Physiological monitoring and related methods
A method of estimating oxygen saturation includes illuminating a target site with light of three different wavelengths, for example green, red and infrared, and detecting light returned from the site at each of the wavelengths. The method distinguishes between cardiac pulsatility and nonpulsatility based on a parameter of the return light of the shortest wavelength. The method formulates an estimate of oxygen saturation as a function of a parameter of the light of the two longer wavelengths taken at both a period of pulsatility and at a period of nonpulsatility. An oximeter adapted to carry out the method is also disclosed. Aspects of the method and oximeter may be used to estimate other physiological parameters.
1. A method of estimating oxygen saturation comprising:
illuminating a target site with light of a first wavelength, light of a second wavelength and light of a third wavelength, the second wavelength being greater than the first wavelength and the third wavelength being greater than the second wavelength;
detecting light returned from the site at each of the wavelengths;
distinguishing between pulsatility and nonpulsatility based on a first parameter of the return light of the first wavelength, wherein the first parameter comprises an intensity of the light of the first wavelength, wherein the nonpulsatility corresponds to the intensity of the light of the first wavelength being greater than a first threshold and the pulsatility corresponds to the intensity of the light of the first wavelength being less than a second threshold; and
formulating a first estimate of oxygen saturation as a function of:
a second pulsatile parameter corresponding to the light returned in response to the illumination with the light of the second wavelength during a pulsatile period,
a second nonpulsatile parameter corresponding to the light returned in response to the illumination with the light of the second wavelength during a nonpulsatile period,
a third pulsatile parameter corresponding to the light returned in response to the illumination with the light of the third wavelength during a pulsatile period, and
a third nonpulsatile parameter corresponding to the light returned in response to the illumination with the light of the third wavelength during a nonpulsatile period.
2. The method of claim 1 comprising formulating a second estimate of oxygen saturation based on the first estimate of oxygen saturation and a calibration relationship.
3. The method of claim 1 wherein the step of formulating a first estimate of oxygen saturation is carried out concurrently with the illuminating step.
4. The method of claim 1 wherein the first parameter, the second pulsatile parameter, the second nonpulsatile parameter, the third pulsatile parameter and the third nonpulsatile parameter are all the same parameter.
5. The method of claim 4 wherein the same parameter is light intensity.
6. The method of claim 1 wherein the first wavelength is in the blue to green portion of the electromagnetic spectrum, the second wavelength is in the red portion of the electromagnetic spectrum, and the third wavelength is in the infrared portion of the electromagnetic spectrum.
7. The method of claim 1 wherein the first wavelength is in the visible green portion of the electromagnetic spectrum, the second wavelength is in the visible red portion of the spectrum and the third wavelength is in the infrared portion of the spectrum.
8. The method of claim 1 wherein the step of illuminating the target site is carried out with pulses of light of the first, second, and third wavelengths such that the pulses of light of the second and third wavelengths are nested between two successive pulses of light of the first wavelength.
9. The method of claim 1 , further comprising determining that the pulsatility and nonpulsatility is indeterminate if the intensity of light of the first wavelength is between the first threshold and the second threshold.
10. The method of claim 9 , wherein, if the pulsatility and nonpulsatility is inderminate, formulating a first estimate of oxygen saturation is postponed until the intensity of light of the first wavelength is either above the first threshold or below the second threshold.
11. The method of claim 1 , further comprising outputting the first estimate of oxygen saturation to a display.
12. The method of claim 1 , further comprising outputting the first estimate of oxygen saturation to an electronic medical record.
13. The method of claim 2 , further comprising outputting the second estimate of oxygen saturation to a display.
14. The method of claim 2 , further comprising outputting the second estimate of oxygen saturation to an electronic medical record.
15. The method of claim 1 , wherein illuminating the target site with light of the first wavelength, light of a second wavelength and light of a third wavelength comprises emitting light of the first, second, and third wavelengths through a same aperture of a housing.
16. A blood oxygenation assessment instrument comprising:
an emitter for emitting light of a first wavelength, a second wavelength, and a third wavelength, the second wavelength being greater than the first wavelength and the third wavelength being greater than the second wavelength;
a light detector for detecting a return light signal at each of the first, second and third wavelengths; and
a processor and executable instructions which, when executed by the processor, cause the processor to:
1) distinguish between pulsatility and nonpulsatility based on an intensity of the first wavelength return light signal, wherein the nonpulsatility corresponds to the intensity of the first wavelength return light signal being greater than a first threshold and the pulsatility corresponds to the intensity of the first wavelength return light signal being less than a second threshold; and
2) formulate a first estimate of oxygen saturation based on:
a) a property of the second wavelength return light signal corresponding to a period of pulsatility;
b) a property of the second wavelength return light signal corresponding to a period of nonpulsatility;
c) a property of the third wavelength return light signal corresponding to a period of pulsatility;
d) a property of the third wavelength return light signal corresponding to a period of nonpulsatility.
17. The instrument of claim 16 wherein the property is light intensity.
18. The instrument of claim 16 , further comprising a housing in which the emitter, the light detector, and the processor are situated.
19. The instrument of claim 18 , wherein the housing includes a first aperture through which the light of the first, second, and third wavelength is emitted from the housing.
20. The instrument of claim 19 , wherein the housing includes a second aperture through which reflected light is returned into the housing.
21. The instrument of claim 16 , further comprising a driver to drive the light sources in a successive manner.
22. The instrument of claim 16 , wherein the first wavelength corresponds to green light, the second wavelength corresponds to red light, and the third wavelength corresponds to infrared light.
23. The instrument of claim 16 , wherein the executable instructions, when executed by the processor, also cause the processor to formulate a second estimate of oxygen saturation based on the first estimate of oxygen saturation and a calibration relationship.