Measurement of absolute values of hemoglobin oxygenation in the brain of small rodents by near infrared reflection spectrophotometry

Abstract

Reflection near infrared spectroscopy (reNIRS) has been proposed as a novel technique for the measurement of absolute values of total hemoglobin (tHb), oxygenated hemoglobin (oxHb), hemoglobin saturation (SO 2), and cytochrome aa 3 oxidation status (oxCyt aa 3) in living tissue. In this study, we evaluated reNIRS during physiological cerebral blood flow conditions in rats ( n=6) and during the induction of global cerebral ischemia in gerbils ( n=6). ReNIRS parameters were assessed over the exposed cerebral cortex and compared to regional cerebral blood flow (rCBF) data obtained by laser Doppler flowmetry. Under physiological conditions, reNIRS measurements reflected the large intra- and interindividual variability of oxHb and tHb in the brain. The absolute values obtained by reNIRS for tHb (6.3±1.7 mg/ml), oxHb (3.7±1.1 mg/ml), and SO 2 (61±5%) matched expected values. In contrast, measurements of oxCyt aa 3 were unstable and results unreliable. reNIRS reliably detected cerebral ischemia, verified by a reduction of rCBF to 11% of baseline. tHb dropped to 74±7% of baseline ( P<0.001), reflecting ischemic microvascular vasoconstriction. oxHb and SO 2 dropped to expected near-zero values (2±4 and 3±5% of baseline, respectively; P<0.001). We conclude that reNIRS provides reliable and reproducible absolute values for brain tissue tHb, oxHb, and SO 2 in small rodents. Determination of physiological values requires measurements at multiple locations, while cerebral ischemia is reliably detected by continuous recordings at a single location.