Signal processing apparatus and method

• US 5,769,785 A
• Filed: 06/07/1995
• Issued: 06/23/1998
• Est. Priority Date: 03/07/1991
• Status: Expired due to Term

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Kiani-Azarbayjany, Esmaiel, Diab, Mohamed Kheir
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): CARTER, RYAN CHRISTOPHER
• Application Number:
  US08/479,918
• Time in Patent Office: 1,112 Days
• Field of Search: 128/633, 128/634, 128/664-667, 356/39-41, 379/410-411
• US Class Current: 600/364
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/7214 using signal cancellation, e.g. based on input of two identical physiological sensors spaced apart, or based on two signals derived from the same sensor, for different optical wavelengths
• CPC Class Codes: A61B 5/7228 Signal modulation applied to the input signal sent to patient or subject; demodulation to recover the physiological signal modulation H03C
• CPC Class Codes: G06F 2218/04 Denoising
• CPC Class Codes: H04B 1/123 using adaptive balancing or compensation means adaptive filter circuits and algorithms H03H

Abstract

A signal processor which acquires a first signal, including a first desired signal portion and a first undesired signal portion, and a second signal, including a second desired signal portion and a second undesired signal portion, wherein the first and second desired signal portions are correlated. The signals may be acquired by propagating energy through a medium and measuring an attenuated signal after transmission or reflection. Alternatively, the signals may be acquired by measuring energy generated by the medium. A processor of the present invention generates a noise reference signal which is a combination of only the undesired signal portions and is correlated to both the first and second undesired signal portions. The noise reference signal is then used to remove the undesired portion of each of the first and second measured signals via an adaptive noise canceler, preferably of the joint process estimator type. The processor of the present invention may be employed in conjunction with an adaptive noise canceler in physiological monitors wherein the known properties of energy attenuation through a medium are used to determine physiological characteristics of the medium. Many physiological conditions, such as the pulse of a patient or the concentration of a constituent in a medium, can be determined from the desired portion of the signal after undesired signal portions, such as those caused by erratic motion, are removed.

Claims

1. A method of determining a noise reference signal from first and second physiological measurement signals, the method comprising the steps of:

generating the first and second physiological measurement signals, the first measurement signal comprising a first desired signal portion and a first noise portion and the second measurement signal comprising a second desired signal portion and a second noise portion;

selecting a signal coefficient which is proportional to a ratio of predetermined attributes of said first desired signal portion and predetermined attributes of said second desired signal portion;

inputting said first measurement signal and said signal coefficient into a signal multiplier wherein said first measurement signal is multiplied by said signal coefficient thereby generating a first intermediate signal; and

inputting said second measurement signal and said first intermediate signal into a signal subtractor wherein said first intermediate signal is subtracted from said second measurement signal thereby generating a reference signal having a primary component which is derived from said first and second noise signal portions.

2. The method of claim 1 wherein said first and second measurement signals collectively represent a blood-gas saturation of a patient.

5. The method of claim 1 wherein the step of generating first and second physiological measurements comprises passing first and second wavelengths of light energy through a light-absorptive physiologic medium and detecting said light energy.

6. The method of claim 5 wherein the absorptive medium comprises living tissue of a human digit.

7. The method of claim 5 wherein the first measurement signal is derived from light energy of the first wavelength, and the second measurement signal is derived from light energy of the second wavelength, and the method further comprises selecting the first and second wavelengths of light such that the first and second desired signal portions are generally linearly dependent.

8. The method of claim 5, further comprising selecting the first wavelength (λ

_A) and the second wavelength (λ

_B) such that the equation ε

_hemoglobin,λ

A /ε

_{oxyhemoglobin},λ

A =ε

_hemoglobin,λ

B /ε

_{oxyhemoglobin},λ

B is substantially satisfied.

9. The method of claim 1 wherein the first and second physiological measurements are taken in close proximity.

10. The method of claim 1 wherein the step of selecting a signal coefficient comprises selecting a coefficient which results in a substantial cancellation of the desired signal portions from the second measurement signal and the intermediate signal in said step of inputting.

11. The method of claim 1 further comprising the step of applying the noise reference signal to at least one of the first and second measurement signals to substantially remove a respective noise portion therefrom.

12. The method of claim 1 wherein each of the first and second desired signal portions contain information necessary for determining a desired physiological parameter.

13. The method of claim 12 further comprising the steps of:

applying the noise reference signal to the first and second measurement signals to extract the first and second desired signal portions; and

combining the first and second desired signal portions extracted in said step of applying to generate a signal indicative of the physiological parameter.

3. A method of removing a motion artifact signal from a signal derived from a physiological measurement comprising the steps of:

passing at least first and second wavelengths of electromagnetic radiation through living tissue;

detecting the first and second wavelengths with a detector following propagation through the living tissue to generate, respectively, a first signal having a physiological measurement component and a motion artifact component and a second signal having a physiological measurement component and a motion artifact component; and

deriving from said first and second signals a motion artifact reference signal which is a primary function of said first and second signals'"'"' motion artifact components.

4. A method as defined in claim 3 further comprising the step of inputting said motion artifact reference signal into an adaptive noise canceler to produce an output signal which is a primary function of said first signal physiological measurement component.

14. The method of claim 3 wherein the first and second wavelengths are selected such that the respective physiological measurement components of the first and second signals are generally linearly dependent.

15. The method of claim 14 wherein the first wavelength (λ

_A) and the second wavelength (λ

_B) are selected such that the equation ε

_hemoglobin,λ

A /ε

_{oxyhemoglobin},λ

A =ε

_hemoglobin,λ

B /ε

_{oxyhemoglobin},λ

B is substantially satisfied.

16. The method of claim 3 wherein the step of deriving comprises multiplying at least one of the first and second signals by a proportionality coefficient, the proportionality coefficient selected such that the respective physiological measurement components of the first and second signals are substantially equal following said step of multiplying.

17. The method of claim 16 wherein the proportionality coefficient is a constant that is based on selected absorption coefficients of the first and second wavelengths.

18. The method of claim 3 wherein the step of passing comprises propagating light energy through a human digit.

19. The method of claim 3 wherein the physiological measurement components of the first and second signals contain information which, in combination, is indicative of a blood-gas saturation of a patient.

20. The method of claim 19 wherein the physiological measurement components of the first and second signals contain information which, in combination, is indicative of a saturation level of oxygen within arterial blood of the patient.

21. A method of taking a physiologic measurement, the method comprising the steps of:

passing light energy of at least first and second wavelengths through a light-absorptive physiologic medium to a light-sensitive detector to generate, respectively, first and second signals, each of the first and second signals comprising a physiologic measurement component and an artifact component, the first and second wavelengths selected based on light absorption characteristics of the physiologic medium such that a generally linear relationship exists between the physiologic measurement components of the first and second signals; and

combining the first and second signals to generate a noise reference signal which is primarily correlated to the artifact components of the first and second signals, the step of combining comprising utilizing the generally linear relationship to substantially cancel the physiologic measurement components.

22. The method of claim 21 wherein the step of combining comprises multiplying the first signal by a proportionality constant to generate an intermediate signal and then combining the intermediate signal with the second signal, the proportionality constant selected such that the physiologic measurement components substantially cancel in said step of combining.

23. The method of claim 21 wherein the first wavelength (λ

_A) and the second wavelength (λ

_B) are selected such that the equation ε

_hemoglobin,λ

A /ε

_{oxyhemoglobin},λ

A =ε

_hemoglobin,λ

B /ε

_{oxyhemoglobin},λ

B is substantially satisfied.

24. The method of claim 21 wherein the step of passing light energy through a light-absorptive physiologic medium comprises passing the light energy through at least a portion of a human digit.

25. The method of claim 21 wherein the physiological measurement components of the first and second signals collectively indicate a blood-gas saturation of a patient.

26. The method of claim 25 wherein the physiological measurement components represent a saturation level of oxygen within blood of the patient.

27. The method of claim 21, further comprising the step of applying the noise reference signal to at least one of the first and the second signals to generate a physiologic measurement signal in which the artifact components are substantially absent.

28. A method of taking a taking a physiological measurement, the method comprising the steps of:

passing light energy of at least first and second wavelengths through a light absorptive physiologic medium to a light-sensitive detector to generate, respectively, first and second signals, each of the first and second signals comprising a desired physiologic measurement component and an artifact component, the first and second wavelengths selected based on light absorption characteristics of the physiologic medium such that a substantially linear relationship exists between the desire physiologic measurement components of the first and second signals;

extracting the desired physiological measurement component from said first or said second signals by multiplying at least one of the first and second signals by a proportionality constant and subtracting said at least one of the first and second signals multiplied by said proportionality constant from the other one of said first and second signals; and

determining the physiological measurement.

29. A method of taking a physiological measurement, the method comprising the steps of:

passing light energy of at least first and second wavelengths through a light-absorptive physiologic medium to a light-sensitive detector to generate, respectively, first and second signals, each of the first and second signals comprising a physiologic measurement portion and a secondary portion, the first and second wavelengths selected based on light absorption characteristics of the physiologic medium such that a substantially linear relationship exists between the secondary portion of the first and second signals; and

combining said first and second signals to generate a signal which is primarily correlated to the physiologic measurement portions, the step of combining comprising using the linear relationship to substantially remove the secondary portion of the signals.

30. The method of claim 29, wherein said secondary portion of said signals contains information indicative of venous blood absorption and said physiologic measurement portion contains information indicative of arterial blood absorption.

31. The method of claim 29, wherein said step of combining comprises multiplying one of said first and second signals by a constant and subtracting the one of said first and second signals from the other one of said first and second signals.

32. The method of claim 29, wherein said step of combining comprises the steps of:

multiplying one of said first or second signal by each of a plurality of possible constants and subtracting each of said multiplied values from the other one of said first and second signals to obtain a plurality of possible signals intermediate signals; and

evaluating each of said plurality of possible signals to calculate one or more that most closely correlate to the physiologic measurement component of said signals.

33. The method of claim 32, wherein said step of evaluating comprises calculating a power value for each of said plurality of possible signals and selecting the signal with the minimum power value.

34. The method of claim 32, wherein said step of evaluating comprises calculating a power value for each of said plurality of possible signals and selecting the signal with the maximum power value.