Pulse oximeter with calibration stabilization

• US 6,711,425 B1
• Filed: 05/28/2002
• Issued: 03/23/2004
• Est. Priority Date: 05/28/2002
• Status: Expired due to Term

• Current Assignee: Prothia
• Original Assignee: OB Scientific Incorporated
• Inventors: Reuss, James L.
• Primary Examiner(s): Hindenburg, Max F.
• Assistant Examiner(s): KREMER, MATTHEW J
• Application Number:
  US10/156,480
• Time in Patent Office: 665 Days
• Field of Search: 600/322-323, 600/331, 600/338
• US Class Current: 600/331
• CPC Class Codes: A61B 2560/0223 of calibration, e.g. protocols for calibrating sensors
• CPC Class Codes: A61B 5/14542 for measuring blood gases A61B5/14551 takes precedence
• CPC Class Codes: A61B 5/14551 for measuring blood gases
• CPC Class Codes: A61B 5/1464 specially adapted for foetal tissue
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

An improved pulse oximeter (sensor and monitor) uses a plurality of wavelengths selected to provide sensitivity to both oxygen saturation and deviations in tissue site characteristic(s) from conditions at calibration. The monitor detects and/or removes the effects of deviations on SpO₂ calibration, of particular value in fetal/newborn monitoring.

Claims

1. A method for determining oxygen saturation in a pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising the following steps:

exposing a tissue site to a plurality of wavelengths of light generated by the pulse oximeter;

defining a tissue site characteristic, other than oxygen saturation, which may influence the determination of oxygen saturation;

defining a ratio wavelength set as a first subset of the plurality of wavelengths for calculation of an oxygen saturation value;

defining a correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic, and including selecting the correction wavelength set to optimize the correction of the deviations;

detecting light of the plurality of wavelengths transmitted by or reflected from components of the tissue site and converting the detected light into a plurality of electrical signals;

digitizing and conditioning the plurality of electrical signals in a signal data acquisition component of the pulse oximeter to yield a plurality of input signals;

defining a set of correction function(s) operating upon the plurality of input signals and substantially removing deviations from the input signals corresponding to the ratio wavelength set;

calculating a plurality of normalized pulse amplitudes from the plurality of input signals in a processing component of the pulse oximeter;

detecting the deviation in input signals corresponding to the correction wavelength set, and if the deviation is present calculating a calibration deviation correction by applying the set of correction function(s) to the input signals corresponding to the correction wavelength set, and;

calculating an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, and utilizing the calibration deviation correction.

2. The method as defined in claim 1, wherein the tissue site characteristic is a fraction of blood in the tissue.

3. The method as defined in claim 1, wherein the tissue site characteristic is a hematocrit of blood in the tissue.

4. The method as defined in claim 1, wherein the tissue site characteristic is a relative proportion of arterial to venous blood in the tissue.

5. The method as defined in claim 1, wherein the specified range of oxygen saturation is 0-100%.

6. The method as defined in claim 1, wherein the specified range of oxygen saturation is 0-70%.

7. The method as defined in claim 1, wherein the specified range of oxygen saturation is 20-40%.

8. The method as defined in claim 1, wherein the selecting the correction wavelength set to optimize the correction of the deviation is based upon a differential sensitivity of a pair of wavelengths from the correction wavelength set to a perturbation of the tissue site characteristic, and the differential sensitivity is calculated as a change in slope of a ratio resulting from perturbation, nornmalized by a value of an unperturbed slope.

9. The method as defined in claim 1, wherein the ratio wavelength set consists of a red wavelength in the range of 650-699 nm, and an infrared wavelength in the range of 850-950 nm.

10. The method as defined in claim 1, wherein the correction wavelength set consists of one or more wavelength(s) of the ratio wavelength set, and another different red wavelength in the range of 650-699 nm.

11. The method as defined in claim 1, wherein the correction wavelength set consists of one or more wavelengths of the ratio wavelength set, and another wavelength, which is near an isobestic point of oxygenated versus reduced hemoglobin absorption, and found in the range of 800-815 nm.

12. The method as defined in claim 1, wherein the correction wavelength set consists of one or more wavelengths of the ratio wavelength set, and another wavelength in the range of 700-799 nm.

13. The method as defined in claim 1, wherein one set of correction function(s) among a plurality of sets of correction function(s) corresponding to a plurality of tissue site characteristic perturbations is chosen for use with the plurality of normalized pulse amplitudes.

14. The method as defined in claim 1, wherein the tissue site is located on a fetus in utero.

15. A method for determining oxygen saturation in a pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising the following steps:

exposing a tissue site to a plurality of wavelengths of light generated by the pulse oximeter;

defining a tissue site characteristic, other than oxygen saturation, which may influence the determination of oxygen saturation;

defining a ratio wavelength set as a first subset of the plurality of wavelengths for calculation of an oxygen saturation value, including selecting the ratio wavelength set to optimize oxygen saturation sensitivity over a specified range;

defining a correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic;

detecting light of the plurality of wavelengths transmitted by or reflected from components of the tissue site and converting the detected light into a plurality of electrical signals;

digitizing and conditioning the plurality of electrical signals in a signal data acquisition component of the pulse oximeter to yield a plurality of input signals;

defining a set of correction functions operating upon the plurality of input signals and substantially removing deviations from the input signals corresponding to the ratio wavelength set;

calculating a plurality of normalized pulse amplitudes from the plurality of input signals in a processing component of the pulse oximeter;

detecting the deviation in input signals corresponding to the correction wavelength set, and if the deviation is present calculating a calibration deviation correction by applying the set of correction functions to the input signals corresponding to the correction wavelength set, and;

calculating an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, and utilizing the calibration deviation correction.

16. The method as defined in claim 15, wherein the step of defining a ratio wavelength set includes determining a slope of a calibration curve of the ratio of the normalized pulse amplitudes versus oxygen saturation, the calibration curve corresponding to a pair of wavelengths from the ratio wavelength set;

and basing the selecting to optimize oxygen saturation sensitivity over the specified range upon maximizing the magnitude of a slope over the specified range of oxygen saturation.

17. A method for determining oxygen saturation in a pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising the following steps:

exposing a tissue site to a plurality of wavelengths of light generated by the pulse oximeter;

defining a tissue site characteristic, other than oxygen saturation, which may influence the determination of oxygen saturation;

defining a ratio wavelength set as a first subset of the plurality of wavelengths for calculation of an oxygen saturation value;

defining a correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic;

detecting light of the plurality of wavelengths transmitted by or reflected from components of the tissue site and converting the detected light into a plurality of electrical signals;

digitizing and conditioning the plurality of electrical signals in a signal data acquisition component of the pulse oximeter to yield a plurality of input signals;

defining a set of correction function(s) operating upon the plurality of input signals and substantially removing deviations from the input signals corresponding to the ratio wavelength set;

calculating a plurality of normalized pulse amplitudes from the plurality of input signals in a processing component of the pulse oximeter;

detecting the deviation in input signals corresponding to the correction wavelength set, and if the deviation is present calculating a calibration deviation correction by applying the set of correction functions to the input signals corresponding to the correction wavelength set, wherein the calibration deviation correction obtained from each correction function(s) is a corrected normalized pulse amplitude computed from the plurality of input signals and;

calculating an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, and utilizing the calibration deviation correction.

18. A method for determining oxygen saturation in a pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising the following steps:

exposing a tissue site to a plurality of wavelengths of light generated by the pulse oximeter;

defining a tissue site characteristic, other than oxygen saturation which may influence the determination of oxygen saturation;

defining a ratio wavelength set as a first subset of the plurality of wavelengths for calculation of an oxygen saturation value;

defining a correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic;

detecting light of the plurality of wavelengths transmitted by or reflected from components of the tissue site and converting the detected light into a plurality of electrical signals;

digitizing and conditioning the plurality of electrical signals in a signal data acquisition component of the pulse oximeter to yield a plurality of input signals;

defining a set of correction function(s) operating upon the plurality of input signals and substantially removing deviations from the input signals corresponding to the ratio wavelength set;

calculating a plurality of normalized pulse amplitudes from the plurality of input signals in a processing component of the pulse oximeter;

detecting the deviation in input signals corresponding to the correction wavelength set, and if the deviation is present calculating a calibration deviation correction by applying the set of correction function(s) to the input signals corresponding to the correction wavelength set, wherein the calibration deviation correction obtained from the correction function is a corrected ratio of normalized pulse amplitudes computed from the plurality of input signals and;

calculating an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, and utilizing the calibration deviation correction.

19. A method for determining oxygen saturation in a pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising the following steps:

exposing a tissue site to a plurality of wavelengths of light generated by the pulse oximeter;

defining a tissue site characteristic, other than oxygen saturation, which may influence the determination of oxygen saturation;

defining a ratio wavelength set as a first subset of the plurality of wavelengths for calculation of an oxygen saturation value;

defining a correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic;

detecting light of the plurality of wavelengths transmitted by or reflected from components of the tissue site and converting the detected light into a plurality of electrical signals;

digitizing and conditioning the plurality of electrical signals in a signal data acquisition component of the pulse oximeter to yield a plurality of input signals;

defining a set of correction function(s) operating upon the plurality of input signals and substantially removing deviations from the input signals corresponding to the ratio wavelength set;

calculating a plurality of normalized pulse amplitudes from the plurality of input signals in a processing component of the pulse oximeter;

detecting the deviation in input signals corresponding to the correction wavelength set, including assessing an approximate oxygen saturation level calculated from an uncorrected normalized pulse amplitudes, and applying the correction function(s) only if the approximate oxygen saturation level is within a range of application for the correction function(s) and if the deviation is present calculating a calibration deviation correction by applying the set of correction function(s) to the input signals corresponding to the correction wavelength set, and;

calculating an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, and utilizing the calibration deviation correction.

20. A pulse oximeter having a calibration stabilization process for reducing a deviation in calibration comprising:

a sensor device comprising light emitting device(s) and detector(s) placed against a tissue site;

a monitor device comprising at least a data acquisition component, a memory storage component, a visual presentation component, and a computing component having a capability for oxygen saturation determination with calibration stabilization for quantifying and/or correcting and/or eliminating a calibration deviation(s) resulting from changes in tissue site characteristics in a pulse oximetry signal, wherein the light emitting device(s) emit a plurality of wavelengths, the pulse oximeter utilizing in the computing component (a) a ratio wavelength set comprising at least one combination of the plurality of wavelengths, the one combination possessing high sensitivity to blood oxygen saturation changes, and (b) a correction wavelength set comprising at least two wavelengths, of which at least one of the two wavelengths is not already employed in the ratio wavelength set and which exhibits substantially different dependence upon a tissue site characteristic other than blood oxygen saturation.

21. The pulse oximeter as defined in claim 20, wherein the capability for, oxygen saturation determination with calibration stabilization includes:

defining a tissue site characteristic, other than oxygen saturation, which may influence the determination of oxygen saturation;

defining the ratio wavelength set as a first subset of a plurality of wavelengths for calculation of an oxygen saturation value;

defining the correction wavelength set as a second subset of the plurality of wavelengths for detection and correction of a deviation in the calibration of the oxygen saturation calculation over a specified range of oxygen saturation, due to a perturbation of the tissue site characteristic;

defining a set of correction function(s); and

wherein the correction function(s) operate upon a plurality of input signals related to detected light received by the detector(s) and substantially remove deviations from the input signals corresponding to the ratio wavelength set; and

wherein the computing component calculates a plurality of normalized pulse amplitudes from the plurality of input signals and detects a deviation in an input signals corresponding to the correction wavelength set, and if the deviation is present calculates a calibration deviation correction by applying the set of correction function(s) to the input signals corresponding to the correction wavelength; and

calculates an oxygen saturation value from a ratio of normalized pulse amplitudes corresponding to the ratio wavelength set, utilizing the calibration deviation correction.

22. The pulse oximeter of claim 20, wherein the monitor device processes signal data acquired for each wavelength of the plurality of wavelengths in order to recalculate signals of the ratio wavelength set, based upon both a signal data of the ratio wavelength set, and a signal data of the correction wavelength set.

23. The pulse oximeter of claim 22, wherein the signal data of the ratio wavelength set and/or the signal data of the correction wavelength set are possibly perturbed.

24. The pulse oximeter as defined in claim 20, wherein the wavelengths of the light emitting device(s) of the sensor device are chosen so that a calibration deviation resulting from changes in more than one tissue site characteristic is quantified and/or corrected by employing more than one correction wavelength set.

25. The pulse oximeter as defined in claim 24, wherein the monitor device determines which correction wavelength set, if any, to employ in recalculating a signal data corresponding to the ratio wavelength set, based upon an estimate of a blood oxygen saturation level derived from the signal data corresponding to the ratio wavelength set.

26. The pulse oximeter as defined in claim 25, wherein the signal data corresponding to the ratio wavelength set is possibly perturbed.

27. The pulse oximeter as defined in claim 24, wherein the monitor device determines which correction wavelength set, if any, to employ in recalculating signal data corresponding to the ratio wavelength set, based upon a pattern of a magnitude(s) and a direction(s) of a deviation(s) in signal data corresponding to the correction wavelength set from an expected mathematical relationship(s) between a member(s) of each of the ratio wavelength set and the correction wavelength set.

28. The pulse oximeter system as defined in claim 20, wherein the sensor device is designed for intrauterine fetal pulse oximetry.

29. A calibration stabilization process for use in reducing calibration deviation in a pulse oximeter, the process comprising the steps of:

deriving a calibration surface and calibration error surface for a selected blood oxygen saturation range for a calibration deviation resulting from a change in a tissue site characteristics for a ratio wavelength set,

applying a correction function utilizing a correction wavelength set to operate on and remove the calibration deviation corresponding to the ratio wavelength set, wherein the correction function is a wavelength correction function; and

reducing the deviation across the blood oxygen saturation range for the change in the tissue site characteristics.

30. A calibration stabilization process for use in reducing calibration deviation in a pulse oximeter, the process comprising the steps of:

deriving a calibration surface and calibration error surface for a selected blood oxygen saturation range for a calibration deviation resulting from a change in a tissue site characteristics for a ratio wavelength set, wherein the ratio wavelength set possesses high sensitivity to blood oxygen saturation change;

applying a correction function utilizing a correction wavelength set to operate on and remove the calibration deviation corresponding to the ratio wavelength set, wherein the correction wavelength set includes at least two wavelengths, of which at least one wavelength is not in the ratio wavelength set and wherein the correction wavelength set exhibits different dependence upon the tissue site characteristics other than blood oxygen saturations;

and reducing the deviation across the blood oxygen saturation range for the change in the tissue site characteristics.

31. The calibration stabilization process as claimed in claim 30, wherein the oxygen saturation range is from 0% to 100%.

32. The calibration stabilization process as claimed in claim 30, wherein the correction function is a ratio correction function.