Resting sympathetic baroreflex sensitivity in subjects with low and high tolerance to simulated hemorrhage with lower body negative pressure

Abstract

Central hypovolemia elicited by hemorrhage or lower body negative pressure (LBNP) triggers autonomic reflexes to maintain organ perfusion; these reflexes fail with severe blood loss resulting in cardiovascular collapse. The ability to tolerate LBNP is variable and factors contributing to tolerance are unclear. We tested the hypothesis that resting sympathetic baroreflex sensitivity (BRS) is attenuated in subjects who have a low tolerance (LT) to LBNP. Muscle sympathetic nerve activity (MSNA) and diastolic arterial pressure (DAP) were recorded in 48 human subjects who subsequently underwent LBNP to presyncope. LT subjects reached presyncope prior to completing −60 mmHg LBNP, and high tolerant (HT) subjects experienced presyncope after −60 mmHg LBNP. BRS was assessed as the slope of the relationship between spontaneous fluctuations in DAP and MSNA during 5 min of supine rest. MSNA burst incidence/ DAP correlations were strong (r>0.5) in 36 subjects (LT: n= 8; HT: n=28), and BRS was not different between LT and HT (−1.91 ± 0.31 vs. −2.24 ± 0.22 bursts·(100 beats)−1·mmHg−1, p=0.45). MSNA total activity/ DAP correlations were strong (r>0.5) in 34 subjects (LT: n= 8; HT: n=26), and BRS was not different between LT and HT (−592 ± 69 vs. −666 ± 70 arbitrary units·(beats)−1·mmHg−1, p=0.58). We conclude that resting MSNA BRS does not influence tolerance to LBNP. Funded by the US Army Medical Research and Materiel Command.