Reduced cross talk pulse oximeter

• US 6,778,923 B2
• Filed: 05/15/2002
• Issued: 08/17/2004
• Est. Priority Date: 11/15/2000
• Status: Active Grant

• Current Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Original Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Inventors: Norris, Mark A., Hanna, D. Alan
• Primary Examiner(s): Nghiem, Michael
• Application Number:
  US10/147,452
• Publication Number:
  US 20030028357A1
• Time in Patent Office: 825 Days
• Field of Search: 702/74, 702/76, 702/79, 702/69, 600/309, 600/310, 600/386, 275/316, 275/346
• US Class Current: 702/74
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• 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

Abstract

A system and method of reducing cross talk in pulse oximetry signals that are attenuated by a patient tissue site are provided. In one embodiment, Red and IR LEDs of a pulse oximeter are separately excited (1010) and the respective Red and IR data vectors output by the detector are measured (1020). The Red and IR data vectors are normalized (1030). Red to IR and IR to Red cross talk vectors are computed (1040). Red and IR demodulation vectors are obtained by subtracting (1050) the respective Red and IR cross talk vectors from the respective normalized Red and IR data vectors. The demodulation vectors are normalized (1060), scaled (1070), and the magnitudes of the Red and IR signal components are obtained by computing (1080) the dot product of the composite signal data vector with the normalized and scaled Red demodulation and IR demodulation vectors, respectively.

Claims

1. A method for demodulating at least first and second attenuated signal components within a composite signal output by at least one detector of a pulse oximeter, the first and second attenuated signal components corresponding to first and second multiplexed signals that are emitted by first and second optical signal sources of the pulse oximeter, attenuated by a patient tissue site and received by the detector, said method comprising the steps of:

generating a first demodulation vector for demodulating the first attenuated signal component from the composite signal, the first demodulation vector being orthogonal to the second attenuated signal component of the composite signal;

generating a second demodulation vector for demodulating the second attenuated signal component from the composite signal, the second demodulation vector being substantially orthogonal to the first attenuated signal component of the composite signal;

demodulating the composite signal with the first demodulation vector to obtain a magnitude of the first attenuated signal component; and

demodulating the composite signal with the second demodulation vector to obtain a magnitude of the second attenuated signal component.

2. The method of claim 1 wherein said steps of generating a first demodulation vector and generating a second demodulation vector are performed prior to using the pulse oximeter and wherein the first and second demodulation vectors remain fixed while the pulse oximeter is used to monitor a patient.

3. The method of claim 1 wherein said steps of generating a first demodulation vector and generating a second demodulation vector are performed prior to using the pulse oximeter to monitor a patient and wherein said method further comprises the step of:

adjusting the first and second demodulation vectors dynamically while the pulse oximeter is used.

4. The method of claim 3 wherein said step of adjusting the first and second demodulation vectors comprises applying at least one correction factor dynamically computed from information included in at least the first and second attenuated signal components of the composite signal.

5. The method of claim 4 wherein said first and second multiplexed signals comprise Red and IR signals and said first and second signal components are Red and IR signal components, and wherein said at least one correction factor is selected from a group consisting of a Red signal to IR signal component cross talk correction factor, an IR signal to Red signal component cross talk correction factor, a Red optical signal source capacitive coupling to Red signal component correction factor, a Red optical signal source capacitive coupling to IR signal component correction factor, an IR optical signal source capacitive coupling to IR signal component correction factor, and an IR optical signal source capacitive coupling to Red signal component correction factor.

6. The method of claim 1 wherein the composite signal comprises a digital signal, wherein the first and second demodulation vectors comprise digital signals, and wherein said steps of demodulating the composite signal with the first demodulation vector and demodulating the composite signal with the second demodulation vector are performed in a digital processor.

7. The method of claim 1 wherein said steps of generating a first and a second demodulation vector comprise the steps of:

operating only the first optical signal source of the pulse oximeter;

recording a first data vector output from the detector when only the first optical signal source is operated;

operating only the second optical signal source of the pulse oximeter;

recording a second data vector output from the detector when only the second optical signal source is operated;

computing a first scalar value corresponding to cross talk from operation of the second optical signal source into the first data vector;

computing a second scalar value corresponding to cross talk from operation of the first optical signal source into the second data vector;

forming a first correction vector having a direction opposite the direction of the first data vector and a magnitude given by the first scalar value;

forming a second correction vector having a direction opposite the direction of the second data vector and a magnitude given by the second scalar value;

subtracting the first correction vector from the first data vector to obtain the first demodulation vector; and

subtracting the second correction vector from the second data vector to obtain the second demodulation vector.

8. The method of claim 7 wherein said step of computing a first scalar value comprises computing a dot product of the first data vector and the second data vector, and wherein said step of computing a second scalar value comprises computing a dot product of the second data vector and the first data vector.

9. The method of claim 1 wherein the first and second multiplexed signals are code division multiplexed signals.

10. The method of claim 1 wherein said steps of generating a first and a second demodulation vector comprises the steps of:

obtaining data output by the detector for a plurality of pulse oximetry probes operated under different drive signal and transmitted light signal levels; and

applying a regression technique to the obtained data to derive the first and second demodulation waveforms.

11. The method of claim 1 wherein said steps of generating a first and a second demodulation vector comprises the steps of:

obtaining data output by the detector for a plurality of pulse oximetry probes operated under different drive signal and transmitted light signal levels; and

applying a singular value decomposition technique to the obtained data to derive the first and second demodulation waveforms.

12. A method of correcting for undesired non-orthogonal signal components and interferences in a composite pulse oximetry signal output by at least one detector of a pulse oximeter, said method comprising the steps of:

demodulating the composite pulse oximetry signal using a first matched filter corresponding to a first signal component present in the composite pulse oximetry signal that is associated with a first wavelength light signal attenuated by a patient tissue site and a second matched filter corresponding to a second signal component present in the composite pulse oximetry signal that is associated with a second wavelength light signal attenuated by a patient tissue site to obtain at least first and second uncorrected demodulated signal components corresponding to the first and second signal components;

demodulating the composite pulse oximetry signal using a third matched filter corresponding to a first interference present in the composite pulse oximetry signal and a fourth matched filter corresponding to a second interference present in the composite pulse oximetry signal to obtain at least first and second demodulated interferences; and

subtracting the second uncorrected demodulated signal component and the first and second demodulated interferences from the first uncorrected demodulated signal component to obtain a first corrected demodulated signal component corresponding to the first signal component.

13. The method of claim 12 wherein the second signal component is associated with an R wavelength light signal attenuated by a patient tissue site, the first interference comprises capacitive coupling from a first wavelength optical signal source into the first signal component and the second interference comprises capacitive coupling from a second wavelength optical signal source into the first signal component.

14. The method of claim 12 further comprising:

demodulating the composite pulse oximetry signal using a fifth matched filter corresponding to a third interference present in the composite pulse oximetry signal and a sixth matched filter corresponding to a fourth interference present in the composite pulse oximetry signal to obtain at least third and fourth demodulated interferences; and

subtracting the first uncorrected demodulated signal component and the third and fourth demodulated interferences from the second uncorrected demodulated signal component to obtain a second corrected demodulated signal component corresponding to the second signal component.

15. The method of claim 14 wherein in the first interference comprises capacitive coupling from a first wavelength optical signal source into the first signal component, the second interference comprises capacitive coupling from a second wavelength optical signal source into the first signal component, the third interference comprises capacitive coupling from the first wavelength optical signal source into the second signal component, and the fourth interference comprises capacitive coupling from the second wavelength optical signal source into the second signal component.

16. The method of claim 12 wherein the composite pulse oximetry signal comprises a digitized signal having a plurality of sample instances and wherein said demodulating and subtracting steps are performed by a digital processor for each sample instance of the digitized composite pulse oximetry signal.

17. The method of claim 12 wherein the first wavelength light signal comprises a Red wavelength light signal and wherein the second wavelength light signal comprises an IR wavelength light signal.

18. A reduced cross talk pulse oximetry system comprising:

at least first and second optical signal sources operable to transmit first and second multiplexed signals;

at least one detector operable to detect the first and second multiplexed signals after the first and second multiplexed signals are attenuated by a patient tissue site and output an analog composite signal including at least first and second attenuated signal components corresponding to the attenuated first and second multiplexed signals;

an analog-to-digital converter operable to convert the analog composite signal to a digital composite signal having a plurality of sample instances; and

a digital processor operable to demodulate the digital composite signal with a first demodulation vector to obtain a magnitude of the first attenuated signal component and demodulate the digital composite signal with a second demodulation vector to obtain a magnitude of the second attenuated signal component, wherein said first demodulation vector is orthogonal to the second attenuated signal component and the second demodulation vector is orthogonal to the first attenuated signal component.

19. The system of claim 18 further comprising:

a demodulation vector unit operable to provide the first and second demodulation vectors to the digital processor.

20. The system of claim 19, wherein the demodulation vector unit is operable to select the first and second demodulation vectors based on an identification of an operating environment.

21. The system of claim 20 wherein the demodulation vector unit is enabled to receive manually entered information from a user of the pulse oximetry system, said manually entered information identifying the operating environment.

22. The system of claim 20 wherein the demodulation vector unit is enabled to automatically identify the operating environment by operating at least one of said first and second optical signal sources to transmit a signal modulated in accordance with a known code and comparing an attenuated signal received by said detector with a library of processed modulated signals.

23. The system of claim 19 wherein said digital processor is further operable to dynamically adjust the first and second demodulation vectors provided by the demodulation vector unit during operation of the pulse oximetry system.

24. The system of claim 23 wherein said digital processor is operable to dynamically adjust the first and second demodulation vectors by subtracting at least one correction factor from said first and second demodulation vectors.

25. The system of claim 24 wherein said digital processor is operable to compute said at least one correction factor for each sample instance of said digital composite signal.

26. The system of claim 24 wherein said first and second optical signal sources comprise Red and IR wavelength optical signal sources, said first and second signals comprise Red and IR signals and said first and second signal components comprise Red and IR signal components, and wherein said at least one correction factor is selected from a group consisting of a Red signal to IR signal component cross talk correction factor, an IR signal to Red signal component cross talk correction factor, a Red optical signal source capacitive coupling to Red signal component correction factor, a Red optical signal source capacitive coupling to IR signal component correction factor, an IR optical signal source capacitive coupling to IR signal component correction factor, and an IR optical signal source capacitive coupling to Red signal component correction factor.

27. The system of claim 18 said first and second optical signal sources are operable to transmit first and second code division multiplexed signals.