Method and apparatus for detecting a physiological parameter

• US 6,574,491 B2
• Filed: 02/06/2001
• Issued: 06/03/2003
• Est. Priority Date: 02/10/2000
• Status: Expired due to Term

• Current Assignee: Dragerwerk AG & Company KGaA (FPS Vermögensverwaltung GmbH)
• Original Assignee: Siemens Medical Systems, Inc. (Siemens AG)
• Inventors: Elghazzawi, Ziad F.
• Primary Examiner(s): Winakur, Eric F.
• Assistant Examiner(s): KREMER, MATTHEW J
• Application Number:
  US09/777,546
• Publication Number:
  US 20020007114A1
• Time in Patent Office: 847 Days
• Field of Search: 600/322-326, 600/330, 600/336
• 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 derived from patient blood, comprising:

a source of the physiological signal;

a pre-processor for detecting peaks from an infra-red frequency domain transform representation of a physiological signal;

a weighting processor for weighting an individual peak of said detected peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak;

a peak selector for selecting a weighted peak according to a predetermined criterion; and

a computational processor for generating a parameter value corresponding to the selected weighted peak.

2. The system of claim 1 further comprising a bandpass filter coupled between the physiological signal source and the pre-processor.

3. The system of claim 1 wherein said weighting processor weights an individual peak of said detected peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak, and exclusive of other factors.

4. The system of claim 1 wherein said weighting processor weights an individual peak of said detected peaks according to a function of both, (a) the infra-red frequency domain transform representation value corresponding to said individual peak and (b) the blood oxygen saturation representative value corresponding to said individual peak.

5. The system of claim 1 wherein said pre-processor exclusively detects peaks from an infra-red frequency domain transform representation of the physiological signal independently of an associated red light frequency domain transform representation of the physiological signal.

6. The system of claim 1 wherein said infra-red frequency domain transform representation value corresponding to said individual peak comprises at least one of, (a) a value representing a magnitude of the infra-red frequency domain transform corresponding to said individual peak, (b) a value representing an amplitude of the infra-red frequency domain transform corresponding to said individual peak and (c) a peak infra-red frequency domain transform value of said individual peak.

7. The system of claim 1 wherein the weighting processor weights said individual peak according to both the magnitude of the infra-red frequency domain transform corresponding to said individual peak and the blood oxygen saturation representative value corresponding to said individual peak.

8. The system of claim 1 wherein the peak selector selects a weighted peak having the largest weight.

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

a source of the physiological signal;

a pre-processor for detecting peaks of an infra-red frequency domain transform representation of said physiological signal exclusive of peaks in a red frequency domain transform corresponding to said physiological signal;

a processor for calculating a blood oxygen concentration representative value corresponding to detected spectral peaks in said infra-red frequency domain transform representation;

a weighting processor for weighting an individual peak of said detected spectral peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak;

a peak selector for selecting a weighted peak having the largest weight; and

an output processor for providing a blood oxygen concentration representative value corresponding to the selected weighted peak.

10. The system of claim 9 further comprising a bandpass filter coupled between the signal source and the pre-processor.

11. The system of claim 10 wherein the passband of the bandpass filter is substantially 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 infra-red signal representing received infra-red light.

13. The system of claim 12 wherein:

said pre-processor for detecting peaks of an infra-red frequency domain transform representation of a physiological signal includes a Fourier transform processor which generates transformed red and infra-red signals.

14. The system of claim 13 wherein:

the Fourier transform processor comprises an FFT circuit.

15. The system of claim 13 wherein the processor for calculating a blood oxygen concentration representative value comprises:

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

$R=\frac{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACred}+\mathrm{DCred}}{\mathrm{DCred}}}{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACir}+\mathrm{DCir}}{\mathrm{DCir}}}$

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 processor weights an individual peak of said detected spectral peaks 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:

a processor 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 than 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 as a percentage;

and the predetermined amount is substantially 2%.

20. The system of claim 17 wherein the predetermined frequency range is from substantially between 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 derived from patient blood, comprising the steps of:

receiving the physiological signal representing the parameter;

detecting peaks from an infra-red frequency domain transform representation of the physiological signal;

weighting an individual peak of said detected peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak;

selecting a weighted peak according to a predetermined criterion; and

generating a physiological parameter corresponding to the selected weighted peak.

23. The method of claim 22 further including the step of limiting the bandwidth of the physiological signal.

24. The method of claim 22 wherein said step of detecting peaks comprises exclusively detecting peaks from an infra-red frequency domain transform representation of the physiological signal independently of an associated red light frequency domain transform representation of the physiological signal.

25. The method of claim 22 wherein said weighting step comprises weighting an individual peak of said detected peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak, and exclusive of other factors.

26. The method of claim 22 wherein said infra-red frequency domain transform representation value corresponding to said individual peak comprises at least one of, (a) a value representing a magnitude of the infra-red frequency domain transform corresponding to said individual peak, (b) a value representing an amplitude of the infra-red frequency domain transform corresponding to said individual peak and (c) a peak infra-red frequency domain transform value of said individual peak.

27. The method of claim 26 wherein the weighting step comprises the step of weighting said individual peak according to both the magnitude of the infrared frequency domain transform corresponding to said individual peak and the blood oxygen saturation representation value corresponding to said individual peak.

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

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

receiving the physiological signal representing the SpO₂ value;

detecting peaks of an infra-red frequency domain transform representation of said physiological signal exclusive of peaks in a red frequency domain transform corresponding to said physiological signal;

calculating the SpO₂ representative value corresponding to detected spectral peaks;

weighting an individual peak of said detected spectral peaks according to at least one of (a) an infra-red frequency domain transform representation value corresponding to said individual peak and (b) a function of a blood oxygen saturation representative value corresponding to said individual peak;

selecting a weighted peak with the highest weight; and

generating the SpO₂ representative value corresponding to the selected weighted peak.

30. The method of claim 29 further including 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 substantially between around 0.5 Hz to around 5 Hz.

32. The method of claim 29 further including a transforming step comprising the step of transforming the red and IR signals to transformed red and IR signals, respectively;

and

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

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

33. 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;

$R=\frac{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACred}+\mathrm{DCred}}{\mathrm{DCred}}}{\log <br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\mathrm{ACir}+\mathrm{DCir}}{\mathrm{DCir}}}$

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 representative value from a lookup table according to the ratio R.

34. The method of claim 32 wherein the weighting step comprises the step of weighting an individual peak of said detected spectral peak according to the equation:

35. 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.

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 representative value is expressed as a percentage;

and the predetermined amount is substantially 2%.

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