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

• US 6,721,584 B2
• Filed: 06/06/2001
• Issued: 04/13/2004
• 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: Baker, Clark R. Jr., Yorkey, Thomas J.
• Primary Examiner(s): Winakur, Eric F.
• Application Number:
  US09/876,004
• Publication Number:
  US 20020045806A1
• Time in Patent Office: 1,042 Days
• Field of Search: 600/310, 600/322, 600/323, 600/331, 600/330, 600/336
• US Class Current: 600/323
• 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 calculating oxygen saturation of hemoglobin in arterial blood using data corresponding to a plurality of wavelengths of electromagnetic energy transmitted through tissue of a patient, the method comprising:

determining extinction coefficients corresponding to the plurality of wavelengths; and

calculating values proportional to total hemoglobin and oxygenated hemoglobin directly from the data and the extinction coefficients, variably weighing the calculated oxygenated hemoglobin value based on its similarity and dissimilarity to past calculated values of oxygenated hemoglobin, wherein calculating said values directly from the data and extinction coefficients is calculating said values without first calculating a quotient of the pulsatile component of light transmission over the constant component of light or a ratio of ratios.

2. The method of claim 1 wherein a value corresponding to oxygen saturation of hemoglobin is calculated from the values proportional to total hemoglobin and oxygenated hemoglobin.

3. A method for calculating oxygen saturation of hemoglobin in arterial blood using data corresponding to a plurality of wavelengths of electromagnetic energy transmitted through tissue of a patient, the method comprising:

determining extinction coefficients corresponding to the plurality of wavelengths; and

calculating values proportional to total hemoglobin and oxygenated hemoglobin directly from the data and the extinction coefficients, variably weighing the calculated oxygenated hemoglobin value based on its similarity and dissimilarity to past calculated values of oxygenated hemoglobin, wherein calculating values comprises;

preprocessing said data to get preprocessed data, wherein said preprocessing comprises;

taking the natural logarithm of the data and band-pass filtering the data to get band-pass-filtered data;

processing said data through a saturation calculator;

calculating, using said saturation calculator, at least a first and a second oxygen saturation value;

where said at least first oxygen saturation value is calculated using a harmonic filter and a Kalman-filter-based cardiac gated averaging (“

CGA”

) and

where said at least second oxygen saturation value is calculated using said band-pass-filtered data;

where both said at least first and second oxygen saturation values are calculated using instantaneous time-based signals; and

selecting one of said at least first and second oxygen saturation values to determine a best saturation value.

4. The method of claim 3 wherein a value corresponding to oxygen saturation of hemoglobin is calculated from the values proportional to total hemoglobin and oxygenated hemoglobin.