Method and apparatus for estimating a physiological parameter from a physiological signal

• US 20020099280A1
• Filed: 07/26/2001
• Published: 07/25/2002
• Est. Priority Date: 11/21/2000
• Status: Active Grant

• Current Assignee: Dragerwerk AG & Company KGaA (FPS Vermögensverwaltung GmbH)
• Original Assignee: Draeger Medical Systems Incorporated (Dragerwerk AG)
• Inventors: Huang, John W.
• Granted Patent: US 6,594,512 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

A system is disclosed which first identifies a plurality of characteristics of a physiological signal any one of which may represent a physiological parameter. A plurality of different techniques are used to provide respective likelihood factors for each such identified characteristic. The resulting likelihood factors are then analyzed to select the one characteristic of the physiological signal which most likely represents the desired physiological parameter. The physiological parameter is then calculated from the selected characteristic of the physiological signal.

Claims

1. Apparatus for estimating a physiological parameter from a physiological signal, comprising:

a source of the physiological signal;

a characteristic identification circuit, coupled to the physiological signal source, for identifying respective characteristics in the physiological signal;

a plurality of likelihood circuits, coupled to the characteristic identification circuit, each circuit producing a plurality of signals, respectively corresponding to the identified characteristics, representing the likelihood that the corresponding identified characteristic represents said physiological parameter;

an arbitrator, responsive to the likelihood representative signals, for selecting one of the identified characteristics; and

a calculating circuit, for estimating the physiological parameter responsive to the selected one of the identified characteristics.

2. The apparatus of claim 1 further comprising a bandpass filter coupled between the physiological signal source and the characteristic identification circuit.

3. The apparatus of claim 1 wherein the characteristic identification circuit comprises a circuit for identifying one or more peaks at respective corresponding frequencies in the spectrum of the physiological signal as characteristics of the physiological signal.

4. The apparatus of claim 3 wherein the peak identifying circuit comprises:

a fast Fourier transform (FFT) circuit, coupled to the physiological signal source, for generating a frequency spectrum, having respective magnitudes at associated frequencies, representing the physiological signal;

a peak locating circuit, responsive to the spectrum, for identifying peaks in the respective magnitudes and the frequency associated with the peaks.

5. The apparatus of claim 4 further comprising a window filter coupled to the peak identification circuit for generating a plurality of time domain signals window filtered at frequencies respectively corresponding to the identified peaks.

6. The apparatus of claim 5 wherein the window filter comprises:

a circuit selecting a subset of magnitudes in the frequency spectrum in the neighborhood of the frequency of each identified peak; and

an inverse fast Fourier transform circuit, coupled to the selecting circuit, for generating a time domain window filtered signal in response to the selected subset of magnitudes

7. The apparatus of claim 5 wherein:

the physiological signal has a frequency range of interest; and

the window filter comprises;

a plurality of bandpass filter, coupled in common to the physiological signal source, each having a relatively narrow passband, the plurality having respective center frequencies set so that the plurality covers the frequency range of interest; and

a multiplexer, having respective input terminals coupled to the plurality of bandpass filters and an output terminal, and responsive to the frequency of a respective corresponding identified peak, for selecting the one of the plurality of bandpass filters with a center frequency closest to the frequency of the respective corresponding identified peak, and coupling the one of the plurality of bandpass filters to its output terminal.

8.he. The apparatus of claim 7 wherein the passbands of the plurality of bandpass filters overlap.

9. The apparatus of claim 3, wherein:

the frequency of each identified peak has an associated period; and

one of the plurality of likelihood circuits comprises;

a circuit for generating a plurality of time domain signals, each signal being the physiological signal window filtered in the neighborhood of the frequency of a respective corresponding identified peak;

a circuit for generating a plurality of time delayed signals respectively corresponding to the plurality of time domain signals delayed by respective time intervals equal to one-half the period associated with the time domain signal; and

a circuit for generating a plurality of likelihood signals, each corresponding to a respective one of the identified peaks, representing the degree of congruence of the time domain signal to the delayed time domain signal corresponding to the one of the identified peaks.

10. The apparatus of claim 9 wherein the time delayed signal generating circuit comprises circuitry for delaying the plurality of time domain signals by respective time intervals equal to (n+½

) times the period associated with the time domain signal where n is an integer greater than or equal to zero.

11. The apparatus of claim 9 wherein the likelihood signal generating circuit comprises an adder, coupled to the time domain signal generator and the delayed time domain signal generator, the adder operating as an accumulator.

12. The apparatus of claim 3, wherein:

the frequency of each identified peak has an associated period; and

one of the plurality of likelihood circuits comprises;

a circuit for generating a plurality of time domain signals, each signal being the physiological signal window filtered in the neighborhood of the frequency of a respective corresponding identified peak;

a circuit for generating a plurality of time delayed signals respectively corresponding to the plurality of time domain signals delayed by respective time intervals equal to the period associated with the time domain signal; and

a circuit for generating a plurality of likelihood signal, each corresponding to a respective one of the identified peaks, representing the degree of congruence of the time domain signal to the delayed time domain signal corresponding to the one of the identified peaks.

13. The apparatus of claim 12 wherein the time delayed signal generating circuit comprises circuitry for delaying the plurality of time domain signals by respective time intervals equal to n times the period associated with the time domain signal where n is an integer greater than or equal to zero.

14. The apparatus of claim 12 wherein the likelihood signal generating circuit comprises a subtractor, coupled to the time domain signal generator and the delayed time domain signal generator, the subtractor operating as an accumulator.

15. The apparatus of claim 3 wherein:

the physiological signal comprises first and second component signals; and

one of the plurality of likelihood circuits comprises;

a first circuit for generating a first plurality of time domain signals, each signal being the first component signal window filtered in the neighborhood of the frequency of a respective corresponding identified peak;

a second circuit for generating a second plurality of time domain signals, each signal being the second component signal window filtered in the neighborhood of the frequency of a respective corresponding identified peak; and

a circuit for generating a plurality of likelihood signals, each corresponding to a respective one of the identified peaks, representing the degree of congruence of the corresponding one of the first plurality of time domain signals, to the corresponding one of the second plurality of time domain signals.

16. The apparatus of claim 15 wherein the likelihood signal generating circuit comprises a subtractor, coupled to the first and second time domain signal generators, the subtractor operating as an accumulator.

17. The apparatus of claim 15 further comprising a scaling circuit coupled between the first and second time domain signal generating circuits and the likelihood signal generating circuit, for scaling the first and second window filtered component signals.

18. The apparatus of claim 15 wherein:

the frequency of each identified peak has an associated period; and

further comprising;

a delay circuit, coupled to the first and second time domain signal generators, for generating respective pluralities of time delayed signals respectively corresponding to the first and second pluralities of time domain signals delayed by respective time intervals equal to the period associated with the time domain signal.

19. The apparatus of claim 3 wherein one of the plurality of likelihood circuits comprises a spectra probability circuit, coupled to the peak identification circuit, for generating a plurality of signals, respectively representing the probability that the corresponding peak represents the physiological parameter.

20. The apparatus of claim 19 wherein the spectra probability circuit comprises:

circuitry, responsive to an accumulated probability density function representing the probability that the physiological parameter is represented by each frequency location in the spectrum of the physiological signal, for extracting the probability that the frequency location of each identified peak in the physiological signal represents the physiological parameter;

a deviation circuit, responsive to a plurality of previously estimated physiological parameters, for determining a degree of spread of the previously estimated physiological parameters; and

a calculating circuit, responsive to the degree of spread, for generating a new probability density function; and

a combining circuit for combining the new probability density function with the accumulated probability density function.

21. The apparatus of claim 20 further comprising circuitry, coupled to the combining circuit, for scaling the accumulated probability density function downward.

22. The apparatus of claim 1 wherein the physiological signal source comprises an SpO₂ sensor.

24. A method according to claim 23, including the step of bandpass filtering said received physiological signal and wherein said identifying step comprises identifying respective characteristics in said bandpass filtered received physiological signal.

23. A method for estimating a physiological parameter from a physiological signal, comprising the steps of:

receiving a physiological signal;

identifying respective characteristics in said received physiological signal;

producing a plurality of signals, respectively corresponding to said identified characteristics, representing a likelihood said corresponding identified characteristic represent said physiological parameter;

selecting one of said identified characteristics in response to said likelihood representative signals; and

estimating said physiological parameter using said selected one of said identified characteristics.