Continuous spectrophotometric measurements of arteriovenous oxygen difference

A relatively inexpensive dual beam optical absorbance monitor was tested and adapted to measure arteriovenous oxygen difference ((a-v)ΔO₂). Since the molar extinction coefficients of oxy and deoxyhemoglobin are markedly different at 660 nm, the difference in optical density between arterial and venous blood, as measured by the absorbance monitor, was shown to be linearly related to (a-v)ΔO₂. When appropriate cuvettes (optical path length = 0.5 mm) were used, the instrument's 90% response time (4.4 sec) was sufficiently rapid for most physical applications such as Fick determinations of the rate of an organ's oxygen consumption. The theoretical basis of this spectrophotometric (a-v)ΔO₂ measurement and sources of error are discussed in detail.