Pulse oximeter

• US 5,995,858 A
• Filed: 11/07/1997
• Issued: 11/30/1999
• Est. Priority Date: 11/07/1997
• Status: Expired due to Term

• Current Assignee: Shenzhen Mindray Bio-Medical Electronics Co., Ltd. (Excelsior Union Ltd.)
• Original Assignee: Datascope Corporation (Getinge AB)
• Inventors: Kinast, Eric
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/969,407
• Time in Patent Office: 753 Days
• Field of Search: 600/310, 600/322, 600/323, 600/330, 600/473, 600/476, 356/41, 356/432, 356/433, 250/339.12, 250/341.1
• US Class Current: 600/323
• CPC Class Codes: A61B 5/14551 for measuring blood gases

Abstract

An improved pulse oximeter incorporating an innovative method for exposing a patient'"'"'s extremity to electromagnetic radiation of two wavelengths, e.g., a red wavelength and an IR wavelength, and detecting the absorbance of the extremity at each of the wavelengths, said method comprising the steps of:

(a) generating overlapping first and second pulse trains of electromagnetic radiation having first and second wavelengths and having the same frequency but differing in phase by 90 degrees;

(b) exposing the extremity to the first and second pulses;

(c) producing an electrical signal corresponding to the electromagnetic radiation intensity of both the first and second pulses added together;

(d) amplifying said electrical signal using a tuned amplifier; and

(e) demodulating said electrical signal using the first pulse and second pulse, independently, as reference signals. In an alternate embodiment of the invention, improved rejection of noise is provided by using pseudo random sequences for the LED drives and the demodulator reference signals.

Claims

1. Apparatus for use in a pulse oximeter for generating and detecting two wavelengths of electromagnetic radiation comprising:

(a) means for generating pulses of electromagnetic radiation having a first wavelength and for generating pulses of electromagnetic radiation of a second wavelength that have the same frequency as the pulses having the first wavelength and are in quadrature with the pulses having the first wavelength;

(b) means for simultaneously exposing a selected portion of a human being or animal to the first and second pulses;

(c) detector means for producing an electrical signal which is representative of the electromagnetic radiation intensity at a selected location in the vicinity of the selected portion of the human being or animal, said detector means being responsive to electromagnetic radiation including the first and second wavelengths;

(d) amplifying means for amplifying the electrical signal;

(e) demodulating means for extracting the portion of the electrical signal created by electromagnetic radiation having the first wavelength and for extracting the portion of the electrical signal created by electromagnetic radiation having the second wavelength;

(f) filtering means for filtering the portion of the electrical signal created by electromagnetic radiation having the first wavelength and the portion of the electrical signal created by electromagnetic radiation having the second wavelength, said filtering means generates a first signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the first wavelength and a second signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the second wavelength; and

(g) error compensation means which corrects residual phase errors by;

1) briefly operating the oximeter allowing only those pulses of light corresponding to the first wavelength to be emitted;

2) during this time measuring the amplitudes of the first and second unadjusted amplitudes, and computing the fractional crosstalk of the first amplitude into the second;

3) briefly operating the oximeter allowing only those pulses of light corresponding to the second wavelength to be emitted;

4) during this time measuring the amplitudes of the first and second unadjusted amplitudes, and computing the fractional crosstalk of the second amplitude into the first;

5) during normal operation, producing a first corrected signal by subtracting from the first unadjusted amplitude a quantity equal to the second amplitude times the fractional crosstalk of the second amplitude into the first;

6) during normal operation, producing a second corrected signal by subtracting from the second unadjusted amplitude a quantity equal to the first amplitude times the fractional crosstalk of the first amplitude into the second.

2. A pulse oximeter comprising:

a) a waveform generator producing a first drive waveform, a first reference waveform having the same phase and frequency as the first drive waveform, a second drive waveform, and a second reference waveform having the same phase and frequency as the second drive waveform, the second waveforms being identical in frequency and waveshape to the first waveforms but differing in phase by 90 degrees;

b) first and second electro-optical emitters respectively energized by the first and second drive waveforms, said first and second electro-optical emitters producing electromagnetic radiation having first and second wavelengths, respectively;

c) a photodetector responsive to radiation of said first and second wavelengths and arranged to receive radiation from said first and second emitters after said radiation has interacted with body tissue;

d) an amplifier for receiving a signal generated by said photodetector and for producing an amplified signal;

e) a first phase-sensitive demodulator which demodulates the amplified signal with respect to the first reference waveform, producing a first demodulated signal;

f) a second phase-sensitive demodulator which demodulates the amplified signal with respect to the second reference waveform, producing a second demodulated signal;

g) filtering means for filtering the portion of the electrical signal created by electromagnetic radiation having the first wavelength and the portion of the electrical signal created by electromagnetic radiation having the second wavelength, said filtering means generating a first filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the first wavelength and a second filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the second wavelength;

h) processing means which receives and analyzes the first and second filtered signals for determining the oxygen saturation of the blood in said perfused tissue; and

i) error correction means for correcting for residual phase errors by;

1) briefly operating the oximeter allowing only those pulses of light corresponding to the first wavelength to be emitted;

2) during this time measuring the amplitudes of the first and second demodulated and filtered signals, and computing the fractional crosstalk of the first demodulated and filtered signal into the second;

demodulated and filtered signal

3) briefly operating the oximeter allowing only those pulses of light corresponding to the second wavelength to be emitted;

4) during this time measuring the amplitudes of the first and second demodulated and filtered signals, and computing the fractional crosstalk of the second demodulated and filter signal into the first demodulated and filtered signal;

5) during normal operation, producing a corrected first signal by subtracting from the first demodulated and filtered signals a quantity equal to the second demodulated and filtered signal times the fractional crosstalk of the second demodulated and filtered signal into the first demodulated and filtered signal; and

6) during normal operation, producing a second corrected signal by subtracting from the second demodulated and filtered signal a quantity equal to the first demodulated and filtered signal times the fractional crosstalk of the first demodulated and filtered signal into the second demodulated and filtered signal.

3. A pulse oximeter comprising:

1) a waveform generator producing a first drive waveform, a first reference waveform having the same frequency and initially the same phase as the first drive waveform, a second drive waveform, and a second reference waveform having the same frequency and initially the same phase as the second drive waveform, the second waveforms being identical in frequency and waveshape to the first waveforms but differing in phase by 90 degrees;

2) first and second electro-optical emitters respectively energized by the first and second drive waveforms, said first and second electro-optical emitters producing electromagnetic radiation having first and second wavelengths, respectively;

3) a photodetector responsive to radiation of said first and second wavelengths and arranged to receive radiation from said first and second emitters after said radiation has interacted with body tissue;

4) an amplifier for receiving a signal generated by said photodetector and for producing an amplified signal;

5) a first phase-sensitive demodulator which demodulates the amplified signal with respect to the first reference waveform, producing a first demodulated signal;

6) a second phase-sensitive demodulator which demodulates the amplified signal with respect to the second reference waveform, producing a second demodulated signal;

g) filtering means for filtering the portion of an electrical signal created by electromagnetic radiation having the first wavelength and the portion of the electrical signal created by electromagnetic radiation having the second wavelength, said filtering means generating a first filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the first wavelength and a second filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the second wavelength;

i) processing means which receives and analyzes the first and second filtered signals for determining the oxygen saturation of the blood in said perfused tissue; and

j) error correction means for correcting for residual phase errors by;

a) briefly operating the oximeter allowing only those pulses of light corresponding to the first wavelength to be emitted;

b) during this time iteratively adjusting the phase of the second reference signal to produce a null on the output of the second demodulator;

c) briefly operating the oximeter allowing only those pulses of light corresponding to the second wavelength to be emitted;

d) during this time iteratively adjusting the phase of the first reference signal to produce a null on the output of the first demodulator;

e) during normal operation, producing a corrected first signal by operating the first demodulator with the adjusted phase of the first reference signal; and

f) during normal operation, producing a second corrected signal by operating the second demodulator with the adjusted phase of the second reference signal.

4. Apparatus for use in a pulse oximeter for generating and detecting two wavelengths of electromagnetic radiation comprising:

(a) means for generating pulses of electromagnetic radiation having a first wavelength and for generating pulses of electromagnetic radiation of a second wavelength that have the same frequency as the pulses having the first wavelength and are in quadrature with the pulses having the first wavelength;

(b) means for simultaneously exposing a selected portion of a human being or animal to the first and second pulses;

(c) detector means for producing an electrical signal which is representative of the electromagnetic radiation intensity at a selected location in the vicinity of the selected portion of the human being or animal, said detector means being responsive to electromagnetic radiation including the first and second wavelengths;

(d) amplifying means for amplifying the electrical signal;

(e) demodulating means for extracting the portion of the electrical signal created by electromagnetic radiation having the first wavelength and for extracting the portion of the electrical signal created by electromagnetic radiation having the second wavelength;

(f) filtering means for filtering the portion of the electrical signal created by electromagnetic radiation having the first wavelength and the portion of the electrical signal created by electromagnetic radiation having the second wavelength, said filtering means generating a first filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the first wavelength and a second filtered signal which indicates the unadjusted amplitude measurement of the electromagnetic radiation having the second wavelength; and

(g) error compensation means which corrects residual phase errors by;

1) briefly operating the oximeter allowing only those pulses of light corresponding to the first wavelength to be emitted;

2) during this time adjusting the phase of the second reference signal to produce a null on the output of the demodulating means

3) briefly operating the oximeter allowing only those pulses of light corresponding to the second wavelength to be emitted;

4) during this time adjusting the phase of the first reference signal to produce a null on the output of the demodulating means;

5) during normal operation, producing a corrected first signal by operating the demodulating means with the adjusted phase of the first reference signal; and

6) during normal operation, producing a second corrected signal by operating the demodulating means with the adjusted phase of the second reference signal.

5. The pulse oximeter as claimed in claim 4, wherein the filtering means comprises a bandpass filter and wherein the error compensation means corrects residual phase errors through adjustment of the bandpass filter.

6. A method for performing pulse oximetry wherein a selected portion of a human being or an animal is exposed to electromagnetic radiation of two different wavelengths, wherein the improvement comprises:

a) generating a first pseudorandom pulse sequence of electromagnetic radiation at a first wavelength and a second pseudorandom pulse sequence at a second wavelength, wherein said first pseudorandom pulse sequence and said second pseudorandom pulse sequence are orthogonal;

b) simultaneously exposing the selected portion of the human being or animal to the first and second pseudorandom radiation pulse sequences;

c) producing an electrical signal corresponding to the electromagnetic radiation intensity in a band of wavelengths including said first and second wavelengths, wherein said electrical signal is received at a selected location in the vicinity of the selected portion; and

d) demodulating the electrical signal by means of a first demodulator producing a first wavelength signal and a second demodulator producing a second wavelength signal, wherein the first demodulator demodulates using a first reference signal, the second demodulator demodulates using a second reference signal, said first reference signal corresponds to the first pseudorandom sequence and said second reference signal corresponds to the second pseudorandom sequence.