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Am J Physiol Heart Circ Physiol (October 2, 2003). doi:10.1152/ajpheart.00735.2002
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Submitted on August 22, 2002
Accepted on September 26, 2003

Hemodynamics of orthostatic intolerance: Implications for gender differences

Qi Fu1, Armin Arbab-Zadeh2, Merja A Perhonen1, Rong Zhang2, Julie H Zuckerman1, and Benjamin D Levine2*

1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA
2 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas, USA; The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA

* To whom correspondence should be addressed. E-mail: BenjaminLevine{at}TexasHealth.org.

Women have a greater incidence of orthostatic intolerance than men. We hypothesized that this difference is related to hemodynamic effects on the regulation of cardiac filling rather than to a reduced responsiveness of vascular resistance during orthostatic stress. We constructed Frank-Starling curves from pulmonary capillary wedge pressure (PCWP) and stroke volume (SV) as well as stroke index 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. We found that 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 a steeper maximal slope of their Frank-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 stroke index (6.31 ± 0.8 vs. 4.29 ± 0.6 ml/m2/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 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 decreased cardiac filling rather than a reduced responsiveness of vascular resistance during an orthostatic challenge. Thus, cardiac mechanics and the Frank-Starling relationship may be important mechanisms underlying the gender difference in orthostatic tolerance.




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