Method and apparatus for demodulating signals in a pulse oximetry system

• US 8,150,487 B2
• Filed: 05/18/2007
• Issued: 04/03/2012
• Est. Priority Date: 04/14/1997
• Status: Expired due to Term

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Diab, Mohamed K., Weber, Walter M., Al-Ali, Ammar
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US11/750,930
• Publication Number:
  US 20070225582A1
• Time in Patent Office: 1,782 Days
• Field of Search: 600/310, 600/322, 600/323, 600/336
• US Class Current: 600/310
• 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 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;

adaptively 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;

and

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

2. The method of claim 1, further comprising reducing a sampling rate of said first and second intensity signals.

3. The method of claim 2, further comprising reducing a sample rate of said composite signal before demodulating said composite signal.

4. The method of claim 2, wherein reducing a sample rate comprises decimating.

5. The method of claim 1, wherein adaptively demodulating comprises adjusting demodulation parameters during operation.

6. The method of claim 5, wherein the demodulation parameters comprise a decimation rate.

7. The method of claim 1, wherein adaptively demodulating comprises using feedback.

8. The method of claim 7, wherein adaptively demodulating using feedback comprises using feedback from previously demodulated signals.

9. The method of claim 1, further comprising low pass filtering and sample rate compressing the composite signal.

10. The method of claim 1, further comprising low pass filtering and sample rate compressing the first and second intensity signals.

11. 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 demodulation module configured to adaptively demodulate 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;

and

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

12. The system of claim 11 further comprising a first reducing module configured to reduce a sample rate of said composite signal.

13. The system of claim 12, wherein said first reducing module is configured to decimate said composite output signal.

14. The system of claim 12 further comprising a second reducing module configured to reduce a sample rate of said first and second intensity signals.

15. The system of claim 14, wherein said second reducing module is configured to decimate said first and second intensity signals.

16. The system of claim 11, wherein the demodulation module adaptively demodulates by adjusting demodulation parameters during operation.

17. The system of claim 16, wherein the demodulation parameters comprise a decimation rate.

18. The system of claim 11, wherein the demodulation module adaptively demodulates by using feedback.

19. The system of claim 18, wherein the demodulation module adaptively demodulates by using feedback from previously demodulated signals.

20. The system of claim 11, further comprising a low pass filter module and a sample rate compression module.