Oversampling pulse oximeter

• US 20050020894A1
• Filed: 06/08/2004
• Published: 01/27/2005
• Est. Priority Date: 12/17/1999
• Status: Active Grant

• Current Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Original Assignee: Datex-Ohmeda Incorporated (General Electric Company)
• Inventors: Hanna, D. Alan, Norris, Mark A.
• Granted Patent: US 7,062,307 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 2560/0214 of power generation or supply
• CPC Class Codes: A61B 5/02422 within occluders
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: A61B 5/7264 Classification of physiological signals or data, e.g. using neural networks, statistical classifiers, expert systems or fuzzy systems neural networks per se G06N3/00; expert systems per se G06N5/00
• CPC Class Codes: A61B 5/7292 Prospective gating, i.e. predicting the occurrence of a physiological event for use as a synchronisation signal

Abstract

An oversampling pulse oximeter includes an analog to digital converter with a sampling rate sufficient to take multiple samples per source cycle. In one embodiment, a pulse oximeter (100) includes two more more light sources (102) driven by light source drives (104) in response to drive signals from a digital signal processing unit (116). The source drives (104) may drive the sources (102) to produce a frequency division multiplex signal. The optical signals transmitted by the light sources (102) are transmitted through a patient'"'"'s appendage (103) and impinge on a detector (106). The detector (106) provides an analog current signal representative of the received optical signals. An amplifier circuit (110) converts the analog current signal to an analog voltage signal in addition to performing a number of other functions. The amplifier circuit (110) outputs an analog voltage signal which is representative of the optical signals from the sources (102). This analog voltage signal is received by a fast A/D converter (112) which samples the analog voltage signal to generate a digital voltage signal which can be processed by the digital signal processing unit (116). The fast A/D converter (112) operates at a rate sufficient to take multiple samples per source cycle and may have a sampling frequency, for example, of over 41 kHz. The digital signal processing unit (116) implements software for averaging the samples over a source cycle for improved measurement consistency, improved signal to noise ratio and reduced A/D converter word length.

Claims

1. A pulse oximeter, comprising:

a first source for emitting light having a first spectral content;

a second source for emitting light having a second spectral content different than said first spectral content;

a drive system for pulsing each of said first and second sources such that said first and second sources output first and second optical signals, respectively, each of said first and second optical signals including a series of high output periods separated by low output periods;

a detector for receiving said first and second optical signals and providing a detector signal representative of said received first and second optical signals; and

a digital sampler, operatively associated with said detector, for providing multiple digital values corresponding to one of a single high output period and a single low output period of one of said first and second sources based on said detector signal; and

a processor for using said digital values in performing processing steps related to determining an oxygen saturation related value.

2. A pulse oximeter as set forth in claim 1, wherein said first source emits red light and said second source emits infrared light.

3. A pulse oximeter as set forth in claim 1, wherein said drive system pulses each of said first and second light sources so that said first and second optical signals define a composite time division multiplexed signal, wherein the respective high output periods of the first and second optical signals are substantially free from temporal overlap.

4. A pulse oximeter as set forth in claim 1, wherein said drive system pulses said first light source based on a first modulating signal and pulses said second light source based on a second modulating signal so that the first and second optical signals define a composite signal including temporal overlap events where the respective high output periods of the first and second optical signals temporally overlap.

5. A pulse oximeter as set forth in claim 1, wherein said detector signal provided by said detector is an analog current signal reflecting an intensity of light incident on said detector as a function of time.

6. A pulse oximeter as set forth in claim 5, further comprising circuitry for receiving said analog current signal and outputting an analog voltage signal representative of said first and second optical signals.

7. A pulse oximeter as set forth in claim 6, wherein said circuitry is further operative for subtracting one of a DC component and a low frequency component from said analog current signal such that said subtracted component is not reflected in said analog voltage signal.

8. A pulse oximeter as set forth in claim 1, wherein said digital sampler has a sampling rate that is sufficiently fast to track a shape of said detector signal, said shape including a profile of a portion of said detector signal corresponding to a single high output period of one of said first and second sources.

9. A pulse oximeter as set forth in claim 1, wherein said digital sampler has a sampling rate sufficient to obtain at least about 3 of said digital values corresponding to instants of single high output period of one of said first and second sources.

10. A pulse oximeter as set forth in claim 1, wherein said digital sampler has a sampling rate sufficient to obtain at least about 20 of said digital values corresponding to a single high output period of one of said first and second sources.

11. A pulse oximeter as set forth in claim 1, wherein said digital sampler outputs a digital signal to said processor including a first portion representative of said first optical signal and a second portion representative of said second optical signal, and said processor implements demodulation logic for extracting from said digital signal a first set of data corresponding to said first portion of said digital signal and a second set of data corresponding to said second portion of said digital signal.

12. A pulse oximeter as set forth in claim 1, wherein said processor implements averaging logic for receiving multiple values corresponding to a single high output period of one of said first and second sources and for determining an intensity related value based on said multiple values.

13. A pulse oximeter as set forth in claim 1, further comprising a monitor for monitoring said digital values of said digital sampler to identify a phase of interest relative to said detector signal, and said processor includes selection logic for selecting a subset of said digital values based on said identified phase, wherein said selected subset is used in determining said oxygen saturation related value.

14. A pulse oximeter as set forth in claim 1, further comprising a monitor for monitoring said digital values of said digital sampler to identify a phase of interest relative to said detector signal, and said processor includes demodulation logic for extracting, from a digital signal received from said digital sampler, a portion of said digital signal corresponding to a contribution to said digital signal from one of said sources, said demodulation logic employing a demodulation function, wherein said demodulation function is synchronized relative to said digital signal based on said identified phase.

15. A pulse oximeter as set forth in claim 1, further comprising means for correcting a phase related to a time difference between measurements first and second high output periods of said first and second sources, respectively.

16. A pulse oximeter, comprising:

a first source for emitting light having a first spectral content;

a second source for emitting light having a second spectral content different than said first spectral content;

a drive system for pulsing each of said first and second sources such that said first and second sources output first and second optical signals, respectively, each of said first and second optical signals including a series of high output periods separated by low output periods;

a detector for receiving said first and second optical signals and providing a detector signal having a detector signal waveform representative of said received first and second optical signals; and

a digital sampler, operatively associated with said detector, for providing a digital value corresponding to a value of said detector signal corresponding to a portion of said detector signal waveform having a time component that is substantially shorter than any of said high output periods of said sources; and

a processor for using said digital value in performing processing steps related to determining an oxygen saturation related value.

17. A pulse oximeter as set forth in claim 16, wherein said time component of said portion of said detector signal waveform is no more than about 50% of one of said high output periods.

18. A pulse oximeter as set forth in claim 16, wherein said time component of said portion of said detector signal waveform is no more than about 10% of one of said high output periods.

19. A pulse oximeter as set forth in claim 16, wherein said drive system pulses each of said first and second light sources so that said first and second optical signals define a composite time division multiplexed signal, wherein the respective high output periods of the first and second optical signals are substantially free from temporal overlap.

20. A pulse oximeter as set forth in claim 16, wherein said drive system pulses said first light source based on a first modulating signal and pulses said second light source based on a second modulating signal so that the first and second optical signals define a composite signal including temporal overlap events where the respective high output periods of the first and second optical signals temporally overlap.

21. A pulse oximeter as set forth in claim 16, wherein said digital sampler has a sampling rate that is sufficiently fast to track a shape of said detector signal, said shape including a profile of a portion of said detector signal corresponding to a single high output period of one of said first and second sources.

23. A pulse oximeter as set forth in claim 21, wherein said drive system pulses each of said first and second sources so that said optical signal is a composite time division multiplexed signal, wherein respective high output periods of the first and second sources are substantially free from temporal overlap.

24. A pulse oximeter as set forth in claim 21, wherein said drive system pulses said first source based on a first modulating signal and pulses said second source based on a second modulating signal so that the optical signal is a composite signal including temporal overlap events where the respective high output periods of the first and second optical signals temporally overlap.

25. A pulse oximeter as set forth in claim 21, wherein said analog to digital converter has a sampling rate sufficient to obtain at least about 1 of said digital values corresponding to a single instant of high output period of one of said first and second sources.

26. A pulse oximeter as set forth in claim 21, wherein said analog to digital converter has a sampling rate sufficient to obtain at least about 3 of said digital values corresponding to a single high output period of one of said first and second sources.

27. A pulse oximeter as set forth in claim 21, wherein said analog to digital converted has a sampling rate sufficient to obtain at least about 20 of said digital values corresponding to a single high output period of one of said first and second sources.

22. An apparatus for use in a pulse oximeter that includes a first source for emitting light at a first wavelength, a second source for emitting light at a second wavelength, a drive system for pulsing each of said sources so as to define an optical signal including a series of “

on”

periods for each of the sources, detector circuitry for receiving the optical signal and providing an analog electrical signal representative thereof, and a processing unit for determining oxygen saturation values based on the electrical signal, said apparatus comprising;

receiving means for receiving at least a portion of said analog electrical signal;

an analog to digital converter for converting said received analog electrical signal portion into a digital signal, said analog to digital converter being operative for sampling said analog electrical signal portion at a sampling rate and providing corresponding digital values;

said sampling rate being sufficient to provide multiple digital values corresponding to a single “

on”

period of one of said sources; and

output means for outputting said digital signal to said processing unit.

28. An apparatus for use in a pulse oximeter that includes a first source for emitting light at a first wavelength, a second source for emitting light at a second wavelength, a drive system for pulsing each of said sources so as to define an optical signal including a series of “

on”

periods for each of the sources, detector circuitry for receiving the optical signal and providing an analog electrical signal representative thereof, and a processing unit for determining oxygen saturation values based on the electrical signal, said apparatus comprising;

an analog to digital converter, operatively disposed between said detector circuitry and said processing unit, for receiving at least a portion of said analog electrical signal and converting said received analog electrical signal portion into a digital signal;

a phase detector for analyzing the digital signal to detect a phase of interest relative to said digital signal; and

processing logic associated with said processing unit for processing said digital signal based on said detected phase of interest.

29. A pulse oximeter as set forth in claim 28, wherein said drive system pulses each of said first and second light sources so that said first and second optical signals define a composite time-division multiplexed signal, wherein the respective high output periods of the first and second optical signals are substantially free from temporal overlap.

30. A pulse oximeter as set forth in claim 28, wherein said drive system pulses said first light source based on a first modulating signal and pulses said second light source based on a second modulating signal so that the first and second optical signals define a composite signal including temporal overlap events where the respective high output periods of the first and second optical signals temporally overlap.

31. A pulse oximeter as set forth in claim 28, wherein said analog to digital converter has a sampling rate sufficient to obtain at least about 3 of said digital values corresponding to an instant of single high output period of one of said first and second sources.

32. A pulse oximeter as set forth in claim 28, wherein said analog to digital converter has a sampling rate sufficient to obtain at least about 20 digital values corresponding to a single on period of one of said first and second sources.

33. A pulse oximeter as set forth in claim 28, wherein said processing logic includes selection logic for selecting a subset of digital values based on said identified phase, wherein said selected subset is used in determining said oxygen saturation related values.

34. A pulse oximeter as set forth in claim 28, wherein said processing logic includes demodulation logic for extracting, from a digital signal received from said analog to digital converter, a portion of said digital signal corresponding to a contribution to said digital signal from one of said sources, said demodulation logic employing a demodulation function, wherein said demodulation function is synchronized relative to said digital signal based on said identified phase.

35. A pulse oximeter as set forth in claim 28, wherein the analog to digital converter has a word length is shorter than that required to support the required dynamic range in the absence of oversampling.