Cardiovascular and autonomic responses to lower body negative pressure: do not explain gender differences in orthostatic tolerance.

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. Subjects (24 females, 22 males) underwent graded LBNP to -100 mmHg or presyncope. At rest, the males had higher SV (+ 21 ml x beat(-1)), cardiac output (+ 1.65 L x min(-1)), systolic blood pressure (+ 29 mmHg) and mean arterial pressure (+ 8 mmHg; all P < 0.05). Neither the CVP responses to LBNP to -60 mmHg, the deltaforearm vascular conductance/deltaCVP with LBNP to -20 mmHg nor the deltaheart rate/deltaCVP with LBNP of -30 to -60 mmHg differed between genders. LBNP tolerance was lower for the females than males (276 +/- 12 vs 337 +/- 14 mmHg x min; LBNP tolerance index; P < 0.01) and, of the resting data, only gender was associated with LBNP tolerance (R(2) = 20 %). Use of gender, the maximal change in HR with LBNP and the change in HR from 0 to -40 mmHg in a regression equation could explain 75 % of the variability in orthostatic tolerance. No cardiovascular parameter differed between genders in the two minutes preceding presyncope. In the last completed stage and at test termination, [epinephrine] and [norepinephrine] were higher in the men. 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 body size do not explain the gender difference although differences in the adrenergic responses at maximal LBNP may play a role.