Method and apparatus for estimating physiological parameters using model-based adaptive filtering

• US 5,853,364 A
• Filed: 06/07/1996
• Issued: 12/29/1998
• Est. Priority Date: 08/07/1995
• Status: Expired due to Term

• Current Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Yorkey, Thomas J., Baker, Clark R. Jr.
• Primary Examiner(s): Bahr, Jennifer
• Assistant Examiner(s): HUANG, STEPHEN D
• Application Number:
  US08/660,510
• Time in Patent Office: 935 Days
• Field of Search: 128/630, 128/633, 128/634, 128/665, 128/666, 128/664, 128/680, 367/135, 600/300, 600/310, 600/342, 600/323, 600/322, 600/324
• US Class Current: 600/300
• CPC Class Codes: A61B 5/0075 by spectroscopy, i.e. measuring spectra, e.g. Raman spectroscopy, infrared absorption spectroscopy A61B5/0071 takes precedence
• CPC Class Codes: A61B 5/024 Detecting, measuring or recording pulse rate or heart rate A61B5/0205, A61B5/021 take precedence
• CPC Class Codes: A61B 5/02416 using photoplethysmograph signals, e.g. generated by infrared radiation A61B5/14552 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/7203 for noise prevention, reduction or removal
• 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
• CPC Class Codes: H03H 21/003 KALMAN filters

Abstract

A method and apparatus for reducing the effects of noise on a system for measuring physiological parameters, such as, for example, a pulse oximeter. The method and apparatus of the invention take into account the physical limitations on various physiological parameters being monitored when weighting and averaging a series of measurements. Varying weights are assigned different measurements, measurements are rejected, and the averaging period is adjusted according to the reliability of the measurements. Similarly, calculated values derived from analyzing the measurements are also assigned varying weights and averaged over adjustable periods. More specifically, a general class of filters such as, for example, Kalman filters, is employed in processing the measurements and calculated values. The filters use mathematical models which describe how the physiological parameters change in time, and how these parameters relate to measurement in a noisy environment. The filters adaptively modify a set of averaging weights to optimally estimate the physiological parameters.

Claims

1. A method for reducing noise effects in a system for measuring a physiological parameter comprising the steps of:

generating a plurality of measurements derived from at least one wavelength of electromagnetic energy transmitted through living tissue;

comparing selected measurements with at least one expected measurement characteristic,

assigning one of a plurality of variable weights to each selected measurement based-on the comparing step, thereby generating a plurality of weighted measurements;

assigning the plurality of variable weights in part, in response to a similarity between each selected measurement and a corresponding previous measurements;

averaging the plurality of weighted measurements to obtain a filtered measurement for use in estimating the physiological parameter; and

selectively adjusting at least one step in the averaging and assigning steps, the adjusting step based on knowledge, derived independently of the plurality of measurements in the generating step, of at least one characteristics of the physiological parameter.

2. The method of claim 1 wherein a number of the differently weighted measurements averaged varies in response to the assigning step.

3. The method of claim 1 wherein a first number of differently weighted measurements are averaged to obtain the filtered measurement, the first number varying according to weights assigned to a plurality of successive differently weighted measurements in the assigning step, and wherein, over time, a plurality of filtered measurements are generated for each wavelength.

4. The method of claim 1 wherein the plurality of measurements comprises first and second sets of measurements corresponding to first and second wavelengths, respectively, and wherein the steps of comparing and assigning are performed on the first set of measurements, and further comprising the step of assigning to the second set of measurements weights identical to those assigned to the first set of measurements, thereby obtaining separate filtered measurements for each of the first and second wavelengths.

5. The method of claim 1 wherein the plurality of measurements are obtained by combining data from at least two wavelengths.

6. The method of claim 1 wherein the variable weight assigned to each selected measurement is based in part on a rate of change between the selected measurement and a previous measurement.

7. The method of claim 1 wherein said generating step comprises:

taking a logarithm of a signal representative of the at least one wavelength of electromagnetic energy, thereby generating a first signal; and

band pass filtering the first signal, thereby generating a second signal from which the plurality of measurements are derived.

8. The method of claim 7 wherein said generating step further comprises normalizing the second signal, thereby generating a third signal from which the plurality of measurements are derived.

9. The method of claim 8 wherein said generating step further comprises taking a derivative of the third signal, thereby generating a fourth signal from which the plurality of measurements are derived.

10. The method of claim 1 wherein a similarity between each selected measurement and the corresponding previous measurement is compared to an expected value.

11. The method of claim 10 wherein the expected value corresponds to a physiological model.

12. The method of claim 10 wherein the expected value corresponds to a rate of change between a plurality of corresponding previous measurements.

13. The method of claim 1 wherein the corresponding previous measurement corresponds to a filtered measurement.

14. The method of claim 1 wherein the at least one expected measurement characteristic comprises a prediction corresponding to at least one previous filtered measurement.

15. The method of claim 1 wherein the selected measurements comprise digital signals.

16. A method for reducing noise effects in a system for measuring a physiological parameter comprising the steps of:

generating a plurality of measurements corresponding to a series of cardiac pulses;

comparing each selected measurement with at least one expected measurement characteristic;

assigning one of a plurality of variable weights to each selected measurement based on the comparing step, thereby generating a plurality of weighted measurements;

averaging the plurality of weighted measurements from successive pulses to obtain a filtered measurement, each weighted measurement corresponding to a particular filtered measurement being similarly situated in a corresponding one of the successive pulses; and

selectively adjusting at least one step in the averaging and assigning steps, the adjusting step based on knowledge, derived independently of the plurality of measurements in the generating step, of at least one characteristics of cardiac pulses.

17. The method of claim 16 wherein a first number of differently weighted measurements are averaged to obtain the filtered measurement, the first number varying according to weights assigned to a plurality of successive differently weighted measurements in the assigning step, and wherein, over time, a plurality of filtered measurements are generated for each wavelength.

18. The method of claim 16 wherein the variable weight assigned to each selected measurement is based in part on a rate of change between the selected measurement and a previous measurement.

19. The method of claim 16 wherein said generating step comprises:

taking a logarithm of a signal representative of the at least one wavelength of electromagnetic energy, thereby generating a first signal;

band pass filtering the first signal, thereby generating a second signal from which the plurality of measurements are derived.

20. An apparatus for reducing noise effects in a system for measuring a physiological parameter, comprising:

means for generating a plurality of measurements corresponding to a series of cardiac pulses;

means for comparing each selected measurement with at least one expected measurement characteristic;

means for assigning one of a plurality of variable weights to each measurement based on the comparing step, thereby generating a plurality of weighted measurements;

means for averaging the plurality of weighted measurements from successive pulses to obtain a filtered measurement, each weighted measurement corresponding to a particular filtered measurement being similarly situated in a corresponding one of the successive pulses; and

means for selectively adjusting at least one means in the averaging and assigning means, the adjusting means being based on knowledge, derived independently of the plurality of measurements in the generating means, of at least one characteristics of cardiac pulses.

21. A method for reducing noise effects in a system for measuring a physiological parameter comprising the steps of:

generating a plurality of measurements derived from at least one wavelength of electromagnetic energy transmitted through living tissue;

comparing selected measurements with at least one expected measurement characteristic;

assigning one of a plurality of variable weights to each selected measurement based on the comparing step, thereby generating a plurality of weighted measurements;

averaging the plurality of weighted measurements to obtain a filtered measurement; and

selectively adjusting, in a non-linear manner, at least one step in the averaging and assigning steps based on characteristics of the physiological parameter.

22. The method of claim 21 wherein a first number of differently weighted measurements are averaged to obtain the filtered measurement, the first number varying according to weights assigned to a plurality of successive differently weighted measurements in the assigning step, and wherein, over time, a plurality of filtered measurements are generated for each wavelength.

23. The method of claim 21 wherein the variable weight assigned to each selected measurement corresponds to a rate of change between the selected measurement and a previous measurement.

24. The method of claim 21 wherein said generating step comprises:

taking a logarithm of a signal representative of the at least one wavelength of electromagnetic energy, thereby generating a first signal;

band pass filtering the first signal, thereby generating a second signal from which the plurality of measurements are derived.

25. The method of claim 24 wherein said generating step further comprises normalizing the second signal, thereby generating a third signal from which the plurality of measurements are derived.

26. The method of claim 25 wherein said generating step further comprises taking a derivative of the third signal, thereby generating a fourth signal from which the plurality of measurements are derived.

32. The method of claim 25 wherein said generating step comprises:

taking a logarithm of a signal representative of the at least one wavelength of electromagnetic energy, thereby generating a first signal;

band pass filtering the first signal, thereby generating a second signal from which the plurality of time-based measurement are derived.

33. The method of claim 32 wherein said generating step further comprises normalizing the second signal, thereby generating a third signal from which the plurality of time-based measurements are derived.

34. The method of claim 33 wherein said generating step further comprises taking a derivative of the third signal, thereby generating a fourth signal from which the plurality of time-based measurements are derived.

27. A method for reducing noise effects in a system for measuring a physiological parameter comprising the steps of:

generating a plurality of time-based measurements which are not event driven, the time-based measurements being derived from at least one wavelength of electromagnetic energy transmitted through living tissue;

comparing selected time-based measurements with at least one expected measurement characteristic;

assigning one of a plurality of variable weights to each selected time-based measurement based on the comparing step, thereby generating a plurality of weighted time-based measurements;

averaging the plurality of weighted time-based measurements to obtain a filtered time-based measurement; and

selectively adjusting at least one step in the averaging and assigning steps, the adjusting step based on knowledge, derived independently of the plurality of time-based measurements in the generating step, of at least one characteristics of the physiological parameter.

28. The method of claim 27 wherein a number of the differently weighted time-based measurements averaged varies in response to the assigning step.

29. The method of claim 27 wherein a first number of differently weighted time-based measurements are averaged to obtain the filtered time-based measurement, the first number varying according to weights assigned to a plurality of successive weighted measurements in the assigning step, and wherein, over time, a plurality of filtered time-based measurements are generated for each wavelength.

30. The method of claim 27 wherein the plurality of time-based measurements are obtained by combining data from at least two wavelengths of electromagnetic energy.

31. The method of claim 27 wherein the variable weight assigned to each time-based measurement corresponds to a rate of change between the time-based measurement and a previous time-based measurement.

35. The method of claim 27 wherein a confidence level is associated with each filtered time-based measurement, the method further comprising the step of arbitrating between the filtered time-based measurement and another measurement with regard to the associated confidence levels to determine a best measurement.

36. The method of claim 27 wherein each filtered time-based measurement corresponds to a current value of a ratio of ratios which is representative of oxygen saturation of hemoglobin in arterial blood.

37. The method of claim 27 wherein each filtered time-based measurement corresponds to a current value of oxygen saturation of hemoglobin in arterial blood.

38. The method of claim 37 wherein the at least one expected measurement characteristic comprises a constant representative of a rate of change of the oxygen saturation value.

39. The method of claim 27 wherein the plurality of time-based measurements are normalized prior to the comparing step, thereby reducing noise effects corresponding to motion artifact on some of the time-based measurements.

40. An apparatus for reducing noise effects in a system for measuring a physiological parameter, comprising:

means for generating a plurality of time-based measurements which are not event driven, the time-based measurements being derived from at least one wavelength of electromagnetic energy transmitted through living tissue;

means for comparing selected time-based measurements with at least one expected measurement characteristic;

means for assigning one of a plurality of variable weights to each selected time-based measurement based on the comparing step, thereby generating a plurality of weighted time-based measurements;

means for averaging the plurality of the weighted time-based measurements to obtain a filtered time-based measurement; and

means for selectively adjusting at least one means in the averaging and assigning means, the adjusting means being based on knowledge, derived independently of the plurality of time-based measurements in the generating means, of at least one characteristics of the physiological parameter.

41. An apparatus for reducing noise effects in a system for measuring a physiological parameter, comprising:

means for generating a plurality of measurements derived from at least one wavelength of electromagnetic energy transmitted through living tissue;

means for providing a signal indicative of the at least one wavelength of electromagnetic energy;

means for comparing selected measurements with at least one expected measurement characteristic;

means for assigning one of a plurality of variable weights to each selected measurement based on the comparing step thereby generating a plurality of differently weighted measurements for each wavelength, the variable weights being assigned, in part, in response to a similarity between each selected measurement and a corresponding previous measurement, the variable weights comprising a plurality of different non-zero numbers;

means for averaging a plurality of the differently weighted measurements to obtain a filtered measurement for use in estimating the physiological parameter; and

means for calibrating the system to measure the physiological parameter in response to the signal indicative of the at least one wavelength of electromagnetic energy.