Synchronized modulation/demodulation method and apparatus for frequency division multiplexed spectrophotometric system

• US 6,363,269 B1
• Filed: 12/17/1999
• Issued: 03/26/2002
• Est. Priority Date: 12/17/1999
• Status: Expired due to Term

• Current Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Original Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Inventors: Hanna, D. Alan, Norris, Mark A.
• Primary Examiner(s): Winakur, Eric F.
• Assistant Examiner(s): KREMER, MATTHEW J
• Application Number:
  US09/465,304
• Time in Patent Office: 830 Days
• Field of Search: 600/309-311, 600/322-324, 600/336, 600/315-316, 600/326, 600/328, 600/330, 356/39-41
• US Class Current: 600/322
• CPC Class Codes: A61B 5/14551 for measuring blood gases

Abstract

An improved method and apparatus is disclosed for use in frequency division multiplexed spectrophotometric systems. In photoplethysmographic applications the invention provides for the modulation of a plurality of light sources at different frequencies and in accordance with a predetermined phase relationship. Light from the sources that is transmitted through a tissue under test is detected at a detector. A composite signal indicative of the intensity of light received at the detector is demodulated based on the different modulation frequencies and predetermined phase relationship to obtain signal portions corresponding with each of the light sources. Modulation and demodulation are synchronized during each measurement period. The modulation waveforms used to modulate the light sources and corresponding demodulation waveforms used to demultiplex the composite signal are symmetrically timed about a center point for each of the measurement periods. The invention reduces artifacts associated with rising/falling light source amplitude levels, thereby reducing system noise sensitivity.

Claims

1. A photoplethysmographic measurement apparatus for measuring a blood analyte level in a tissue under test, comprising:

a plurality of light sources for emitting a corresponding plurality of light signals at different corresponding wavelengths to said tissue under test;

modulation means for modulating the light signals according to a predefined plurality of different periodic modulation waveforms at corresponding different frequencies and in accordance with a predetermined phase relationship therebetween;

a detector for detecting at least a portion of said light signals transmitted through said tissue under test;

demodulation means for demodulating a composite signal indicative of an intensity of said detected portion of said plurality of light signals according to a predefined plurality of different periodic demodulation waveforms which correspond to said plurality of periodic modulation waveforms, wherein said demodulation is based on said different frequencies and said predetermined phase relationship to obtain signal portions corresponding with each of said light signals, and wherein said signal portions are employable to determine a blood analyte level in said tissue under test; and

synchronization means for synchronizing operation of said modulation means and demodulation means during each of one or more measurement periods so that each of said modulation waveforms and demodulation waveforms are symmetrically timed about a center point for each of said one or more measurement periods.

2. An apparatus as recited in claim 1, wherein each modulation waveform is orthogonal to all other modulation waveforms, and each demodulation waveform is orthogonal to all other demodulation waveforms.

3. An apparatus as recited in claim 2, wherein each of said modulation waveforms and demodulation waveforms are of a type selected from a group consisting of:

sinusoidal waveforms;

square waveforms; and

combinations of sinusoidal and square waveforms.

4. An apparatus as recited in claim 1, wherein said modulation waveforms each define a different exact integer number of cycles for each of said one or more measurement periods.

5. An apparatus as recited in claim 4, wherein said demodulation waveforms each define a different integer number of cycles that is equal to said different integer number of cycles defined by said modulation waveforms, respectively, for each of said one or more measurement periods.

6. An apparatus as recited in claim 1, further comprising:

a sampling means for sampling a detector output signal to provide said composite signal, wherein said sampling means samples at a rate at least 2 times greater than a highest one of said different light source modulation frequencies.

7. An apparatus as recited in claim 6, wherein said demodulation means comprises:

a processor for separately processing a first plurality of values and a second plurality of values comprising said first and second signal portions, respectively, for each of said one or more measurement periods, to determine said blood analyte level in said tissue under test.

8. A method for use in a photoplethysmographic measurement system comprising a plurality of pulsed light sources for illuminating a tissue under test and a detector for receiving at least a portion of the light pulses that is transmitted by the tissue under test and outputting a detector signal indicative of the intensity of the received portion of the light pulses, the method comprising:

modulating each of the plurality of light sources according to a predefined plurality of different periodic modulation waveforms at a corresponding different one of a plurality of pulsing frequencies and in accordance with a predetermined phase relationship therebetween;

demodulating a composite signal according to a predefined plurality of different periodic demodulation waveforms which correspond to said plurality of modulation waveforms, wherein said composite signal corresponds with said detector output signal based on said different pulsing frequencies and predetermined phase relationship, wherein the modulating and demodulating steps corresponding with each given one of said plurality of pulsing frequencies are synchronized so that each of said plurality of periodic modulation and demodulation waveforms are symmetrically timed about a center point of a blood analyte measurement period, and wherein demodulated signal portions corresponding with each of said plurality of light sources are obtained for said blood analyte measurement period.

9. A method as recited in claim 8, wherein said plurality of periodic modulation waveforms and demodulation waveforms completes an integer number of cycles during said measurement period.

10. A method as recited in claim 8, wherein each of said corresponding modulation and demodulation waveforms are orthogonal to all other modulation and demodulation waveforms comprising said plurality of periodic modulation waveforms and demodulation waveforms.

11. A method as recited in claim 8, further comprising:

sampling said detector output signal at a predetermined rate to obtain said composite signal, wherein said composite signal comprises a series of measurement values for said measurement period, and wherein said predetermined rate is at least 2 times greater than a greatest one of said plurality of pulsing frequencies.

12. A method as recited in claim 11, further comprising:

separately multiplying each of said series of measurement values by corresponding-in-time relative values extracted from each of said plurality of different periodic demodulating waveforms to obtain said demodulated signal portions; and

separately summing said demodulated signal portions corresponding with each of said plurality of light sources for said measurement period to obtain a corresponding plurality of values which are each indicative of an intensity of said received portion of the light pulses corresponding with a different one of said plurality of pulsed light sources.

13. A method for determining a blood analyte level in tissue under test, the method comprising:

emitting first and second light signals at different corresponding wavelengths into the tissue over at least one predetermined time period, wherein said first and second light signals are emitted according to first and second periodic modulation waveforms having first and second frequencies, wherein said first and second periodic modulation waveforms are timed to be symmetric about a center point of said time period;

detecting said first and second light signals relative to said tissue and generating a composite signal indicative thereof;

processing said composite signal using corresponding-in-time values from first and second periodic demodulation waveforms which correspond in phase and frequency with said first and second periodic modulation waveforms, respectively, to determine a first plurality of values and a second plurality of values corresponding to said first and second light signals applied to the tissue; and

employing said first plurality of values and said second plurality of values to determine a blood analyte level in the tissue.

14. A method as recited in claim 13, wherein said periodic modulation waveforms and demodulation waveforms complete an integer number of cycles during said time period.

15. A method as recited in claim 13, wherein each of said corresponding first modulation and first demodulation waveforms are orthogonal to said second modulation and second demodulation waveforms.