Device and method for monitoring of absolute oxygen saturation and total hemoglobin concentration
A medical device for monitoring of oxygen saturation includes an optical sensor adapted for positioning adjacent to a tissue volume. The optical sensor has a light emitting portion capable of emitting light at a plurality of wavelengths and a light detecting portion capable of generating an electrical output signal corresponding to light incident on the detecting portion. A control module coupled to the optical sensor controls the light emitted by the light emitting portion. A monitoring module receives the output signal from the light detecting portion and computes a volume-independent measure of oxygen saturation in the volume of tissue using the output signal.
1. A medical device for monitoring oxygen saturation, comprising:
a sealed housing comprising a first opening and a second opening;
a light emitting portion comprising a first lens sealed in the first opening of the housing, the light emitting portion emitting light at a plurality of wavelengths including at least four spaced apart wavelengths through the lens into a volume of a patient's body tissue adjacent to the lens;
a light detecting portion comprising a second lens sealed in the second opening of the housing, the light detecting portion generating an electrical output signal corresponding to light incident on the light detecting portion, the incident light comprising light emitted by the emitting portion, scattered by the body tissue volume, and passing through the second lens;
a control module coupled to the light emitting portion to provide control signals controlling the light emitted by the light emitting portion; and
a monitoring module coupled to the light detecting portion to receive the output signal and to compute a volume-independent measure of oxygen saturation in the volume of tissue in response to the output signal, wherein each of the light emitting portion and the light detecting portion comprise a light source for emitting light and a light detector for detecting light and the functionality of the light emitting portion and the light detecting portion as one of emitting and detecting can be selectively switched.
2. The device of claim 1 , wherein the light emitting portion comprises at least four light sources each emitting light at separate spaced apart wavelengths.
3. The device of claim 2 , wherein the light emitting portion comprises a light diffusing material to diffuse the separate spaced apart wavelengths.
4. The device of claim 1 , wherein the separate spaced apart wavelengths comprises:
a first wavelength spacing between a first wavelength and a second wavelength; and
a second wavelength spacing between the second wavelength and a third wavelength, wherein the first and the second wavelength spacings are approximately equal.
5. The device of claim 2 , wherein the separate spaced apart wavelengths comprises:
a first wavelength spacing between a first wavelength and a second wavelength; and
a second wavelength spacing between the second wavelength and a third wavelength, wherein the first and the second wavelength spacings are unequal.
6. The device of claim 1 , wherein the light emitting portion comprises a reference photodetector providing a reference signal corresponding to an intensity of emitted light.
7. The device of claim 6 wherein the control module receives the reference signal, the control module configured to adjust a drive signal applied to the light emitting portion in response to the reference signal.
8. The device of claim 6 wherein the monitoring module receives the reference signal and computes the volume-independent measure of oxygen saturation using the reference signal.
9. The device of claim 1 , wherein the light emitting portion and the light detecting portion comprises:
a first light source and light detector pair spaced apart at a first emitting-to-detecting distance; and
a second light source and light detector pair spaced apart at a second emitting-to-detecting distance different than the first emitting-to-detecting distance.
10. The device of claim 1 , wherein the selective switching of the emitting and detecting functionality of the light emitting portion and the light detecting portion results in a change in an emitting-to-detecting distance between a light source selected to emit light and a light detector selected to detect light.
11. The device of claim 1 , wherein the housing comprises a cuff having a first surface and a second surface opposing the first surface, the light emitting portion positioned along the first surface and the light detecting portion positioned along the second surface in opposition to the light emitting portion.
12. The device of claim 1 , wherein the light emitting portion comprises a plurality of light sources each emitting light at separate spaced apart wavelengths.
13. The device of claim 1 , wherein the monitoring module determines an unacceptable monitoring site in response to the measurement of oxygen saturation.
14. The device of claim 1 , further comprising an electrode positioned along the housing to stimulate the volume of body tissue.
15. A method for using a medical device comprising an optical sensor for monitoring oxygen saturation, the method comprising:
controlling a light emitting portion of the optical sensor to emit light at a plurality of wavelengths including at least four spaced apart wavelengths into a volume of a patient's body tissue;
receiving an output signal from a light detecting portion corresponding to light incident on the light detecting portion, the incident light comprising light emitted by the emitting portion and scattered by the body tissue volume; and
computing a volume-independent measure of oxygen saturation in the volume of tissue in response to the output signal, wherein each of the light emitting portion and the light detecting portion comprise a light source for emitting light and a light detector for detecting light, the method further comprising selectively switching a functionality of the light emitting portion and the light detecting portion.
16. The method of claim 15 , wherein controlling the light emitting portion comprises controlling emission of at least four light sources each emitting light at separate spaced apart wavelengths.
17. The method of claim 16 , wherein the separate spaced apart wavelengths comprises:
a first wavelength spacing between a first wavelength and a second wavelength; and
a second wavelength spacing between the second wavelength and a third wavelength, wherein the first and the second wavelength spacings are approximately equal.
18. The method of claim 16 , wherein the separate spaced apart wavelengths comprises:
a first wavelength spacing between a first wavelength and a second wavelength; and
a second wavelength spacing between the second wavelength and a third wavelength, wherein the first and the second wavelength spacings are unequal.
19. The method of claim 15 , further comprising:
receiving a reference signal produced by a reference photodetector included in the light emitting portion, the reference signal corresponding to an intensity of the emitted light; and
adjusting a drive signal applied to the light emitting portion to control light emission in response to the reference signal.
20. The method of claim 19 , further comprising computing the volume-independent measure of oxygen saturation using the reference signal.
21. The method of claim 15 , wherein computing the volume-independent measure of oxygen saturation comprises:
computing an attenuation spectra using the output signal; and
computing a second derivative of attenuation with respect to wavelength.
22. The method of claim 21 , further comprising computing a measure of total hemoglobin volume fraction using the measure of oxygen saturation.
23. The method of claim 21 , further comprising:
storing calibration data relating the second derivative of attenuation data to oxygen saturation and total hemoglobin volume fraction, and
computing an absolute oxygen saturation using the calibration data.
24. The method of claim 15 , further comprising determining an unacceptable monitoring site in response to the measurement of oxygen saturation.
25. The method of claim 21 , further comprising:
storing an acceptable measurement range;
comparing a measure of total hemoglobin volume fraction to the acceptable measurement range; and
filtering the measure of total hemoglobin volume fraction in response to the comparison.
26. The method of claim 15 , further comprising electrically stimulating the volume of body tissue.
27. A non-transitory computer readable medium having computer executable instructions for performing a method comprising:
controlling a light emitting portion of the optical sensor to emit light at a plurality of wavelengths including at least four spaced apart wavelengths into a volume of a patient's body tissue;
receiving an output signal from a light detecting portion corresponding to light incident on the light detecting portion, the incident light comprising light emitted by the emitting portion and scattered by the body tissue volume; and
computing a volume-independent measure of oxygen saturation in the volume of tissue in response to the output signal, wherein each of the light emitting portion and the light detecting portion comprise a light source for emitting light and a light detector for detecting light, the method further comprising selectively switching a functionality of the light emitting portion and the light detecting portion.