Method, apparatus and system for removing motion artifacts from measurements of bodily parameters

• US 7,072,702 B2
• Filed: 06/22/2004
• Issued: 07/04/2006
• Est. Priority Date: 10/15/1998
• Status: Expired due to Fees

• Current Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: RIC INVESTMENTS LLC (Respironics Incorporated)
• Inventors: DelFavero, John R., Jaffe, Michael B., Gunneson, Paul B., Allo, August J. Jr., Edgar, Reuben W. Jr., Martin, Jesus D.
• Primary Examiner(s): WINAKUR, ERIC FRANK
• Application Number:
  US10/873,758
• Publication Number:
  US 20050033129A1
• Time in Patent Office: 742 Days
• Field of Search: 600/336, 600310-326, 600/330, 600/331, 600/473, 600/476
• 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 5/7257 using Fourier transforms
• CPC Class Codes: A61B 5/726 using Wavelet transforms

Abstract

A method for removing motion artifacts from devices for sensing bodily parameters and apparatus and system for effecting same. The method includes analyzing segments of measured data representing bodily parameters and possibly noise from motion artifacts. Each segment of measured data may correspond to a single light signal transmitted and detected after transmission or reflection through bodily tissue. Each data segment is frequency analyzed to determine up to three candidate peaks for further analysis. Each of the up to three candidate frequencies may be filtered and various parameters associated with each of the up to three candidate frequencies are calculated. The best frequency, if one exists, is determined by arbitrating the candidate frequencies using the calculated parameters according to predefined criteria. If a best frequency is found, a pulse rate and SPO₂ may be output. If a best frequency is not found, other, conventional techniques for calculating pulse rate and SpO₂ may be used. The above method may be applied to red and infrared pulse oximetry signals prior to calculating pulse rate and/or pulsatile blood oxygen concentration. Apparatus and systems disclosed are configured to perform methods disclosed according to the invention.

Claims

1. A method of removing motion artifacts, comprising:

transforming a plurality of electrical signals representative of a plurality of attenuated light signals into frequency domain data;

identifying a plurality of candidate peaks from the frequency domain data;

eliminating candidate peaks from the plurality of candidate peaks such that no two of the plurality of candidate peaks comprise harmonics of one another;

filtering each of the plurality of candidate peaks;

developing selection parameters associated with each of the plurality of candidate peaks; and

selecting a best frequency from at least some of the plurality of candidate peaks by employing at least some of the selection parameters.

2. The method of claim 1, wherein selecting the best frequency comprises analyzing each of the plurality of candidate peaks with respect to at least some of the selection parameters to find the best frequency.

3. The method of claim 1, wherein identifying the plurality of candidate peaks further comprises:

assigning a largest power amplitude as a primary candidate peak;

assigning a next largest power amplitude as a secondary candidate peak; and

assigning a previous non-zero pulse rate as a tertiary candidate peak if the previous non-zero pulse rate is neither the primary candidate peak nor the secondary candidate peak.

4. The method of claim 3, wherein selecting the best frequency comprises rejecting a candidate peak of the plurality of candidate peaks if a number of peaks in a pulse window associated with the candidate peak is two or less, or a window pulse rate associated with the candidate peak is zero.

5. The method of claim 4, wherein selecting the best frequency further comprises:

selecting the primary candidate peak;

if the primary candidate peak pulse window confidence is greater than the secondary candidate peak pulse window confidence by a first threshold, and the primary candidate peak pulse rate history percentage is greater than a second threshold;

or

if the secondary candidate peak frequency is a rough harmonic of the primary candidate peak frequency and the primary candidate peak pulse window confidence is no more than a specified number of points greater than the secondary candidate peak pulse window confidence;

selecting the secondary candidate peak if the secondary candidate peak pulse window confidence is greater than the primary candidate peak pulse window confidence by the first threshold, and the secondary candidate peak pulse rate history percentage is greater than the second threshold;

selecting the tertiary candidate peak if the tertiary candidate peak pulse window confidence is less than a pulse window confidence for either the primary candidate peak or the secondary candidate peak; and

selecting no candidate peak;

if the primary candidate peak frequency is zero;

or

if the primary candidate peak and the secondary candidate peak have both been rejected.

6. The method of claim 1, wherein transforming the plurality of electrical signals is performed with a technique selected from the group consisting of a fast Fourier transform, a periodogram, a correlogram, autoregressive methods, Prony'"'"'s method, minimum variance methods, maximum likelihood methods, a discrete cosine transform, a wavelet transform, a discrete Hartley transform, and a Gabor transform.

7. The method of claim 1, further comprising conditioning the plurality of electrical signals to reduce spectral leakage prior to the transforming act.

8. The method of claim 7, wherein the conditioning includes filtering the plurality of electrical signals.

9. The method of claim 8, wherein the filtering comprises a band pass filter using a Hanning window.

10. The method of claim 1, wherein the filtering comprises using a narrow band filter selected from the group consisting of finite impulse response filtering, and infinite impulse response filtering.

11. The method of claim 1, wherein the filtering comprises using one of n narrow band filters to mask influence of candidate frequencies not under evaluation.

12. The method of claim 11, wherein n=8 and wherein each of the eight narrow band filters is separated by a fixed difference in center frequency in a range of approximately 25 bpm to approximately 30 bpm.

13. The method of claim 11, wherein each of the n narrow band filters is separated by a variable difference in center frequency in a range of approximately 25 bpm to approximately 30 bpm.

14. The method of claim 1, wherein the filtering comprises using a narrow band filter of variable center frequency.

15. The method of claim 1, wherein the filtering comprises using a narrow band filter wherein filter coefficients are generated and adjusted so that a center frequency of the narrow band filter is approximately a center frequency associated with each of the candidate peaks.

16. The method of claim 1, wherein the selection parameters includes at least one of:

pulse width variability;

SpO₂ variability;

pulse window SpO₂;

pulse peak amplitude variability;

pulse rate history percentage;

window pulse rate; and

pulse window confidence.

17. The method of claim 1, wherein the selection parameters include at least one of:

pulse width variability calculated as a sum of absolute differences between individual pulse widths and an average pulse width normalized by the average pulse width;

SpO₂ variability calculated as a sum of absolute difference between individual SpO₂ values and an average SpO₂ for a given pulse window;

pulse window SpO₂ calculated by taking a measure of central tendency of all individual SpO₂ calculations in a given pulse window;

pulse peak amplitude variability calculated as a sum of differences between individual pulse peak amplitudes and average pulse peak amplitude for a given pulse window;

pulse rate history percentage calculated as a percentage of time that a pulse rate corresponding to a candidate peak has occurred in a given period of time;

pulse window confidence calculated as a weighted sum of pulse width variability, SpO₂ variability, pulse peak amplitude variability and pulse rate history percentage; and

window pulse rate calculated by dividing a sum of all pulse width times of all peaks in a data segment by a quantity of peaks detected in the data segment.

18. A method of determining parameters related to blood flow, comprising:

acquiring a segment of red data by transforming an electrical signal representative of an attenuated red light signal;

acquiring a segment of infrared data by transforming an electrical signal representative of an attenuated infrared light signal;

transforming the segment of red data and the segment of infrared data into red frequency domain data and infrared frequency domain data, respectively;

identifying a plurality of candidate peaks from the red frequency domain data and the infrared frequency domain data;

eliminating harmonic frequencies from the plurality of candidate peaks such that no two of the plurality of candidate peaks comprise harmonics of one another;

filtering each of the plurality of candidate peaks;

developing selection parameters associated with each of the plurality of candidate peaks;

selecting a best frequency from at least some of the plurality of candidate peaks by employing at least some of the selection parameters; and

repeating the above acts for a new segment of red data and a new segment of infrared data.

19. The method of claim 18, further comprising outputting at least one of a pulse rate, SpO₂, and a blood oxygen saturation, relating to the best frequency.

20. The method of claim 18, wherein selecting the best frequency comprises analyzing each of the plurality of candidate peaks with respect to at least some of the selection parameters to find the best frequency.

21. The method of claim 18, wherein the identifying the plurality of candidate peaks comprises:

assigning a largest power amplitude as a primary candidate peak;

assigning a next largest power amplitude as a secondary candidate peak; and

assigning a previous non-zero pulse rate as a tertiary candidate peak if the previous non-zero pulse rate is neither the primary candidate peak nor the secondary candidate peak.

22. The method of claim 21, wherein selecting the best frequency comprises rejecting a candidate peak of the plurality of candidate peaks if a number of peaks in a pulse window associated with the candidate peak is two or less, or a window pulse rate associated with the candidate peak is zero.

23. The method of claim 22, wherein selecting the best frequency further comprises:

selecting the primary candidate peak;

if the primary candidate peak pulse window confidence is greater than the secondary candidate peak pulse window confidence by a first threshold, and the primary candidate peak pulse rate history percentage is greater than a second threshold;

or

if the secondary candidate peak frequency is a rough harmonic of the primary candidate peak frequency and the primary candidate peak pulse window confidence is no more than a specified number of points greater than the secondary candidate peak pulse window confidence;

selecting the secondary candidate peak if the secondary candidate peak pulse window confidence is greater than the primary candidate peak pulse window confidence by the first threshold, and the secondary candidate peak pulse rate history percentage is greater than the second threshold;

selecting the tertiary candidate peak if the tertiary candidate peak pulse window confidence is less than a pulse window confidence for either the primary candidate peak or the secondary candidate peak; and

selecting no candidate peak;

if the primary candidate peak frequency is zero;

or

if the primary candidate peak and the secondary candidate peak have both been rejected.

24. The method of claim 18, wherein the selection parameters include at least one of:

pulse width variability calculated as a sum of absolute differences between individual pulse widths and an average pulse width normalized by the average pulse width;

SpO₂ variability calculated as a sum of absolute difference between individual SpO₂ values and an average SpO₂ for a given pulse window;

pulse window SpO₂ calculated by taking a measure of central tendency of all individual SpO₂ calculations in a given pulse window;

pulse peak amplitude variability calculated as a sum of differences between individual pulse peak amplitudes and average pulse peak amplitude for a given pulse window;

pulse rate history percentage calculated as a percentage of time that a pulse rate corresponding to a candidate peak has occurred in a given period of time;

pulse window confidence calculated as a weighted sum of pulse width variability, SpO₂ variability, pulse peak amplitude variability and pulse rate history percentage; and

window pulse rate calculated by dividing a sum of all pulse width times of all peaks in a data segment by a quantity of peaks detected in the data segment.

25. A motion artifact analyzer, comprising:

a processor for processing at least one computer program; and

a memory operably coupled to the processor and storing the at least one computer program, the at least one computer program including instructions for implementing a method of removing motion artifacts, the method comprising;

acquiring a segment of red data by transforming an electrical signal representative of an attenuated red light signal;

acquiring a segment of infrared data by transforming an electrical signal representative of an attenuated infrared light signal;

transforming the segment of red data and the segment of infrared data into red frequency domain data and infrared frequency domain data, respectively;

identifying a plurality of candidate peaks from the red frequency domain data and the infrared frequency domain data;

eliminating harmonic frequencies from the plurality of candidate peaks such that no two of the plurality of candidate peaks comprise harmonics of one another;

filtering each of the plurality of candidate peaks;

developing selection parameters associated with each of the plurality of candidate peaks;

selecting a best frequency from at least some of the plurality of candidate peaks by employing at least some of the selection parameters; and

repeating the above acts for a new segment of red data and a new segment of infrared data.

26. The motion artifact analyzer of claim 25, wherein selecting the best frequency comprises analyzing each of the plurality of candidate peaks with respect to at least some of the selection parameters to find the best frequency.

27. The motion artifact analyzer of claim 25, wherein the processor is a digital signal processor.

28. The motion artifact analyzer of claim 25, wherein the method further comprises calculating and outputting at least one of pulsatile blood oxygen concentration, SpO₂, and pulse rate, using the best frequency.

29. A pulse oximeter including an input device, an output device, and a motion artifact analyzer, wherein the motion artifact analyzer comprises:

a processor for processing at least one computer program; and

a memory operably coupled to the processor and storing the at least one computer program, the at least one computer program including instructions for implementing a method of removing motion artifacts, the method comprising;

acquiring a segment of red data by transforming an electrical signal representative of an attenuated red light signal;

acquiring a segment of infrared data by transforming an electrical signal representative of an attenuated infrared light signal;

transforming the segment of red data and the segment of infrared data into red frequency domain data and infrared frequency domain data, respectively;

identifying a plurality of candidate peaks from the red frequency domain data and the infrared frequency domain data;

eliminating harmonic frequencies from the plurality of candidate peaks such that no two of the plurality of candidate peaks comprise harmonics of one another;

filtering each of the plurality of candidate peaks;

developing selection parameters associated with each of the plurality of candidate peaks;

selecting a best frequency from at least some of the plurality of candidate peaks by employing at least some of the selection parameters; and

repeating the above acts for a new segment of red data and a new segment of infrared data.

30. The pulse oximeter of claim 29, wherein selecting the best frequency comprises analyzing each of the plurality of candidate peaks with respect to at least some of the selection parameters to find the best frequency.

31. The pulse oximeter of claim 29, wherein the processor is a digital signal processor.

32. The pulse oximeter of claim 29, wherein the method further comprises calculating and outputting at least one of pulsatile blood oxygen concentration, SpO₂, and pulse rate, using the best frequency.

33. A computer readable medium having stored thereon a plurality of computer instructions, which cause a computing system to perform a method of removing motion artifacts, the method comprising:

acquiring a segment of red data by transforming an electrical signal representative of an attenuated red light signal;

acquiring a segment of infrared data by transforming an electrical signal representative of an attenuated infrared light signal;

transforming the segment of red data and the segment of infrared data into red frequency domain data and infrared frequency domain data, respectively;

identifying a plurality of candidate peaks from the red frequency domain data and the infrared frequency domain data;

eliminating harmonic frequencies from the plurality of candidate peaks such that no two of the plurality of candidate peaks comprise harmonics of one another;

filtering each of the plurality of candidate peaks;

developing selection parameters associated with each of the plurality of candidate peaks;

selecting a best frequency from at least some of the plurality of candidate peaks by employing at least some of the selection parameters; and

repeating the above acts for a new segment of red data and a new segment of infrared data.

34. The computer readable medium of claim 33, wherein selecting the best frequency comprises analyzing each of the plurality of candidate peaks with respect to at least some of the selection parameters to find the best frequency.

35. The computer readable medium of claim 33, wherein the computing system includes a digital signal processor.

36. The computer readable medium of claim 33, wherein the method further comprises calculating and outputting at least one of pulsatile blood oxygen concentration, SpO₂, and pulse rate, using the best frequency.