MECHANISMS UNDERLYING GENDER DIFFERENCES IN ORTHOSTATIC TOLERANCE

Abstract

PURPOSE: This study tested the hypotheses that differences in the adrenergic, central venous pressure (CVP), cardiovascular, or baroreflex responses to lower body negative pressure (LBNP) or differences in body size would be associated with gender differences in orthostatic tolerance. METHODS: Subjects (24 females, 22 males) underwent graded LBNP to −100 mmHg or presyncope while CVP, heart rate, stroke volume (SV), blood pressure, forearm blood flow and [norepinephrine] were assessed. RESULTS: At rest, the males had higher SV (+21 ml·beat−1), cardiac output (+1.65 L·min−1), and mean arterial pressure (+8 mmHg; all P < 0.05). Neither the CVP responses to LBNP to −60 mmHg, the ΔFVC/ΔCVP with LBNP to −20 mmHg nor the ΔHR/ΔCVP with LBNP of −30 to −60 mmHg differed between genders. LBNP tolerance was lower for the females than males (276 ± 12 vs 337 ± mmHg·min; LBNP tolerance index; P < 0.01) and, of the resting data, only gender was associated with LBNP tolerance (R2 = 20%). No cardiovascular parameter differed between genders in the 2 mins preceding presyncope. However, [norepinephrine] was higher in the men than the women at test termination (1.64 ± 0.24 vs 1.01 ± 0.14 nmol·1−1, P < 0.05). CONCLUSION: These data suggest that the CVP, cardiovascular and baroreflex responses to graded LBNP are similar in men and women despite marked differences in LBNP tolerance. Differences in tolerance are not due to differences in body size although differences in noradrenergic responses at maximal LBNP may play a role.