Variability in mechanisms associated with tolerance to progressive reductions in central blood volume (1157.3)

High tolerance to reductions in central blood volume has been associated with higher heart rate (HR), sympathetic nerve activity (SNA), and cardiac baroreflex sensitivity (BRS). Using subjects classified as high tolerance to lower body negative pressure (LBNP), we tested the hypothesis that subjects with BRS > 1.0 would demonstrate greater LBNPtolerance associated with higher HR, PVR, and SNA than subjects with BRS <1.0. Methods: 112 subjects underwent LBNP to presyncope. SNA was measured via microneurography (N= 19) and arterial pressure (AP) was measured by finger photoplethysmography. BRS was auto‐calculated (WinCPRS) as ∆RR/∆AP. BRS was used to dichotomize subjects into two groups (Group 1 = BRS > 1.0, N = 49, and Group 2 = BRS < 1.0, N = 67). Results: Both groups had similar tolerance times (1849±260 vs. 1839 ±253 sec), AP (78±11 vs. 79±12 mmHg), and cardiac output (4.5±1.2 vs. 4.7 ± 1.1 L/min) at presyncope. Group 1 (BRS>1.0) had lower HR (107 ± 19 vs. 131 ± 20 beats/min), higher SV (45 ± 15 vs. 36 ± 15 ml), less SNA (45 ± 13 vs. 53 ± 7 bursts/min), and less increase in PVR (4.1 ± 1.3 vs. 4.5 ± 2.6) compared to Group 2 (BRS < 1.0). Conclusions: Contrary to our hypothesis, higher HR, SNA and BRS were not associated with greater tolerance to reduced central blood volume. In a cohort of high tolerant subjects, some individuals rely on cardiac filling and vagal withdrawal to defend AP while others rely on sympathoexcitation. Grant Funding Source : Department of Defense