G-15D Free Communication/Slide Hypotension and Orthostasis
To test the hypothesis that a greater incidence of orthostatic intolerance in young women compared with men is related to hemodynamics rather than to a reduced vascular responsiveness during orthostatic stress.
We constructed Frank-Starling curves from pulmonary capillary wedge pressure (PCWP) and stroke volume (SV) during lower body negative pressure (LBNP) and rapid saline infusion in 10 healthy, sedentary women (29.6 ± 1.9 yrs, mean ± SE) and 13 control men (25.5 ± 1.4 yrs). Orthostatic tolerance was determined by progressive LBNP to presyncope with continuous hemodynamic monitoring including PCWP and right atrial pressure.
LBNP tolerance was significantly lower in females than males (626.8 ± 55.0 vs. 927.7 ± 53.0 mmHg x min, P <0.01). Women had steeper slopes of the linear portion of Frank-Starling curves than men (12.5 ± 2.0 vs. 7.1 ± 1.5 ml/mmHg, P <0.05). During progressive LBNP, PCWP dropped quickly at low levels of LBNP, then more slowly and reached a plateau at high levels of LBNP near presyncope in all the subjects. At presyncope, SV was 35% and stroke index was 29% lower for the females compared with the males (both P <0.05). Coincident with the smaller SV, females had higher heart rates (P <0.05), but similar mean arterial pressures compared with males at the onset of presyncope. Vascular resistance was similar and plasma norepinephrine concentration did not differ between the genders.
These results suggest that lower orthostatic tolerance in women is associated with a smaller SV at any given cardiac filling pressure, rather than a lower responsiveness of vascular resistance during an orthostatic challenge. Thus, the Frank-Starling relationship may be an important mechanism underlying the gender difference in orthostatic tolerance. Supported by NASA SCORT grant NAGW-3582, and NASA OLMSA grant NAG5–4846.