Resting Sympathetic Baroreflex Sensitivity in Subjects with Low and High Tolerance to Central Hypovolemia Induced by Lower Body Negative Pressure
ARMY INST OF SURGICAL RESEARCH FORT SAM HOUSTON TX
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Central hypovolemia elicited by orthostasis or hemorrhage triggers sympathetically-mediated baroreflex responses to maintain organ perfusion these reflexes are less sensitive in patients with orthostatic intolerance, and during conditions of severe blood loss, may result in cardiovascular collapse decompensatory or circulatory shock. The ability to tolerate central hypovolemia is variable and physiological factors contributing to tolerance are emerging. We tested the hypothesis that resting muscle sympathetic nerve activity MSNA and sympathetic baroreflex sensitivity BRS are attenuated in male and female subjects who have low tolerance LT to central hypovolemia induced by lower body negative pressure LBNP. MSNA and diastolic arterial pressure DAP were recorded in 47 human subjects who subsequently underwent LBNP to tolerance onset of presyncopal symptoms. LT subjects experienced presyncopal symptoms prior to completing LBNP of 60mm Hg, and subjects with high tolerance HT experienced presyncopal symptoms after completing LBNP of 60mm Hg. Contrary to our hypothesis, resting MSNA burst incidence was not different between LT and HT subjects, and was not related to time to presyncope. BRS was assessed as the slope of the relationship between spontaneous fluctuations in DAP and MSNA during 5 min of supine rest. MSNA burst incidenceDAP correlations were greater than or equal to 0.5 in 37 subjects LT n 9 HT n 28, and BRS was not different between LT and HT 1.8 0.3 vs. 2.2 0.2 bursts 100 beats 1 mm Hg 1, p 0.29. We conclude that tolerance to central hypovolemia is not related to either resting MSNA or sympathetic BRS.
- Anatomy and Physiology
- Medicine and Medical Research