Cardiovascular and autonomic responses to lower body negative pressure

Abstract

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.

Forebrain neural patterns associated with sex differences in autonomic and cardiovascular function during baroreceptor unloading

“…The comparable physiological adjustments to 15 mmHg LBNP between men and women support previous observations (13), highlighting the lack of sex-based differences in the autonomic and cardiovascular responses to mild levels of orthostatic stress. However, at the higher level of LBNP, the increases in heart rate and MSNA were greater in men than in women.…”

Lower capacitance response and capillary fluid absorption in women to defend central blood volume in response to acute hypovolemic circulatory stress

“…The total hypovolemic load was similar between sexes (30,49). This finding is supported by Franke et al (13), who measured the effect of LBNP on central venous pressure and thoracic impedance and found similar levels of central hypovolemic circulatory stress in women and men. Not only the applied hypovolemic stimulus but also the speed by Percentage of resting values are means Ϯ SE.…”

“…Young women are more susceptible to hypovolemic circulatory stress than young men (6,13,17,18,48). The mechanisms underlying this susceptibility in women are not clear, however, and are probably multifactorial (17).…”

“…Women are more susceptible to hypovolemic circulatory stress than men (6,13,17,18,48), and clinical studies have found decreased survival in women after penetrating injury as well as burn injuries (19,35). The mechanisms underlying the susceptibility are not entirely clear and are probably multifactorial (17).…”

“…A decreased baroreceptor sensitivity has been proposed in women by several authors (13,27,42), and women seem to respond with diminished arterial vasoconstriction to the infusion of ␣-receptor agonists (4, 14, 26). Furthermore, a more pronounced decrease in stroke volume and cardiac output has been postulated as the main mechanism for the susceptibility to hypovolemic circulatory stress, due to smaller and functionally stiffer hearts and impeding cardiac filling (6, 17, 18) as well as lower relative circulating blood volume in women (6,18,49).…”