System and method for the extractment of physiological signals

• US 5,662,105 A
• Filed: 05/17/1995
• Issued: 09/02/1997
• Est. Priority Date: 05/17/1995
• Status: Expired due to Term

• Current Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Original Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Inventors: Tien, Jonathan
• Primary Examiner(s): Cohen, Lee S.
• Assistant Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/442,834
• Time in Patent Office: 839 Days
• Field of Search: 128/633, 128/664, 128/665, 356/41, 364/413.01, 364/413.02, 364/413.03
• US Class Current: 600/336
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7207 of noise induced by motion artifacts
• CPC Class Codes: A61B 5/7239 using differentiation including higher order derivatives
• CPC Class Codes: A61B 5/725 using specific filters therefor, e.g. Kalman or adaptive filters A61B5/316 take precedence

Abstract

A technique for the accurate determination of artifact free pulse oximetry signals utilizes an adaptive signal processor and peak detector circuit. A reference signal for the adaptive signal processor is derived based on mathematical relations of the detected light signals and the true intensity of light signals. The derivation of the appropriate reference signal requires the determination of peak values for two ratio constants. The two ratio constants lie within a known physiological range of values. When the value of the ratio constants has been determined, the reference signal equals the true intensity. The output of the adaptive signal processor is provided to the peak detector. The peak detector subdivides the known physiological range into two intervals and determines in which interval a peak exists. The remaining interval is discarded, and the peak detector repeats the measurement on the remaining interval until a peak has been detected within the desired accuracy. The value corresponding to the detected peak is one of the ratio constants. In a second alternative embodiment, the output of the adaptive filter is fed directly to the peak detector to determine values for the two ratio constants. In yet a third embodiment, two adaptive signal processors are coupled to a ratio processor. The output of the ratio processor is at a minimum when the appropriate value is selected for each of the two ratio constants.

Claims

1. A system for the enhancement of physiological signals for the measurement of blood oxygen in a subject, the system comprising:

first and second light sources to direct light toward the subject, said first and second light sources producing first and second light signals of first and second wavelengths, respectively;

a light detector positioned to detect said first and second light signals after interaction with the subject and to generate first and second signals indicative of an intensity of said first and second detected light signals, respectively, said first generated signal having a first portion arising from light transmitted from said first source and a second portion arising from a first interference light source, said second generated signal having a first portion arising from light transmitted from said second source and a second portion arising from a second interference light source;

an adaptive signal processor having a signal input coupled to said light detector to receive said first generated signal, an adaptive filter having an input to receive a reference signal, and an output, and an error output to generate an error signal, wherein said error output is coupled to said adaptive filter to adjust said adaptive filter so that said error signal has a minimum value;

a storage location containing a mathematical relationship of said first and second portions of said first and second generated signals and a first ratio constant;

a reference signal generator to generate said reference signal based on an estimated value of said first ratio constant; and

a peak detector to receive an output signal from said adaptive signal processor and determine a calculated value for said first ratio constant corresponding to a first peak value of said output signal over a predetermined range of possible ratios, said reference signal generator generating said first portion of said first detected signal and said first portion of said second detected signal based on said mathematical relationship and said calculated value of said first ratio constant.

2. The system of claim 1 wherein said peak detector includes means for subdividing said predetermined range into first and second substantially equal ranges to determine a peak location in either said first or second ranges, said peak detector continuing to subdivide one of said first or second ranges containing said peak until said peak detector determines said calculated value of said first ratio constant corresponding to said first peak value.

3. The system of claim 1 wherein said output signal received by said peak detector is selected from a set of output signals comprising said error signal output and said adaptive filter output.

4. The system of claim 1, further including an oxygen saturation calculating circuit to determine blood oxygen saturation of the subject based on said calculated value of said first ratio constant.

5. The system of claim 1, further including a data table interrelating said calculated value of said first ratio constant with blood oxygen saturation level.

6. The system of claim 1 wherein said calculated value of said first ratio constant is a time-varying signal, the system further including a peak saturation calculating circuit to calculate a peak oxygen saturation level from said time-varying signal.

7. The system of claim 1 wherein said first and second wavelengths are in the red and near-infrared wavelength range, respectively.

8. The system of claim 1 wherein said mathematical relationship has the following form:

##EQU11## where R*(t) corresponds to said first portion of said first generated signal, R(t) corresponds to said first generated signal, including said first and second portions of said first generated signal, r(t) corresponds to said second generated signal, including said first and second portions of said second generated signal, α

is said first ratio constant and corresponds to a ratio of said first portion of said first generated signal to said first portion of said second generated signal, and β

is a second ratio constant and corresponds to a ratio of said second portion of said first generated signal to said second portion of said second generated signal.

9. The system of claim 1 wherein said mathematical relationship has the following form:

##EQU12## where r*(t) corresponds to said first portion of said second generated signal, R(t) corresponds to said first generated signal, including said first and second portions of said first generated signal, r(t) corresponds to said second generated signal, including said first and second portions of said second generated signal, α

is said first ratio constant and corresponds to a ratio of said first portion of said first generated signal to said first portion of said second generated signal, and β

is a second ratio constant and corresponds to a ratio of said second portion of said first generated signal to said second portion of said second generated signal.

10. A method for the enhancement of physiological signals for the measurement of blood oxygen in a subject, the method comprising the steps of:

directing light from first and second light sources of different wavelengths toward the subject;

detecting signals from said first and second light sources after interaction with the subject and generating first and second signals corresponding to an intensity of said first and second detected signals, respectively, said first generated signal having a first portion arising from light transmitted from said first source and a second portion arising from a first interference light source, said second generated signal having a first portion arising from light transmitted from said second source and a second portion arising from a second interference light source;

coupling said first generated signal to a signal input of an adaptive signal processor having an adaptive filter having an input to receive a reference signal, and an output, and an error output generating an error signal wherein said error signal is coupled to said adaptive filter to adjust said adaptive filter so that said error signal has a minimum value;

coupling an output signal from said adaptive signal processor to a peak detector and calculating a first ratio value corresponding to a first detected peak value of said error signal over a predetermined range of possible ratio values;

generating a first reference signal based on a mathematical relationship of said first and second portions of said first and second generated signals, and said first ratio value; and

coupling said first reference signal to said adaptive filter input wherein said filter output generates said first portion of said first generated signal.

11. The method of claim 10 wherein said output signal from said adaptive signal processor is said error signal and said calculated first ratio value is based on said first detected peak value in said error signal.

12. The method of claim 10 wherein said output signal from said adaptive signal processor is derived from said adaptive filter output and said calculated first ratio value is based on said first detected peak value in said output signal derived from adaptive filter output.

13. The method of claim 10 wherein said peak detector performs the steps of subdividing said predetermined range into first and second substantially equal ranges and detecting said first peak value in either said first or second ranges, said peak detector continuing to subdivide one of said first or second ranges containing said first detected peak value until said peak detector detects said first peak value within a predetermined threshold.

14. The method of claim 12, further including the step of generating said first portion of said second generated signal based on said mathematical relationship and said calculated first ratio value.

15. The method of claim 14 wherein said first ratio value is a ratio of said first portion of said first generated signal to said first portion of said second generated signal.

16. The method of claim 15 wherein said calculated first ratio value is a time-varying signal, the method further including the step of calculating a peak oxygen saturation level from said time-varying signal.

17. The method of claim 10, further including the step of determining a blood oxygen saturation level of the subject based on said calculated first ratio value.

18. The method of claim 10, further including the step of determining a blood oxygen saturation level of the subject using a data table interrelating said calculated first ratio value with blood oxygen saturation level.

19. The method of claim 10 wherein said mathematical relationship has the following form:

##EQU13## where R*(t) corresponds to said first portion of said first generated signal, R(t) corresponds to said first generated signal, including said first and second portions of said first generated signal, r(t) corresponds to said second generated signal, including said first and second portions of said second generated signal, α

is said first ratio constant and corresponds to a ratio of said first portion of said first generated signal to said first portion of said second generated signal, and β

is a second ratio constant and corresponds to a ratio of said second portion of said first generated signal to said second portion of said second generated signal.

20. The method of claim 10 wherein said mathematical relationship has the following form:

##EQU14## where r*(t) corresponds to said first portion of said second generated signal, R(t) corresponds to said first generated signal, including said first and second portions of said first generated signal, r(t) corresponds to said second generated signal, including said first and second portions of said second generated signal, α

is said first ratio constant and corresponds to a ratio of said first portion of said first generated signal to said first portion of said second generated signal, and β

is a second ratio constant and corresponds to a ratio of said second portion of said first generated signal to said second portion of said second generated signal.