CEREBRAL HEMOGLOBIN AND OPTICAL PATHLENGTH INFLUENCE NEAR-INFRARED SPECTROSCOPY MEASUREMENT OF CEREBRAL OXYGEN-SATURATION

Near-infrared spectroscopy (NIRS) is a noninvasive optical technique t o monitor cerebral oxygen saturation at the bedside. Despite its appli cability, NIRS has had limited clinical use because of concerns about accuracy, noted by intersubject variability in slope and intercept of the line between NIRS- and weighted-average arterial-cerebrovenous sat uration (SMO2). This study evaluated transcranial optical pathlength a nd cerebral hemoglobin concentration as sources for this intersubject variability. Experiments were performed in an in vitro brain model and in piglets. Optical pathlength and cerebral hemoglobin concentration were measured by time-resolved spectroscopy (TRS). NIRS and TRS were r ecorded in the model, as perfusate blood saturation was varied (0%-100 %) at several hemoglobin concentrations, and in piglets, as SMO2 was v aried (15%-90%) before and after hemodilution. In the model, hemoglobi n concentration significantly altered the NIRS versus blood saturation line slope and intercept, as well as optical pathlength. In piglets ( before hemodilution), there was significant intersubject variability i n NIRS versus SMO2 line slope (0.73-1.4) and intercept (-24 to 36) and in transcranial optical pathlength (13.4-16 cm) and cerebral hemoglob in concentration (0.58-1.1 g/dL). By adjusting the NIRS algorithm with optical pathlength or cerebral hemoglobin measurements, intersubject variability in slope (0.9-1.2) and intercept (-9 to 18) decreased sign ificantly. Hemodilution significantly changed NIRS versus SMO2 line sl ope and intercept, as well as transcranial optical pathlength and cere bral hemoglobin concentration (before versus after hemodilution: slope 0.9 vs 0.78, intercept 13 vs 19, pathlength 13.9 vs 15.6 cm, cerebral hemoglobin 0.98 vs 0.73 g/dL). By adjusting the NIRS algorithm with t he cerebral hemoglobin measurements, slope and intercept remained unch anged by hemodilution. These data indicate that intersubject variabili ty in NIRS originates, in part, from biologic variations in transcrani al optical pathlength and cerebral hemoglobin concentration. Instrumen ts to account for these factors may improve NIRS cerebral oxygen satur ation measurements.