Abstract
Increased tolerance to central hypovolemia is generally associated with greater sympathoexcitation, high-frequency oscillatory patterns of mean arterial pressure (MAP), and tachycardia. On average, women are less tolerant to central hypovolemia than men; however, the autonomic mechanisms governing these comparisons are not fully understood. We tested the hypothesis that women with relatively high tolerance (HT) to central hypovolemia would display similar physiological reserve capacity for sympathoexcitation and oscillations in MAP at presyncope compared to HT men. About 10 men and five women were exposed to progressive lower body negative pressure (LBNP) until the presence of presyncopal symptoms. Based on our previous classification system, all subjects were classified as HT because they completed at least -60 mmHg LBNP. Muscle sympathetic serve activity (MSNA) was measured directly from the peroneal nerve via microneurography and arterial pressure (AP) was measured at the finger by photoplethysmography. LBNP time to presyncope was less (P < 0.01) in women (1727 ± 70 sec) than in men (2022 ± 201 sec). At presyncope, average MSNA in men (50 ± 12 bursts/min) and women (51 ± 7 bursts/min) was similar (P = 0.87). Coincident with similar stroke volume (SV) at presyncope, women had similar MAP and heart rates. However, women had less physiological reserve capacity for SV, AP-MSNA coherence, and oscillations in the high-frequency (HF) components of arterial pressure compared to men. Contrary to our hypothesis, lower tolerance to central hypovolemia in women was not associated with sympathoexcitation, but can be explained, in part by lower physiological reserve to elicit oscillatory patterns in AP, maintenance of AP-MSNA coherence and SV when compared to men.