System and method for the algebraic derivation of physiological signals

• US 5,687,722 A
• Filed: 07/26/1995
• Issued: 11/18/1997
• Est. Priority Date: 05/17/1995
• Status: Expired due to Term

• Current Assignee: Spacelabs Healthcare LLC (OSI Systems Incorporated)
• Original Assignee: Spacelabs Medical Incorporated (OSI Systems Incorporated)
• Inventors: Tien, Jonathan, Marble, David R.
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): YARNELL, BRYAN KEITH
• Application Number:
  US08/507,754
• Time in Patent Office: 846 Days
• Field of Search: 128/633, 128/664-7, 356/39-41
• US Class Current: 600/323
• 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 algebraic derivation of a pulse oximetry signal uses the mathematical relations of detected light signals. First and second light sources transmit light through the patient'"'"'s finger or reflects light off the blood vessels in the patient'"'"'s finger. A light detector detects light from each of the light sources and generates a measured intensity signal. The measured intensity signal includes the true intensity of light transmitted from each of the light sources as well as noise introduced during the measurement process. A data sample from each of the light sources is digitized and subdivided into a plurality of data windows. The mathematical relationship is applied to each of the data windows to determine a ratio value indicative of oxygen saturation in the patient. The ratio values are statistically analyzed to determine the best estimate for the correct ratio value. This ratio value is subsequently processed to determine the oxygen saturation within the patient.

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 light of first and second wavelengths, respectively;

a light detector positioned to detect first and second light signals after interacting 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;

a window generator to generate first and second data windows derived from data comprising said first and second generated signals;

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

a calculator using said mathematical relationship to generate first and second functions using said first and second data windows, respectively, said first and second functions having a point of intersection indicative of said ratio.

2. The system of claim 1 wherein said ratio is indicative of blood oxygen saturation in the subject, the system further including a look-up table containing data relating said ratio to said blood oxygen saturation.

3. The system of claim 1 wherein said first data window is a derivative of said first and second generated signals.

4. The system of claim 1 wherein said first data window is an alternating current (AC) component of said first and second generated signals.

5. The system of claim 1 wherein said first data window is derived from a group of signals comprising said first generated signal, a mathematical derivative of said first generated signal, said second generated signal, a mathematical derivative of said second generated signal, a combination of said first and second generated signals, and a mathematical derivative of said combination of said first and second generated signals.

6. The system of claim 5 wherein said window generator calculates an average value for each of said group of signals and data having a data value less than said average for a particular one of said group of signals is placed in said first data window and data having said data value greater than or equal to said average for said particular one of said group of signals is placed in said second data window.

7. The system of claim 5 wherein said window generator calculates an average value for each of said group of signals and data having a data value less than said average for a first one of said group of signals is placed in said first data window and data having said data value greater than or equal to said average for a second one of said group of signals is placed in said second data window.

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

##EQU14## where α

is said ratio, i=1 to 2 for said first and second data windows, respectively, R_i (t) is said first generated signal in the ith data window, r_i (t) is said second generated signal in the ith data window, and β

represents a ratio variable of a portion of said first generated signal to a corresponding portion of said second generated signal.

9. The system of claim 8 wherein said calculator is configured to solve for a pair of values for α

and β

, respectively, by setting F₁ (β

)=F₂ (β

) and solving for the (α

, β

) pair using a mathematical function having the following form;

##EQU15##

10. The system of claim 1 wherein the physiological signals are also indicative of blood carbon monoxide levels, the system further comprising a third light source generating a third wavelength of light different from said first and second wavelengths, said detector being positioned to detect a third light signal from said third light source after interacting with the subject and to generate a third signal indicate of an intensity of said third detected light signal, said third generated signal having a first portion arising from light transmitted from said third source and a second portion arising from a third interference light source, said window generator generating a third data window derived from said third generated signal, said first and second functions having a point of intersection indicative of said first ratio related to blood oxygen saturation, said calculator also using said mathematical relationship to generate a third function using said second and third data windows, respectively, said second and third functions having a point of intersection indicative of said second ratio related to blood carboxyhemoglobin saturation.

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

directing first and second light sources toward the subject, said first and second light sources producing light of first and second wavelengths, respectively;

detecting first and second light signals after interaction with the subject and generating signals indicative of an intensity of said first and second detected light signals, 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;

generating a plurality of data windows derived from data comprising said first and second generated signals; and

using a mathematical relationship of said first and second portions of said first and second generated signals and a ratio of said first portion of said first generated signal to said first portion of said second generated signal to generate a plurality of functions from said plurality of data windows, respectively, said plurality of functions having a solution value indicative of said ratio.

12. The method of claim 11 wherein said plurality of functions comprises first and second functions, said first and second functions having an intersection indicative of said ratio.

13. The method of claim 11 wherein said plurality of functions comprises first, second, and third functions, said ratio being statistically derived based on said solution values for said first, second, and third functions.

14. The method of claim 11 wherein said ratio is indicative of blood oxygen saturation in the subject, the method further including the step of using a look-up table containing data relating said ratio to said blood oxygen saturation to determine the blood oxygen level in the subject.

15. The method of claim 11 wherein a first of said plurality of data windows is a derivative of said first generated signal.

16. The method of claim 11 wherein a first of said plurality of data windows is an alternating current (AC) component of said first generated signal.

17. The method of claim 11 wherein a first of said plurality of data windows is derived from a group of signals comprising said first generated signal, a mathematical derivative of said first generated signal, said second generated signal, a mathematical derivative of said second generated signal, a combination of said first and second generated signals, and a mathematical derivative of said combination of said first and second generated signals.

18. The method of claim 11, further including the steps of calculating an average value for each of said group of signals and placing data whose value is less than said average for a particular one of said group of signals in a first of said plurality of data windows and placing data whose value is greater than or equal to said average for said particular one of said group of signals in a second of said plurality of data windows.

19. The method of claim 11, further including the steps of calculating an average value for each of said group of signals and placing data whose value is less than said average for a particular one of said group of signals in a first of said plurality of data windows and placing data whose value is greater than or equal to said average for a second one of said group of signals in a second of said plurality of data windows.

20. The method of claim 11 wherein the physiological signals are also indicative of blood carbon monoxide levels and said step of directing light sources includes directing first, second, and third light sources toward the subject and said step of detecting includes detecting first, second, and third light signals after interaction with the subject and generating first, second and third signals indicative of an intensity of said first, second and third detected light signals, respectively, said third generated signal having first portion arising from light transmitted from said third light source and a second portion arising from a third interference light source, said step of generating data windows generating first, second, and third data windows derived from said first, second and third generated signals, respectively, said step of generating said plurality of functions using said mathematical relationship to generate a second ratio of said first portion of a first of said plurality of generated signals to said first portion of a third of said plurality of said generated signals, and using said mathematical relationship to generate first and second functions using said first and second data windows, respectively, said first and second functions having a point of intersection indicative of said first ratio related to blood oxygen saturation and using said mathematical relationship to generate a third function using said second and third data windows, respectively, said second and third functions having a point of intersection indicative of said second ratio related to blood carboxyhemoglobin saturation.

21. 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 light of first and second wavelengths, respectively;

a light detector positioned to detect 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;

a window generator to generate a plurality of data windows derived from data comprising said first and second generated signals;

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

a calculator using said mathematical relationship to generate a plurality of functions from said plurality of data windows, respectively, said plurality of functions having a final solution value indicative of said ratio.

22. The system of claim 21 wherein said ratio is indicative of blood oxygen saturation in the subject, the system further including a look-up table containing data relating said ratio to said blood oxygen saturation.

23. The system of claim 21 wherein a first of said plurality of data windows is derived from a group of signals comprising said first generated signal, a mathematical derivative of said first generated signal, said second generated signal, a mathematical derivative of said second generated signal, a combination of said first and second generated signals, and a mathematical derivative of said combination of said first and second generated signals.

24. The system of claim 23 wherein said window generator calculates an average value for each of said group of signals and data having a data value less than said average for a particular one of said group of signals is placed in said first data window and data having said data value greater than or equal to said average for said particular one of said group of signals is placed in a second of said plurality of data windows.

25. The system of claim 23 wherein said window generator calculates an average value for each of said group of signals and data having a data value less than said average for a first one of said group of signals is placed in said first data window and data having said data value greater than or equal to said average for a second one of said group of signals is placed in a second of said plurality of data windows.

26. The system of claim 21 wherein each plurality of functions results in a solution value, said calculator determining said final solution value based on said solution values for said plurality of functions.

27. The system of claim 26 wherein said calculator includes a statistical analyzer to evaluate said solution values for said plurality of functions and determine said final solution value based on statistical analysis of said solution values for said plurality of functions.

28. The system of claim 21 wherein the physiological signals are indicative of blood carbon monoxide levels, the system further comprising a third light source generating a third wavelength of light different from said first and second wavelengths, said detector being positioned to detect a third light signal from said third light source after interaction with the subject and to generate a third signal indicative of an intensity of said third detected signal, said third generated signal having a first portion arising from light transmitted from said third source and a second portion arising from a third interference light source, said window generator generating first, second, and third data windows derived from said first, second and third generated signals, said mathematical relationship also including a first ratio of said first portion of said first generated signal to said first portion of said second generated signal and a second ratio of said first portion of said first generated signal to said first portion of said third generated signal, said calculator using said mathematical relationship to generate first and second functions using said first and second data windows, respectively, said first and second functions having a solution value indicative of said first ratio related to blood oxygen saturation, said calculator using said mathematical relationship to generate a third function using said first and third data windows, respectively, said second and third functions having a solution value indicative of said second ratio related to blood carboxyhemoglobin saturation.