apparatus for determining the location in time of maxima and/or minima of an oscillatory waveform, the apparatus including sensing means to sense an oscillatory waveform and generate an oscillatory signal representative of an oscillatory waveform and a computer processing means in communication with the sensing means and having an instruction set to cause said computer processing means to:

i) identify local maxima or minima that have a preselected measure indicative of the size of the maxima or minima, which exceeds a certain threshold;

ii) compute an average interval between local maxima or minima identified in step i);

iii) compute the duration of one or more exclusion periods dependent on said average interval;

iv) compute a location in time of the one or more exclusion periods based on the observed signal;

v) identify local maxima or minima in the observed signal and record identified maxima or minima except any maximum or minimum that is located in time within an exclusion period.

33. Apparatus for determining the location in time of maxima and/or minima of an oscillatory waveform, the apparatus including sensing means to sense an oscillatory waveform and generate an oscillatory signal representative of an oscillatory waveform and a computer processing means in communication with the sensing means and having an instruction set to cause said computer processing means to:

e) compute a first and a second moving average of the oscillatory signal, the first moving average having a first averaging time interval and the second moving average having a second averaging time interval longer than the first averaging time interval;

f) compute the location in time of a plurality of crossing points, wherein a crossing point is a point of intersection between the first and second moving averages;

g) define at least one measurement period as the interval between two crossing points; and

h) compute the location in time of an absolute maximum or minimum of the oscillatory signal within the or each measurement period, thereby determining the location in time of each periodic maxima or minima respectively of the oscillatory signal.

36. A method of detecting maxima and/or minima in an oscillatory signal substantially as herein described and with reference to FIGS. 1 to 12 or FIGS. 13 to 31.

37. Apparatus for detecting maxima and/or minima in an oscillatory signal substantially as herein described and with reference to FIGS. 1 to 12 or FIGS. 13 to 31.