Vasoconstrictor Reserve and Sympathetic Neural Control of Orthostasis

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Abstract
Background— We tested the hypothesis that individual variability in orthostatic tolerance is dependent on the degree of neural and vasomotor reserve available for vasoconstriction. Methods and Results— Muscle sympathetic nerve activity (MSNA) and hemodynamics were measured in 12 healthy young volunteers during 60° head-up tilt (HUT), followed by a cold pressor test (CPT) in HUT. Orthostatic tolerance was determined by progressive lower-body negative pressure (LBNP) to presyncope. The same protocols were performed randomly in normovolemic and hypovolemic conditions. We found that mean arterial pressure increased and stroke volume decreased, whereas heart rate (HR), MSNA, and total peripheral resistance (TPR) increased during HUT (all P P r =0.442, P =0.039). Those who had greater increases in MSNA had greater increases in TPR during the CPT and longer time to presyncope (both P Conclusions— These results support our hypothesis and suggest that vasoconstrictor capability is a contributor to orthostatic tolerance in humans. Vasoconstrictor reserve therefore may be one mechanism underlying individual variability in orthostatic intolerance.