Relation between hematocrit and optical density in pulse oximetry -In vitro study with Waseda mock circulatory system-

Optical properties of living tissues have not been well established even today, and biooptic instrumentations have to be based on empirical formulae. In order to examine optical properties of the tissue having pulsating blood perfusion, we investigated the relation between optical density (defined as O.D.) of whole blood and hematocrit by transmission spectrophotometry. We used Waseda mock circulatory system that simulates blood circulation in the tissue. It was found that with increasing light path length, O.D. per unit light path length due to scattering and absorption effect, tended to become constant in each hematocrit. For wavelengths of 660, 805 and 940 nm, the relations between O.D. of whole blood and hematocrit predicted by Twersky´s equation, Loewinger´s equation and photon diffusion equation fitted to the data obtained. Meanwhile, for 1300 nm, the relation predicted by Loewinger´s equation gave the best fit to the data