Role of Light Scatterng in Whole Blood Oximetry

We investigated the role of light scattering in whole blood oximetry by transmission spectrophotometry. To delineate the role of scattering and absorbance in the measurement of oxyhemoglobin saturation, we applied Twersky´s theory of radiation scattering and measured the apparent optical density of whole blood and hemoglobin solutions. The optical density versus hematocrit relationship predicted by Twersky´s theory was found to give a good fit to the data obtained at 660, 813, 880, and 940 nm. A semi-empirical variation of Twersky´s equation and photon diffusion equations were also compared to the data, and Twersky´s original equation was found to give the best fit. Therefore, Twersky´s equation was employed throughout the rest of the data analysis. Total scattering effects were shown to be wavelength and oxygenation dependent. Moreover, the relationship between total scattering effects and percent O2 saturation was approximately linear, and it had a greater slope (at 660 nm) than absorbance versus O2 saturation. Thus, scattering effects in the red-infrared range do not detract from the linearity of whole blood oximeters. By contrast, scattering effects increase the sensitivity of oximeters by contributing linearly to the total O. D. change that occurs with altered oxygenation.