Adaptive calibration for pulse oximetry

• US 20030114738A1
• Filed: 12/04/2002
• Published: 06/19/2003
• Est. Priority Date: 12/06/2001
• Status: Active Grant

• Current Assignee: Philips RS North America LLC (Koninklijke Philips N.V.)
• Original Assignee: RIC INVESTMENTS LLC (Koninklijke Philips N.V.)
• Inventors: Shankar, Uday S., Iyer, Vijay K., Zonios, George
• Granted Patent: US 6,839,580 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

A method for calibrating a pulse oximeter device and an apparatus incorporating the method and a system for utilizing the method. The method is based on modeling light propagation in tissue at two wavelengths, typically, one in the red and one in the infrared range of the spectrum. A formula is derived relating the arterial oxygen saturation to a ratio R commonly measured by standard pulse oximeters. A specific parameter is identified and utilized in the calibration of the oximeter. This parameter is formulated in terms of the DC signals measured by the pulse oximeter at the two wavelengths. An empirical method for estimating this parameter based on experimental data is also described.

Claims

1. A method of calibrating a pulse oximeter comprising:

selecting a probe having an emitter, a detector, and a pathlength between the emitter and detector;

measuring transmission of light through a tissue of interest with the probe;

compiling a database of measured values characterizing the transmission of light through such a tissue;

formulating a value for a parameter k_DC as a function of at least one of the measured values; and

incorporating the parameter k_DC into a calibration equation for determining a value for SpO₂.

2. The method of claim 1, wherein the tissue of interest is a living being.

3. The method of claim 2, wherein the steps of measuring, compiling, formulating, and incorporating are repeated for the living being over a range of SaO₂ levels.

4. The method of claim 3, wherein the range of SaO₂ levels is greater than 70%.

5. The method of claim 3, wherein the range of SaO₂ levels is between about 15% and about 65%.

6. The method of claim 1, wherein the probe is an invasive probe.

7. The method of claim 1, wherein the probe'"'"'s pathlength is approximately 1 mm to about 5 mm.

8. The method of claim 1, wherein, in the formulating step, the measured value includes a DC value.

9. The method of claim 1, wherein, in the compiling step, the measured values includes an AC and a DC component.

10. The method of claim 1, wherein, before the incorporating step, the measured values are used to calculate a Ratio R.

11. A method of performing optical blood oximetry comprising:

measuring the transmission of light through tissue;

determining at least one DC value based upon the transmission of light measurements;

selecting a calibration parameter based upon the determined DC value;

estimating SaO₂ based upon the selected calibration parameter.

12. The method of claim 11, further comprising the step of providing updated oxygen saturation estimates adapted to changes in the measured transmission.

13. The method of claim 11, wherein the calibration parameter is k_DC.

14. The method of claim 13, wherein k_DC includes the partial derivation of values for D_r.

15. The method of claim 11, further comprising providing a pulse oximeter probe that includes a light source capable of emitting light at two or more wavelengths, a detector, and a measurable distance between the light source and the detector.

16. The method of claim 11, wherein the step of determining further includes:

quantifying an AC value based upon the transmission of light;

calculating a Ratio R; and

,

relating R to the calibration parameter that accounts for background absorption and scattering of light.

17. The method of claim 16, wherein the relating step further includes determining a partial derivative D_r value, and using the D_r value to relate R to the calibration parameter.

18. The method of claim 16, wherein the calibration parameter is k_DC.

19. The method of claim 11, wherein the estimating step comprises:

quantifying a k_DC value; and

selecting a corresponding SaO₂ based upon previously determined clinical data.

20. The method of claim 16, wherein the step of quantifying includes estimating a partial derivative D_r value.

21. A physiological condition measuring device comprising:

light generating means;

light detecting means, wherein an optical path having pathlength is defined between the light generating means and the light detecting means; and

a processing system that measures light incident upon the light detecting means and returns a solution derived from previously obtained clinical data and being indicative of the physiological condition based on the measured light.

22. A measuring device according to claim 21, wherein the physiological condition is oxygen saturation.

23. A measuring device of claim 21, wherein the processing system produces a measurement of SpO₂.

24. A measuring device according to claim 21, wherein the optical pathlength is less than about 1 cm.

25. A measuring device according to claim 21, wherein the processing system is calibrated to measure SaO₂ in a range of about 10% to about 70%.

27. The apparatus of claim 22 further comprising means for adaptively calibrating the predicted arterial oxygen saturation in response to the signals received from the probe.

28. The apparatus of claim 27, wherein the parameters calculated by the computing means include a k_DC parameter

26. An apparatus for determining oxygen saturation of hemoglobin in arterial blood using signals received from a probe, the signals being indicative of the light absorption of arterial blood, which has pulsatile and non-pulsatile components, at two or more light wavelengths, the apparatus comprising:

means responsive to the signals received from the probe;

means for converting the signals received into data;

memory means for storing the data;

computing means for calculating parameters from the data; and

means for relating the calculated parameters to a predicted arterial oxygen saturation.

29. A system for performing pulse oximetry comprising:

a probe including at least one light source for transmitting light and at least one detector for detecting the transmitted light;

a processing system in communication with the probe so as to receive signals from the probe, wherein the processing system determines data corresponding to

     1) variables indicative of the optical parameters of a tissue being probed based on the received signals and

     2) variables related to a determinable parameter that accounts for the scattering and absorption of light by the tissue, wherein the variables are mathematically formulated for comparison to a previously obtained database of variables that a value for SpO₂ can be output that is indicative of SaO₂.