RELATIONSHIPS BETWEEN HEMODYNAMIC AND SYMPATHETIC NEURAL RESPONSES TO HEAD‐UP TILT DURING MODERATE DEHYDRATION IN HUMANS

Abstract

Dehydrationcan cause an increase in supine sympathetic nerve activity (SNA) and heart rate(HR). Reflex sympatho excitation and tachycardia are classic baroreflex responses to orthostasis, increasing peripheral resistance and cardiac output, respectively, to maintain arterial pressure during upright posture. We hypothesized that the sympathetic nervous system would have a greater contribution to blood pressure regulation during orthostasis in dehydrated people compared to conditions of euhydration. We studied 14 young healthy subjects (4 F, 10 M) divided into 2groups: euhydrated (EUH, n = 6), previously exercised in 40 ºC while maintaining hydration; and dehydrated (DEH, n = 8), previously exercised in 40 ºC who lost ~4% of body weight via sweat loss. We measured supine muscle SNA (MSNA), HR, blood pressure (BP) during a 7minute baseline period, followed by MSNA, HR and BP responses to 30 and 45º HUT(2 minutes each). Supine MSNA was not different between groups (24 ± 9 vs 28 ±11 bursts/min, P > 0.05). HR and MSNA increased in all subjects during HUT, and mean responses were not different between groups (P>0.05). Individual changes in HR (ΔHR) were directly related to changes in MSNA (ΔMSNA) during 45ºHUT in both hydration conditions (EUH: R=0.92; DEH: R=0.92), suggesting similar responsiveness in both arms of the baroreflex. The MSNA ‐ HR relationships were weaker at 30º HUT in both groups. During DEH, ΔMSNA and ΔBP were positively related(30º HUT: R = 0.57; 45º HUT: R = 0.88); this was not seen in EUH. Our data suggest that, during exercise induced dehydration, there is an augmented contribution of the sympathetic nervous system to BP regulation.Support or Funding InformationFunded by USAMRMC; author views not official US Army or DOD policy.