METHOD AND APPARATUS FOR DEMODULATING SIGNALS IN A PULSE OXIMETRY SYSTEM

• US 20070225582A1
• Filed: 05/18/2007
• Published: 09/27/2007
• Est. Priority Date: 04/14/1997
• Status: Active Grant

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Al-Ali, Ammar, Diab, Mohamed, Weber, Walter
• Granted Patent: US 8,150,487 B2
• Time in Patent Office: Days
• US Class Current: 600/336
• CPC Class Codes: A61B 2560/0242 adapted to measure environmental factors, e.g. temperature, pollution
• CPC Class Codes: A61B 2562/0238 Optical sensor arrangements for performing transmission measurements on body tissue
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7217 of noise originating from a therapeutic or surgical apparatus, e.g. from a pacemaker
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: A61B 5/7232 involving compression of the physiological signal, e.g. to extend the signal recording period
• CPC Class Codes: H04J 14/08 Time-division multiplex systems
• CPC Class Codes: H04J 3/10 Arrangements for reducing cross-talk between channels

Abstract

A method and an apparatus measure blood oxygenation in a subject. A light source is activated to cause a first emission at a first wavelength and a second emission at a second wavelength. A detector detects a composite signal indicative of an attenuation of the first and second wavelengths by tissue of a patient. The composite signal is demodulated into a first intensity signal and a second intensity signal. Blood oxygenation in the subject is determined from the first and second intensity signals. In one embodiment, demodulation is based at least in part on a period when at least one of the wavelengths is not activated. In one embodiment, a modulation of the first and second wavelengths is determined in order to avoid frequencies of ambient noise. In one embodiment, the composite signal'"'"'s sampling rate is reduced before and/or after demodulation.

Claims

1. A method of determining an indication of a physiological parameter by demodulating a composite output signal from a detector of a noninvasive optical sensor, the method comprising:

activating at least one light source to cause a first emission at a first wavelength;

activating the at least one light source to cause a second emission at a second wavelength at least once before a reactivation of the at least one light source;

wherein activating includes inactivating the at least one light source to cause no signal emissions;

detecting the composite output signal indicative of an attenuation by said body tissue of said first and second emissions;

demodulating said composite output signal into a first intensity signal responsive to said attenuation of said first emission, into a second intensity signal responsive to said attenuation of said second emission and into a third signal representative of a non-physiological portion of the output signal, wherein said demodulation is based at least in part on the inactivation; and

determining an indication of a physiological parameter of said body tissue based on one or more of said first and second intensity signals.

2. The method of claim 1, wherein demodulating further comprises using a demodulation constraint.

3. The method of claim 2, wherein using a demodulation constraint further comprises adjusting said demodulation constraint based at least in part on the output signal.

4. The method of claim 3, wherein adjusting said demodulation constraint based at least in part on the output signal further comprises using feedback.

5. A system for determining an indication of a physiological parameter by demodulating a composite output signal from a detector of a noninvasive oximetry optical sensor, the system comprising:

an input configured to receive a composite signal indicative of an attenuation by body tissue of a first and second emission, said composite signal including at least one inactivation of said first and second emissions; and

a processor configured to demodulate said composite signal into a first intensity signal responsive to said attenuation of said first emission and into a second intensity signal responsive to said attenuation of said second emission and into a third signal representative of a non-physiological portion of the output signal, wherein said demodulation is based at least in part on the inactivation;

wherein said processor is further configured to determine an indication of a physiological parameter of said body tissue based on one or more of said first and second intensity signals.

6. The system of claim 5, wherein said processor is further configured to determine a demodulation constraint and used said demodulation constraint to demodulate said composite signal.

7. The method of claim 6, wherein said demodulation constraint is adjusted based at least in part on the output signal.

8. The method of claim 7, wherein said demodulation constraint is adjusted using feedback.

9. A method of determining an indication of a physiological parameter by demodulating a composite output signal from a detector of a noninvasive optical sensor, the method comprising:

receiving a composite output signal indicative of an attenuation by a body tissue of a first and second emission of a light source, said composite signal including at least one inactivation of said first and second emissions;

demodulating said composite output signal into a first intensity signal responsive to said attenuation of said first emission, into a second intensity signal responsive to said attenuation of said second emission and into a third signal representative of a non-physiological portion of the output signal, wherein said demodulation is based at least in part on the inactivation; and

determining an indication of a physiological parameter of said body tissue based on one or more of said first and second intensity signals.

10. The method of claim 9, wherein demodulating further comprises using a demodulation constraint.

11. The method of claim 10, wherein using a demodulation constraint comprises determining a demodulation coefficient which reduces crosstalk.

12. The method of claim 9, wherein demodulating further comprises demodulating a predetermined number of harmonics of said first and second emissions.

13. A method of determining an activation frequency for two or more emissions of different wavelengths in order to minimize effects of ambient noise in a physiological parameter calculation, the method comprising:

determining frequencies of ambient noise;

determining an activation frequency for activating a light source to cause a plurality of first emissions at a first wavelength and a plurality of second emissions at a second wavelength;

wherein the activation frequency is determined to substantially avoid the frequencies of the ambient noise

activating said light source in proximity to tissue;

detecting a composite signal representative of an attenuation of said first and second emissions by tissue;

determining a physiological measurement responsive to the composite signal.

14. The method of claim 13, wherein determining frequencies of the ambient noise further comprises determining at least one harmonic frequency of the ambient noise.

15. The method of claim 13, wherein said noise comprises light noise.

16. The method of claim 15, wherein determining frequencies of the ambient light noise further comprises determining at least one harmonic frequency of the ambient light noise.

17. The method of claim 13, further comprising demodulating said composite signal into a first intensity signal representative of said attenuation of said first emission and a second intensity signal representative of said attenuation of second signal.

18. A method of demodulating a detected light signal representative of light attenuated by tissue of a patient, the method comprising:

receiving a composite signal representative of an attenuation of a first and second emissions by body tissue;

demodulating said composite signal into at least a first intensity signal representative of said attenuation of said first emission by body tissue and a second intensity signal representative of said attenuation of said second emission by body tissue;

reducing a sample rate of said first and second intensity signals; and

determining a physiological measurement based on the first and second intensity signals.

19. The method of claim 18, further comprising reducing a sample rate of said composite signal before demodulating said composite signal.

20. The method of claim 18, wherein reducing a sample rate comprises decimating.

21. A system for demodulating a detected light signal representative of light attenuated by tissue of a patient, the system comprising:

an input for receiving a composite signal representative of an attenuation of a first and second emissions by body tissue;

a first reducing module configured to reduce a sample rate of said composite signal;

a demodulation module configured to demodulate said reduced sample rate composite signal into at least a first intensity signal representative of said attenuation of said first emission by body tissue and a second intensity signal representative of said attenuation of said second emission by body tissue;

a second reducing module configured to reduce a sample rate of said first and second intensity signals; and

a processor configured to determine and output a physiological measurement based on the first and second intensity signals.

22. The system of claim 21, wherein said first reducing module is configured to decimate said composite output signal.

23. The system of claim 21, wherein said second reducing module is configured to decimate said first and second intensity signals.