In-vitro evaluation of a dual oxygen saturation/hematocrit intravascular fiberoptic catheter

The authors describe the results of an in-vitro evaluation of a prototype new three-fiber intravascular fiberoptic catheter that is suitable for in-vivo measurement of mixed venous oxygen saturation and hematocrit simultaneously. The catheter consists of a single source and two unequally spaced (near and far) detecting fibers. The study showed that the ratio of backscattered light intensities measured at an isobestic wavelength by the two detecting fibers (IRnear/IRfar) can be used as a correcting factor for reducing the dependence of oxygen saturation measurements on hematocrit and also as a means for measuring hematocrit independently. The ability to measure hematocrit in vivo is important in critical care situations where fluids are administered intravenously and blood transfusions are performed routinely. Therefore, the purpose of this study was to investigate the quantitative effect of variations in red blood cell volume induced by changing the pH of the blood. The IRnear/IRfar ratio was found to be affected by variations in bicarbonate ion and carbon dioxide concentrations. The reflected light intensity detected by the catheter was found to be significantly more sensitive to changes in red blood cell volume induced by variations in sodium bicarbonate than to changes induced by variations in carbon dioxide levels in the blood. Clinically, these variations would amount to errors in hematocrit estimations in the order of 2.7%/0.1 pH unit when pH variations are induced by the addition of sodium bicarbonate to the blood and only 0.3%/0.1 pH unit when carbon dioxide is used to vary blood pH.