Method and apparatus for processing a physiological signal

• US 20020099281A1
• Filed: 01/14/2002
• Published: 07/25/2002
• Est. Priority Date: 09/30/1999
• Status: Active Grant

• Current Assignee: Prothia
• Original Assignee: Dennis E. Bahr, James L. Reuss
• Inventors: Reuss., James L., Bahr, Dennis E.
• Granted Patent: US 6,647,280 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/1455 using optical sensors, e.g. spectral photometrical oximeters
• CPC Class Codes: A61B 5/1464 specially adapted for foetal tissue
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

A signal processing method, preferably for extracting a fundamental period from a noisy, low-frequency signal, is disclosed. The signal processing method generally comprises calculating a numerical transform for a number of selected periods by multiplying signal data by discrete points of a sine and a cosine wave of varying period and summing the results. The period of the sine and cosine waves are preferably selected to have a period substantially equivalent to the period of interest when performing the transform.

Claims

1. A method for determining a fundamental period from an input signal, the method comprising the following steps:

selecting a predetermined maximum period and dividing the period into a plurality of incremental periods; and

determining a transform at each of the plurality of incremental periods by;

summing the values resultant from multiplying the input signal by a plurality of points spaced equidistantly on a first periodic waveform having a period substantially equivalent to the incremental period; and

summing the values resultant from multiplying the input data by a plurality of points spaced equidistantly on a second periodic waveform having a period substantially equivalent to the incremental period, wherein the second periodic waveform is substantially orthogonal to the first periodic waveform.

2. The method as defined in claim 1, further comprising the step of calculating an index into the first and second periodic waveforms, where the index is based upon an increment which is inversely proportional to the incremental period, permitting decimation of the first and second periodic waveforms to match the incremental period.

3. The method as defined in claim 1, further comprising the step of calculating an ensemble average to reconstruct the input wave.

4. The method as defined in claim 1, further comprising the step of calculating the power spectrum for each transform.

5. The method as defined in claim 1, further comprising the step of tracking a period of interest in the input data through time by locating and tracking its peak in the power versus period buffer.

6. The method as defined in claim 1, wherein the input data is a plesthymographic wave.

7. The method as defined in claim 1, wherein the input data is a plethysmographic signal from a pulse oximetery sensor.

8. The method as defined in claim 1, wherein the input data is a plethysmographic signal from a fetal pulse oximeter.

9. The method as defined in claim 1, wherein the input data is slid through the transform such that a new data point is added and an old data point is subtracted for each iteration.

10. A method for calculating a digital numerical transform for a low frequency periodic signal thereby deriving power versus period in real-time, the method comprising the following steps:

obtain sampled data points from a continuous periodic waveform at a predetermined time interval;

for each period between a pre-selected minimum period and a pre-selected maximum period, wherein each period is separated from the adjacent period by the pre-selected time interval;

(a) calculate an imaginary transform component value equal to input data times a selected point on a sine wave having a period equal to the period;

(b) add to a sum of previous imaginary transform component values for the same period;

(c) calculate a real transform component value equal to input data times a selected point on a cosine wave having a period equal to the period;

(d) add to a sum of previous real transform component values for the same period;

(e) increment the selected point on the sine and cosine waves; and

(f) get new sample of input data;

repeat while calculations are desired.

11. The method as defined in claim 10, further comprising the steps of scaling the real and imaginary component sums to give all periods equal weighting and adding the squares of the sums of real and imaginary components for a given period to arrive at the relative power corresponding to that period in the input signal.

12. The method as defined in claim 10, further comprising the steps of storing points of a sine wave and a cosine wave of the predetermined maximum period taken at the time interval;

calculating an index to the sine and cosine waves, where the index is inversely proportional to the selected period; and

indexing the sine wave and the cosine wave to selectively vary the period of the sine and cosine waves.

13. The method as defined in claim 10, wherein the input data is initially calculated as a difference between the oldest data sample in a buffer and the newest data sample in the buffer and the input data is continually updated by adding the newest data sample and subtracting the oldest data sample.

15. The oximeter as defined in claim 14, wherein the processor is further programmed to index through a buffer storing the first periodic waveform and a buffer storing the second periodic waveform, where the index is inversely proportional to a selected period to selectively decimate the length of the period.

16. The oximeter as defined in claim 14, wherein the processor is further programmed to calculate a cardiac rate based on a counted number of pulsatile components found in a blood oxygenation calculation.

17. The oximeter as defined in claim 16, wherein the processor is programmed to calculate the numerical transform only when the cardiac rate determined by the oxygenation calculation is insufficient.

18. The oximeter as defined in claim 14, wherein the sensor is a fetal sensor.

20. The method as defined in claim 19, further comprising the steps of storing successive sampled points of a cosine wave and sampled points of a sine wave, where the sampled points are separated by a time interval equivalent to the sampling rate.

21. The method as defined in claim 19, further comprising the step of adding a new data point and subtracting an old data point at each time interval.

22. The method as defined in claim 20, further comprising the step of calculating an index to the stored sine and cosine wave sampled points where the index is inversely proportional to the period such that for small periods the index into the stored data is large and for large periods it is small.

14. An oximeter comprising:

an input port for receiving a plethsymographic signal from a sensor;

an analog to digital converter;

a processor programmed to calculate a numerical transform of the plethsymographic wave to determine the fundamental period of the cardiac activity, wherein the numerical transform is calculated at each of a plurality of incremental periods by multiplying samples of the plethsymographic waveform by first and second periodic orthogonal waveforms of period equal to the selected incremental period.

19. A method for determining a cardiac signal from a noisy plethsymographic input signal, the method comprising the following steps:

sampling the input plethsymographic signal at a predetermined time interval;

calculating a numerical transform of the plethsymographic signal for each of a plurality of periods up to a maximum period where each of the period are separated by the predetermined time interval; and

calculating a relative strength for each period as the power of each period.