Measurement of blood oxygen saturation

Abstract

Oxygenation of a subject's blood is determined by sensing an absorption spectrum of light directed either invasively or non-invasively into the blood, and then calculating an oxygenation value by evaluating a cost function of the remitted spectrum relative to at least two pre-determined reference absorption spectra representing different, known levels of blood oxygenation. The source of light preferably uses stable, long-life, white LEDs, in which case white-balancing of the remitted spectrum can be accomplished by predetermining and storing the spectrum of the LEDs, one time for all, and then adjusting the remitted spectrum accordingly to compensate for deviations of the LED spectrum from the constant ideal.

1 . A sensor device comprising a light source for emitting a light beam, a photodetector for receiving the light beam after passing through or being reflected within living tissue and arranged to provide signals corresponding to the intensities of the respective wavelength of light received by the photodetector wherein the sensor device is configured to measure blood oxygen saturation. 
     
     
         2 . A sensor device according to Claim I, wherein the sensor is configured to measure a plurality of wavelengths. 
     
     
         3 . A sensor device according to  claim 2 , wherein the sensor uses a spectral wavelength of from 500 to 600 mn. 
     
     
         4 . A sensor device according to  claim 3 , wherein the sensor uses a special wavelength of from 526 to 586 nm. 
     
     
         5 . A sensor device according to  claim 2 , wherein the different wavelengths bear a predetermined relationship with each other. 
     
     
         6 . A sensor device according to  claim 2 , wherein the sensor uses  3  or more different wavelengths. 
     
     
         7 . A sensor device according to  claim 6 , wherein the number of wavelengths used is 5 or 6. 
     
     
         8 . A sensor device according to  claim 2 , wherein at least one of the wavelengths is all isobestic wavelength. 
     
     
         9 . A sensor device according to  claim 8 , wherein most of the wavelengths are isobestic wavelengths. 
     
     
         10 . A sensor device according to  claim 9 , wherein the rive wavelengths are isobestic and one wavelength provides the maximum absorption difference between oxygenated haemoglobin and deoxygenated haemoglobin. 
     
     
         11  A sensor device according to  claim 7 , wherein the number of wavelengths used are selected from 500, 528, 550, 560, 572 and 586 nm. 
     
     
         12 . A sensor device according to  claim 7 , wherein the scattered light is transmitted along 6 separate fibres to 6 photodetectors via narrow-band optical filtera all in the range 500 to 600 nm. 
     
     
         13 . A sensor device according to  claim 12 , wherein the optical filters are all in the range 526 and 586 nm. 
     
     
         14 . A sensor device according to  claim 7 , wherein the scattered light is transmitted along a single fibre of from 50 to 150 nm in diameter used with one to three white LEDs. 
     
     
         15 . A sensor device according to  claim 1 , wherein the sensor device operates on reflectance. 
     
     
         16 . A sensor device according to  claim 1 , wherein the sensor device is coupled to an oximeter. 
     
     
         17 . A blood oxygenation monitoring system comprising:
 a sensor configured to transmit light containing a plurality of wavelengths into blood and to measure a remitted spectrum over the plurality of wavelengths; and   a monitoring device connected in communication with the sensor, said processor configured to:
 calculate a measured blood absorption spectrum from the remitted spectrum; 
 estimate local rates of change in the measured blood absorption spectrum at a plurality of the wavelengths, including at least one isobestic wavelength; and, 
 calculate an estimate of SO 2  as a function of absolute values of the local rates of change of the measured blood absorption spectrum. 
   
     
     
         18 . A monitoring system of  claim 17 , wherein the plurality of wavelengths include at least five isobestic wavelengths. 
     
     
         19 . A monitoring system of  claim 17 , wherein the plurality of wavelengths lie in a range of 500 to 600 nm. 
     
     
         20 . A monitoring system of  claim 17 , wherein the processor is further configured to apply a Kubelka Monk transformation to the measured blood absorption spectrum. 
     
     
         21 . All SO 2  monitoring system, comprising:
 a sensor configured to transmit light containing a plurality of wavelengths into the blood and measure a remitted spectrum over the plurality of wavelengths; and   a monitoring device connected in communication with the sensor and configured to:
 calculate an estimate of SO 2  in blood to be monitored; 
 correct said estimate of SO 2  in blood by a scaling factor; 
 calculate a measured blood absorption spectrum from the remitted spectrum; 
 estimate local rates of change in the measured blood absorption spectrum at a plurality of the wavelengths, including at least one isobestic wavelength; and, 
 calculate the estimate of SO 2  as a function of absolute values of the local rates of change of the measured blood absorption spectrum. 
   
     
     
         22 . A monitoring system of  claim 21 , wherein the monitoring device is configured to, before calculating the estimate of SO 2 , remove effects of light scattering from the measured blood absorption spectrum; calculate an area under the measured blood absorption spectrum after removing effects of light scattering; and, normalize the measured blood absorption spectrum by the area under the measured blood absorption spectrum. 
     
     
         23 . A monitoring system of  claim 21 , wherein the monitoring device is configured to apply a Kubelka Monk transformation to the measured blood absorption spectrum. 
     
     
         24 . A monitoring system of  claim 21 , wherein the monitoring device is, when calculating the estimate of SO 2 , configured to compute a hemoglobin index value as a function of absolute values of the local rates of change of the measured blood absorption spectrum between a plurality of pairs of isobestic points, whereby the hemoglobin index value is independent of blood oxygenation; compute an oxygenation parameter as a function of absolute values of the local rates of change of the measured blood absorption spectrum between a plurality of isobestic points and at least one non-isobestic point, whereby the oxygenation parameter is dependent on blood oxygenation; normalize the oxygenation parameter by the hemoglobin index value; and, estimate SO 2  as a measure of the level of the normalized oxygenation parameter relative to a predetermined fully deoxygenated reference value and a fully oxygenated reference value. 
     
     
         25 . A blood oxygenation monitoring system comprising:
 a sensor configured to transmit light containing the plurality of wavelengths into blood and measure a remitted spectrum over the plurality of wavelengths; and   a monitoring device configured to:
 determine a first reference spectrum over a plurality of wavelengths; 
 determine a second reference spectrum over the plurality of wavelengths; 
 calculate a measured blood absorption spectrum as a function of the remitted spectrum, the first reference spectrum and the second reference spectrum; and, 
 remove effects of light scattering from the measured blood absorption spectrum by calculating a correction function that is a function of a plurality of isobestic points of the measured blood absorption spectrum by correcting the measured blood absorption spectrum by the correction function, normalizing the measured blood absorption spectrum following the correcting step, calculating an optimal spectrum as a function of a substantially oxygenated reference absorption spectrum and a substantially deoxygenated reference absorption spectrum, so that the optimal spectrum best matches the measured blood absorption spectrum in a determined sense and, calculating an estimate of SO 2  as a function of the optimal spectrum. 
   
     
     
         26 . A monitoring system of  claim 25 , wherein the first reference spectrum is a spectrally neutral “white” spectrum and the second reference spectrum represents an ambient “dark” spectrum.