THE ROLE OF DIFFERENTIAL OXYGEN DISTRIBUTION BETWEEN THE BRAIN AND PERIPHERAL TISSUES ON TOLERANCE TO INDUCED CENTRAL HYPOVOLEMIA

Purpose: Maintenance of cerebral perfusion and oxygenation is essential for survival from hemorrhage. As individuals vary in tolerance to reduced blood volume (e.g., central hypovolemia), we hypothesized that subjects with high tolerance (HT) will maintain cerebral oxygenation compared to low tolerant (LT) subjects, which will be associated with greater peripheral vasoconstriction. Methods: 13 human subjects were instrumented for cerebral (ScO2) and muscle (SmO2) oxygenation (near-infrared spectroscopy, NIRS), then completed a step-wise presyncopal-limited lower body negative pressure (LBNP) protocol. Total hemoglobin (HbT) was calculated as an index of local peripheral resistance (inverse relationship) at the muscle. Results: Progressive decreases in ScO2 were observed in LT subjects (n=5) from -45 mmHg LBNP (P≤0.01 compared to baseline), while Sc02 did not change throughout LBNP for the HT subjects (n=8; P≥0.44). Sm02 decreased at -30 and -60 mmHg LBNP (P≤0.01) in the HT group with concomitant increases in resistance (decreases in HbT; P≤0.03). In contrast, both Sm02 (P=0.01) and HbT (P=0.06) only decreased at -60 mmHg LBNP in the LT group. Conclusions: In support of our hypothesis, increased tolerance to progressive central hypovolemia was associated with maintained cerebral oxygenation, due, in part, to earlier onset of peripheral vasoconstriction.