Method and apparatus for detecting a physiological parameter

• US 20020007114A1
• Filed: 02/06/2001
• Published: 01/17/2002
• Est. Priority Date: 02/10/2000
• Status: Active Grant

• Current Assignee: Dragerwerk AG & Company KGaA (FPS Vermögensverwaltung GmbH)
• Original Assignee: Draeger Medical Systems Incorporated (Dragerwerk AG)
• Inventors: Elghazzawi, Ziad
• Granted Patent: US 6,574,491 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/1455 using optical sensors, e.g. spectral photometrical oximeters
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

A system for detecting a physiological parameter from a physiological signal, includes a source of the physiological signal. Circuitry, coupled to the signal source, detects spectral peaks in the physiological signal. Calculating circuitry, coupled to the spectral peak detecting circuitry, calculates a parameter value corresponding to each detected spectral peak. Weighting circuitry, coupled to the calculating circuitry and the spectral peak detecting circuit, assigns a weight to each peak according to a feature of a signal and the parameter value corresponding to that peak. Circuitry, coupled to the weighting circuitry, selects the peak according to a predetermined criterion. Output circuitry, coupled to the selecting circuitry and the calculating circuitry, then generates the parameter value corresponding to the selected peak.

Claims

1. A system for detecting a physiological parameter from a physiological signal, comprising:

a source of the physiological signal;

circuitry, coupled to the signal source, for detecting spectral peaks in the physiological signal;

calculating circuitry, coupled to the spectral peak detecting circuitry, for calculating a parameter value corresponding to each detected spectral peak;

weighting circuitry, coupled to the calculating circuitry, for weighting each peak according to a signal feature and the parameter value corresponding to that peak;

circuitry, coupled to the weighting circuitry, for selecting a peak according to a predetermined criterion; and

output circuitry, coupled to the selecting circuitry and the calculating circuitry, for generating the parameter value corresponding to the selected peak.

2. The system of claim 1 further comprising a bandpass filter coupled between the signal source and the spectral peak detecting circuit.

3. The system of claim 1 wherein the spectral peak detecting circuitry comprises:

a transform circuit, coupled to the signal source, for transforming the physiological signal to the frequency domain; and

circuitry for detecting spectral peaks in the frequency domain transform of the physiological signal.

4. The system of claim 3 wherein the transform circuit comprises a Fourier transform circuit.

5. The system of claim 4 wherein:

the Fourier transform circuit comprises an FFT circuit; and

the spectral peak detecting circuitry comprises circuitry for calculating the magnitude of the FFT transform of the physiological signal, and a circuit for detecting peaks in the magnitude FFT transform.

6. The system of claim 1 wherein the weighting circuitry weights each peak according to respective signal features of one or more signals and the parameter value corresponding to that peak.

7. The system of claim 1 wherein the weighting circuitry weights each peak according to the magnitude of the detected spectral peak and the parameter value corresponding to that peak.

8. The system of claim 1 wherein the selecting circuitry selects the peak having the largest weight.

9. A system for detecting blood oxygen concentration (SpO₂) from a physiological signal, comprising:

a source of the physiological signal;

circuitry, coupled to the signal source, for detecting spectral peaks in the physiological signal;

calculating circuitry, coupled to the spectral peak detecting circuitry, for calculating the SpO₂ value corresponding to each detected spectral peak;

weighting circuitry, coupled to the calculating circuitry and the spectral peak detecting circuit, for weighting each peak according to the magnitude of the detected spectral peak and the SpO₂ value corresponding to that peak;

circuitry, coupled to the weighting circuitry, for selecting the peak having the largest weight; and

output circuitry, coupled to the selecting circuitry and the calculating circuitry, for generating the parameter value corresponding to the selected peak.

10. The system of claim 9 further comprising a bandpass filter coupled between the signal source and the detecting circuitry.

11. The system of claim 10 wherein the passband of the bandpass filter is between around 0.5 Hz and around 5 Hz.

12. The system of claim 9 wherein the signal source comprises a source of a red signal representing received red light and a source of an IR signal representing received IR light.

13. The system of claim 12 wherein:

the spectral peak detecting circuitry comprises a Fourier transform circuit which generates transformed red and IR signals corresponding to the red and IR signals, respectively; and

the detecting circuitry detects peaks in the transformed IR signal.

14. The system of claim 13 wherein:

the Fourier transform circuit comprises an FFT circuit; and

the peak detecting circuit further comprises a circuit for generating the magnitude transform of the IR signal, and detecting peaks in the magnitude transform of the IR signal.

15. The system of claim 13 wherein the calculating circuitry comprises:

circuitry for calculating a ratio R value according to the equation;

$\begin{array}{cc}R=\frac{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACred}+\mathrm{DCred}}{\mathrm{DCred}}}{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACir}+\mathrm{DCir}}{\mathrm{DCir}}}& \left(1\right)\end{array}$

wherein ACred represents the magnitude of the transformed red signal at each peak, DCred represents the DC magnitude of transformed red signal, ACir represents the magnitude of the transformed IR signal at each peak, and DCir represents the DC magnitude of the IR signal.; and

a lookup table for looking up the SpO₂ value according to the ratio R.

16. The system of claim 13 wherein the weighting circuitry comprises circuitry for calculating the weight W according to the equation:

W=(SpO₂)²·

ACir

wherein ACir represents the magnitude of the transformed IR signal at each peak.

17. The system of claim 9 wherein each detected peak is further associated with a frequency, and the system further comprises:

circuitry for determining if the frequency associated with the selected peak is within a predetermined frequency range and if the frequency associated with the selected peak is within the predetermined frequency range, then;

checking if the transformed signal at one-half the frequency associated with the selected peak is also a peak, and if the signal at one-half the frequency associated with the selected peak is also a peak, then;

comparing the respective magnitudes associated with the selected peak and the peak at one-half the frequency associated with the selected peak, and

if the magnitude of the peak at one-half the frequency associated with the selected peak is greater the magnitude of the selected peak by a predetermined factor, and the SpO₂ value associated with the peak at one-half the frequency associated with the selected peak is greater than the SpO₂ value associated with the selected peak by a predetermined amount, then selecting the peak at one-half the frequency of the selected peak.

18. The system of claim 17 wherein the predetermined factor is substantially two.

19. The system of claim 17 wherein the SpO₂ value is expressed at a percentage;

and the predetermined amount is substantially 2%.

20. The system of claim 17 wherein the predetermined frequency range is from around 0.5 Hz to around 1.4 Hz.

21. The system of claim 9 comprised in a computer system, operating under control of a control program.

22. A method for detecting a physiological parameter from a physiological signal, comprising the steps of:

receiving the physiological signal representing the parameter;

detecting spectral peaks in the physiological signal;

calculating the physiological parameter corresponding to each peak;

weighting each peak according to a signal feature and the physiological parameter corresponding to that peak;

selecting a peak according to a predetermined criterion; and

generating the physiological parameter corresponding to the selected peak.

23. The method of claim 22 further comprising, before peak detecting step, the step of limiting the bandwidth of the physiological signal.

24. The method of claim 22 wherein the peak detecting step comprises the steps of:

calculating the Fourier transform of the physiological signal; and

detecting peaks in the transform of the physiological signal.

25. The method of claim 24 wherein the peak detecting step further comprises the step of calculating the magnitude Fourier transform of the physiological signal.

26. The method of claim 22 wherein the weighting step comprises the step of weighting each peak according to respective features of one or more signals and the physiological parameter corresponding to the peak.

27. The method of claim 26 wherein the weighting step comprises the step of weighting each peak according to the magnitude of the peak and the physiological parameter corresponding to the peak.

28. The method of claim 22 wherein the selecting step comprises the step of selecting the peak having the largest weight.

29. A method for detecting blood oxygen concentration (SpO₂) from a physiological signal, comprising the steps of:

receiving the physiological signal representing the SpO₂ value;

detecting spectral peaks in the physiological signal;

calculating the SpO₂ value corresponding to each peak;

weighting each peak according to magnitude of the peak and the SpO₂ value corresponding to that peak;

selecting the peak with the highest weight; and

generating the SpO₂ value corresponding to the selected peak.

30. The method of claim 29 further comprising, before the detecting step, the step of limiting the bandwidth of the physiological signal.

31. The method of claim 30 wherein the limiting step comprises the step of limiting the bandwidth to from around 0.5 Hz to around 5 Hz.

32. The method of claim 29 wherein:

the receiving step comprises the step of receiving a red signal representing received red light and an IR signal representing received IR light;

the transforming step comprises the step of transforming the red and IR signals to transformed red and IR signals, respectively; and

the detecting step comprises the step of detecting peaks in the transformed IR signal.

33. The method of claim 32 wherein the transforming step further comprises the step of transforming the red and IR signals to magnitude transformed red and IR signals, respectively.

34. The method of claim 32 wherein the transformed physiological signal has a DC magnitude, and the calculating step comprises the steps of:

calculating a ratio R value according to the equation;

$\begin{array}{cc}R=\frac{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACred}+\mathrm{DCred}}{\mathrm{DCred}}}{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACir}+\mathrm{DCir}}{\mathrm{DCir}}}& \left(1\right)\end{array}$

wherein ACred represents the magnitude of the transformed red signal at each peak, DCred represents the DC magnitude of transformed red signal, ACir represents the magnitude of the transformed IR signal at each peak, and DCir represents the DC magnitude of the IR signal.; and

looking up the SpO₂ value from a lookup table according to the ratio R.

35. The method of claim 32 wherein the weighting step comprises the step of calculating the weight W according to the equation:

W=(SpO₂)²·

ACir

wherein ACir represents the magnitude of the transformed IR signal at each peak.

36. The method of claim 29 wherein each detected peak is further associated with a frequency, and the method further comprises, after the selecting step, the steps of:

determining if the frequency associated with the selected peak is within a predetermined frequency range and if the frequency associated with the selected peak is within the predetermined frequency range, then performing the steps of;

checking if the transformed signal at one-half the frequency associated with the selected peak is also a peak and if the signal at one-half the frequency associated with the selected peak is also a peak, then performing the steps of;

comparing the respective magnitudes associated with the selected peak and the peak at one-half the frequency associated with the selected peak; and

if the magnitude of the peak at one-half the frequency associated with the selected peak is greater than a predetermined factor times the magnitude of the selected peak, and the SpO₂ value associated with the peak at one-half the frequency associated with the selected peak is greater than the SpO₂ value associated with the selected peak by a predetermined amount, then selecting the peak at one-half the frequency of the selected peak.

37. The method of claim 36 wherein the predetermined factor is substantially two.

38. The method of claim 36 wherein the SpO₂ value is expressed at a percentage;

and the predetermined amount is substantially 2%.

39. The method of claim 36 wherein the predetermined frequency range is from about 0.5 Hz to about 1.4 Hz.