Extraction Of Physiological Measurements From A Photoplethysmograph (PPG) Signal

• US 20100087720A1
• Filed: 10/02/2008
• Published: 04/08/2010
• Est. Priority Date: 10/02/2008
• Status: Active Grant

• Current Assignee: Nellcor Puritan Bennett Ireland
• Original Assignee: Nellcor Puritan Bennett Ireland
• Inventors: Addison, Paul Stanley
• Granted Patent: US 9,078,609 B2
• Time in Patent Office: Days
• US Class Current: 600/345
• CPC Class Codes: A61B 5/0205 Simultaneously evaluating both cardiovascular conditions and different types of body conditions, e.g. heart and respiratory condition
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• 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 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives

Abstract

The present disclosure relates to signal processing and, more particularly, to determining the value of a physiological parameter, such as the blood oxygen saturation (SpO₂) of a subject. In an embodiment, a first baseline for a first waveform and a second baseline for a second waveform are determined. In this embodiment, the first and second waveforms are indicative of the physiological parameter of the subject. The first and second waveforms are filtered to obtain a first direct-current (DC) component for the first waveform and a second DC component for the second waveform. A measured value for the physiological parameter is derived from the first and second baseline signals, and the first and second DC components. In an embodiment, the measured value for the physiological parameter is determined based on a ratio of the normalized difference between the DC component and the baseline signal for the first waveform with respect to same for the second waveform.

Claims

1. A method for measuring the value of a physiological parameter of a subject, the method comprising:

determining a first baseline for a first waveform and a second baseline for a second waveform, the first and second waveforms being indicative of the physiological parameter of the subject;

filtering the first and second waveforms to obtain a first direct-current (DC) component for the first waveform and a second DC component for the second waveform;

calculating a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each waveform is based at least in part upon a difference between the DC component and baseline signal of the waveform; and

deriving a measured value for the physiological parameter from the first and second difference signals.

2. The method of claim 1, further comprising calculating a first normalized difference signal A*_R for the first waveform and a second normalized difference signal A*_IR for the second waveform, based on the difference signals A_R and A_IR, respectively.

3. The method of claim 2, wherein calculating a normalized difference signal for a waveform comprises dividing the difference signal for the waveform by the DC component of the waveform.

4. The method of claim 2, wherein calculating a normalized difference signal for a waveform comprises dividing the difference signal for the waveform by the baseline signal for the waveform.

5. The method of claim 2, wherein deriving a measured value for the physiological parameter comprises deriving a measured value for the physiological parameter from a ratio of the first normalized difference signal A*_R with respective to the second normalized difference signal A*_IR.

6. The method of claim 5, wherein deriving a measured value for the physiological parameter comprises deriving a measured value from a lookup table based on the ratio of the normalized difference signals.

7. The method of claim 1, wherein determining a baseline signal for a waveform comprises determining a best-fit line through the trough points of the waveform.

8. The method of claim 1, wherein the physiological parameter is indicative of the blood oxygen saturation of the subject.

9. The method of claim 1 further comprising generating an alarm if the measured value for the physiological parameter meets one or more predetermined conditions.

10. A method for measuring the blood oxygen saturation of a subject, the method comprising:

receiving a first waveform (R) corresponding to a red light and a second waveform (IR) corresponding to an infrared light the waveforms being indicative of the blood oxygen saturation of the subject;

determining a first baseline B_R for the first waveform and a second baseline and B_IR for the second waveform;

filtering the first and second waveforms to obtain a first direct-current (DC) component D_R for the first waveform and second DC component D_IR for the second waveform;

obtaining a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each of the first and second waveforms corresponds to a difference between the DC component and baseline signal of the waveform;

calculating a first normalized difference signal A*_R for the first waveform and a second normalized difference signal A*_IR for the second waveform; and

deriving a measured value for the physiological parameter from a ratio of the first normalized difference signal A*_R with respective to the second normalized difference signal A*_IR.

11. A system for measuring the value of a physiological parameter of a subject, the system comprising a computer processor that:

determines a first baseline for a first waveform and a second baseline for a second waveform, the first and second waveforms being indicative of the physiological parameter of the subject;

filters the first and second waveforms to obtain a first direct-current (DC) component for the first waveform and a second DC component for the second waveform;

calculates a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each waveform is based at least in part upon a difference between the DC component and baseline signal of the waveform; and

derives a measured value for the physiological parameter from the first and second difference signals.

12. The system of claim 11, wherein the processor further calculates a first normalized difference signal A*_R for the first waveform and a second normalized difference signal A*_IR for the second waveform, based on the difference signals A_R and A_IR, respectively.

13. The system of claim 12, wherein the processor calculates a normalized difference signal for the waveform by dividing the difference signal for the waveform by the DC component of the waveform.

14. The system of claim 12, wherein the processor calculates a normalized difference signal for the waveform by dividing the difference signal for the waveform by the baseline signal for the waveform.

15. The system of claim 12, wherein the processor derives a measured value for the physiological parameter by deriving a measured value for the physiological parameter from a ratio of the first normalized difference signal A*_R with respective to the second normalized difference signal A*_IR.

16. The system of claim 15, wherein the processor derives a measured value for the physiological parameter by deriving a measured value from a lookup table based on the ratio of the normalized difference signals.

17. The system of claim 11, wherein the processor determines a baseline signal for a waveform by determining a best-fit line through the trough points of the waveform.

18. The system of claim 11, wherein the physiological parameter is indicative of the blood oxygen saturation of the subject.

19. The system of claim 11, wherein the processor further generates an alarm if the measured value for the physiological parameter meets one or more predetermined conditions.

20. A system for measuring the blood oxygen saturation of a subject, the system comprising a computer processor that:

receives a first waveform (R) corresponding to a red light and a second waveform (IR) corresponding to an infrared light, the waveforms being indicative of the blood oxygen saturation of the subject;

determines a first baseline B_R for the first waveform and a second baseline and B_IR for the second waveform;

filters the first and second waveforms to obtain a first direct-current (IC) component D_R for the first waveform and second DC component D_IR for the second waveform;

obtains a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each of the first and second waveforms corresponds to a difference between the DC component and baseline signal of the waveform;

calculates a first normalized difference signal A*_R for the first waveform and a second normalized difference signal A*_IR for the second waveform; and

derives a measured value for the physiological parameter from a ratio of the first normalized difference signal A*_R with respective to the second normalized difference signal A*_IR.

21. Computer-readable medium for use in measuring the value of a physiological parameter of a subject, the computer-readable medium having computer program instructions recorded thereon for:

determining a first baseline for a first waveform and a second baseline for a second waveform, the first and second waveforms being indicative of the physiological parameter of the subject;

filtering the first and second waveforms to obtain a first direct-current (DC) component for the first waveform and a second DC component for the second waveform;

calculating a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each waveform is based at least in part upon a difference between the DC component and baseline signal of the waveform; and

deriving a measured value for the physiological parameter from the first and second difference signals.

22. Computer-readable medium for use in measuring the blood oxygen saturation of a subject the computer-readable medium having computer program instructions recorded thereon for:

receiving a first waveform (R) corresponding to a red light and a second waveform (IR) corresponding to an infrared light, the waveforms being indicative of the blood oxygen saturation of the subject;

determining a first baseline B_R for the first waveform and a second baseline and B_IR for the second waveform;

filtering the first and second waveforms to obtain a first direct-current (DC) component D_R for the first waveform and second DC component D_IR for the second waveform;

obtaining a first difference signal A_R for the first waveform and a second difference signal A_IR for the second waveform, wherein the difference signal for each of the first and second waveforms corresponds to a difference between the DC component and baseline signal of the waveform;

calculating a first normalized difference signal A*_R for the first waveform and a second normalized difference signal A*_IR for the second waveform; and

deriving a measured value for the physiological parameter from a ratio of the first normalized difference signal A*_R with respective to the second normalized difference signal A*_IR.