The role of cerebral oxygenation on tolerance to central hypovolemia (1183.12)

Abstract

Background: Tolerance to central hypovolemia varies between individuals, and recent studies have shown that protection of absolute cerebral blood flow is not an underlying mechanism. We hypothesized that subjects with high tolerance (HT) to central hypovolemia maintain cerebral oxygenation(ScO2) at higher levels of lower body negative pressure (LBNP) compared to their low tolerant (LT) counterparts, despite similar reductions in absolute flow. Methods: 15 healthy human subjects (10 male; 5 female) were instrumented for assessment of ScO2 (via near‐infrared spectroscopy, NIRS) and mean middle cerebral artery velocity (MCAv; via transcranial Doppler, TCD). All subjects completed a presyncopal‐limited lower body negative pressure (LBNP) protocol with an onset rate of 3 mmHg/min. Subjects who made it to 蠅80mmHg LBNP were classified as HT, and subjects who made it to 蠄70 mmHg LBNP were classified as LT. Results: The minimum difference in LBNP tolerance between the HT (N=6) and LT (N=9) group was 206 s (LT=1400±104 s vs. HT=2080±65 s; P=0.0003). Up to ‐45 mmHg LBNP, ScO2 was maintained in HT subjects (P蠅0.538), while the LT (N=9) subjects had a progressive decrease in ScO2 (P蠄0.016) from baseline. MCAv decreased from baseline in both HT and LT subjects (P蠄0.022). There was a strong linear relationship between %∆ MCAv and %∆ ScO2 within the LT group (R2=0.98; P=0.013), whereas a weaker association between perfusion and oxygenation (R2=0.53; P=0.271) was observed in the HT group. Conclusion: In support of our hypothesis, higher tolerance to progressive central hypovolemia was associated with the protection of ScO2, despite an early and significant reduction in cerebral blood flow. This may have important clinical implications for the monitoring of cerebral perfusion and oxygenation in trauma patients.Grant Funding Source: US Army Medical Research and Materiel Command