Role of Light Scattering in Spectrophotometric Measurements of Arteriovenous Oxygen Difference

The role of light scattering in the spectrophotometric measurement of arteriovenous oxygen difference (a -vO2) in whole blood was investigated. Optical absorbance of whole blood and corresponding hemoglobin solutions was measured to elucidate the contributions of true absorbance and "total scattering effects" to the total absorbance of whole blood. Twersky's radiation scattering theory, which provides a mathematical separation of absorbance and total scattering effects, was applied to the data. It is shown that, although the optical density of whole blood is a highly nonlinear function of hematocrit, the difference in optical density between venous and arterial blood is approximately a linear function of the arteriovenous difference in oxygen content. Furthermore, "total scattering effects" contribute significantly more than true absorbance to the total optical density of whole blood and hence to the actual spectrophotometric measurement of a -vO2. Twersky's equation fits the data remarkably well and thus provides a theoretical basis for whole blood measurements. The validity of whole blood spectrophotometric arteriovenous oxygen difference analyzers is thus demonstrated both empirically and theoretically.