Near-infrared monitoring of cerebral tissue oxygen saturation and blood volume in newborn piglets

Near-infrared spectrophotometry (NIRS) potentially provides a tool for noninvasive tissue oxygenetion and blood volume monitoring. Cerebral monitoring could be useful in the prevention of hypoxic ischemic brain injury in newborns. This study sought to validate such NIRS measurements in normoventilated, hypocapnic, and hypoxemic states in the brain of newborn piglets vs. arterial (Sa(O₂)) and sagittal sinus blood hemoglobin saturation (Sss(O2)) and blood volume measurements with ^99mTc-labeled erythrocytes. NIRS measurements of cerebral blood volume (CBV) were performed with both oxyhemoglobin and indocyanine green as tracers, and changes in CBV were monitored by following the change in the concentration of total hemoglobin (i.e., oxyhemoglobin + deoxyhemoglobin). NIRS CBV measurements did not correlate well with the radioactive measurements. NIRS measurements of oxygenetion, however, correlated well with a weighted mean value of Sa(O₂) and Sss(O₂) (r = 0.90; P < 0.0001). Multiple linear regression of the oxygenation index (i.e., oxyhemoglobin - deoxyhemoglobin) on Sa(O₂) and Sss(O₂) suggested that NIRS sees hemoglobin in tissue in a venous-to- arterial ratio of 2:1. Therefore, in this study, NIRS reliably monitored changes in cerebral tissue oxygenetion but not in CBV.