Hemodynamics of orthostatic intolerance: implications for gender differences

Women have a greater incidence of orthostatic intolerance than men. We hypothesized that this difference is related to hemodynamic effects on regulation of cardiac filling rather than to reduced responsiveness of vascular resistance during orthostatic stress. We constructed Frank-Starling curves from pulmonary capillary wedge pressure (PCWP), stroke volume (SV), and stroke index (SI) during lower body negative pressure (LBNP) and saline infusion in 10 healthy young women and 13 men. Orthostatic tolerance was determined by progressive LBNP to presyncope. LBNP tolerance was significantly lower in women than in men (626.8 ± 55.0 vs. 927.7 ± 53.0 mmHg × min, P < 0.01). Women had steeper maximal slopes of Starling curves than men whether expressed as SV (12.5 ± 2.0 vs. 7.1 ± 1.5 ml/mmHg, P < 0.05) or normalized as SI (6.31 ± 0.8 vs. 4.29 ± 0.6 ml·m–2·mmHg–1, P < 0.05). During progressive LBNP, PCWP dropped quickly at low levels, and reached a plateau at high levels of LBNP near presyncope in all subjects. SV w...