Signal processing apparatus

• US 6,263,222 B1
• Filed: 10/06/1997
• Issued: 07/17/2001
• Est. Priority Date: 03/07/1991
• Status: Expired due to Fees

• Current Assignee: Masimo Corporation
• Original Assignee: Masimo Corporation
• Inventors: Weber, Walter M, Diab, Mohamed K., Kiani, Massi E.
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US08/943,511
• Time in Patent Office: 1,380 Days
• Field of Search: 600/300, 600/310, 600/322, 600/323, 600/330, 600/336, 600/473, 600/476, 600/479, 600/485, 600/500, 600/501, 356/39-41
• US Class Current: 600/310
• 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 5/726 using Wavelet transforms
• 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 primary signal portion and a first secondary signal portion, and a second signal, including a second primary signal portion and a second secondary signal portion, wherein the first and second primary 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 primary or secondary reference signal which is a combination, respectively, of only the primary or secondary signal portions. The secondary reference signal is then used to remove the secondary portion of each of the first and second measured signals via a correlation canceler, such as an adaptive noise canceler, preferably of the joint process estimator type. The primary reference signal is used to remove the primary portion of each of the first and second measured signals via a correlation canceler. The processor of the present invention may be employed in conjunction with a correlation 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, or blood pressure of a patient or the concentration of a constituent in a medium, can be determined from the primary or secondary portions of the signal after other signal portion is removed.

Claims

1. A physiological monitoring method comprising the steps of:

receiving at least two measured intensity signals generated by the detection of at least two wavelengths of light transmitted through body tissue, each of said at least two intensity signals having a first portion dependent on attenuation of said light due to arterial blood and a second portion dependent on attenuation of said light due to motion induced variation in the body tissue; and

determining arterial oxygen saturation during motion by filtering at least one of said intensity signals wit a Kalman filter to generate an approximation of arterial oxygen saturation during motion, and selecting a resulting arterial oxygen saturation based upon knowledge about oxygen saturation in body tissue and upon the approximation of arterial oxygen saturation.

2. The method of claim 1, wherein said step of filtering comprises substantially removing the second portion of at least one of said at least two intensity signals.

3. The method of claim 2, wherein said step of determining an arterial oxygen saturation comprises at least the step of determining a ratio between said at least two measured intensity signals.

4. The physiological monitoring method of claim 1, wherein said attenuation of light due to motion is substantially dependent upon the attenuation of said light by venous blood in the tissue during motion.

5. A pulse oximeter configured to determine arterial oxygen saturation of a living patient, said oximeter configured to connect to a pulse oximeter sensor having a source of light and a detector for said light, said source of light providing at least two wavelengths, said pulse oximeter comprising:

an input configured to connect to said pulse oximeter sensor and receive at least two measured intensity signals based on said at least two wavelengths after transmission through the tissue of said living patient, each of said at least two measured intensity signals having a first portion substantially dependent upon attenuation of the light due to arterial blood, and during motion a second portion substantially dependent upon attenuation of the light dependent upon motion of the patient;

a Kalman filter which receives as an input at least one of said measured intensity signals, said Kalman filter having an output which provides an estimate of oxygen saturation related to at least one of said measured intensity signals; and

a processor responsive to said estimate at an input to derive an oxygen saturation value representative of the arterial oxygen saturation of blood in said tissue during motion.

6. The pulse oximeter of claim 5, wherein said attenuation dependent upon motion of the patient represents attenuation due to the movement of venous blood.

7. The pulse oximeter of claim 6, wherein said processor determines said arterial oxygen saturation value based on knowledge about arterial oxygen saturation and possible variation over time.

8. A physiological monitor comprising an input configured to receive at least two measured intensity signals generated by the detection of at least two wavelengths of light transmitted through body tissue, said intensity signals each having a portion indicative of at least one physiological parameter;

a Kalman filter responsive to said intensity signals, said Kalman filter attenuating selected frequencies present in said physiological signal, said frequencies comprising substantially motion noise in said physiological signals; and

a processor responsive to the output of said Kalman filter to derive a physiological parameter based upon said output of said Kalman filter, wherein said processor further determines said physiological parameter based upon knowledge about the physiological parameter and possible variation over time.

9. The physiological monitor of claim 8, wherein said physiological parameter comprises blood oxygen saturation.

10. The physiological monitor of claim 8, wherein said physiological parameter comprises heart rate.

11. The physiological monitor of claim 8, wherein said physiological parameter comprises heart rate and blood oxygen saturation.

12. The physiological monitor of claim 8, wherein said motion noise is substantially dependent upon the movement of venous blood due to said motion.

13. A method of determining oxygen saturation, said method comprising the steps of:

receiving an input of at least two measured intensity signals generated by the detection of at least two wavelengths of light transmitted through body tissues, said intensity signals each having a portion substantially dependent on the attenuation of said light due to arterial blood and a portion substantially dependent upon attenuation due to during motion of the body tissue;

adaptively filtering said intensity signals;

calculating oxygen saturation during motion based upon the result of said filtering.

14. The method of claim 13, wherein said step of calculating comprises the step of generating a plurality of values for oxygen saturation based upon said physiological signals and scanning said plurality of values to find at least one value indicative of arterial blood oxygen saturation.

15. The method of claim 13, wherein said selection is based upon knowledge about said physiological parameter.

16. A pulse oximeter comprising:

an input configured to receive at least two measured intensity signals generated by the detection of at least two wavelengths of light transmitted through body tissue having flowing blood, said intensity signals each having a first portion substantially dependent upon attenuation of said light due to arterial blood, and during motion, a second portion substantially dependent upon the attenuation of said light due to motion induced noise; and

a processor responsive to the at least two intensity signals to determine an approximation of arterial oxygen saturation in the presence of motion induced noise, wherein the processor comprises a Kalman filter.

17. A physiological monitor that computes arterial oxygen saturation in tissue material having arterial and venous blood, the physiological monitor comprising:

a light emitter which emits light of at least first and second wavelengths;

a light detector responsive to light from said light emitter which has passed through body tissue having arterial and venous blood, said light detector providing at least first and second intensity signals associated with said at least first and second wavelengths, each of said first and second intensity signals having, during motion of the tissue, at least a first signal portion indicative of arterial blood and a second signal portion indicative of motion induced noise; and

a signal processor responsive to the first and second intensity signals to calculate arterial oxygen saturation without significant interference in the calculation from the motion induced noise portion of the first and second intensity signals.

18. The physiological monitor of claim 17, wherein said motion induced noise is indicative of the attenuation due to venous blood in the tissue during motion.

19. The physiological monitor of claim 18, wherein the signal processor comprises an adaptive signal processor.

20. The physiological monitor of claim 19, wherein said signal processor comprises an adaptive filter.

21. The physiological monitor of claim 17, wherein said signal processor comprises an adaptive signal processor.

22. The physiological monitor of claim 21, wherein said signal processor comprises an adaptive filter.

23. The physiological monitor of claim 22, wherein said adaptive filter comprises a Kalman filter.