This (Armstrong Laboratory) report compares functions of baroreflex control of heart rate and vascular resistance, adrenoreceptor responsiveness, indices of baseline vagal and sympathetic tone, plasma volume, and venous compliance, in men and women, to test the hypothesis that greater orthostatic intolerance in women is associated with impairment of specific mechanisms of blood pressure regulation.
Heart rate (HR), stroke volume (SV), cardiac output (Q), mean arterial blood pressure (MAP), forearm (FVR) and leg (LVR) vascular resistance, catecholamines (NE), and changes in leg volume (%LV), are measured during various protocols of lower body negative pressure (LBNP), carotid stimulation, and infusions of adrenoreceptor agonists, in 10 females and 10 males matched for age and fitness. LBNP tolerance for women is found to be 35% lower than for men. At presyncope, SV, Q, MAP, and %LV, are lower in females compared to males, while HR, FVR, and TPR are similar in both groups. Lower LBNP tolerance in females is associated with: impairment of the heart-rate response to carotid baroreceptor stimulation; lower baseline cardiac vagal activity; greater decline in Q and SV induced by LBNP; increased Beta1-adrenoreceptor responsiveness; greater vasoconstriction under equal LBNP; lower levels of NE at presyncope; and lower blood volume.
The results support the hypothesis that women have significant deficiencies in mechanisms that underlie blood-pressure regulation under orthostatic challenge. These findings should be considered in selection and training of women for military combat, especially in combat missions requiring high-G aerial maneuvers.
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