Hemodynamics of orthostatic intolerance: implications for gender differences

Abstract

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 FrankStarling 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 Ϯ 55.0 vs. 927.7 Ϯ 53.0 mmHg ϫ min, P Ͻ 0.01). Women had steeper maximal slopes of Starling curves than men whether expressed as SV (12.5 Ϯ 2.0 vs. 7.1 Ϯ 1.5 ml/mmHg, P Ͻ 0.05) or normalized as SI (6.31 Ϯ 0.8 vs. 4.29 Ϯ 0.6 ml⅐m Ϫ2 ⅐mmHg Ϫ1 , P Ͻ 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 was 35% and SI was 29% lower in women at presyncope (both P Ͻ 0.05). Coincident with the smaller SV, women had higher heart rates but similar mean arterial pressures compared with men at presyncope. Vascular resistance and plasma norepinephrine concentration were similar between genders. We conclude that lower orthostatic tolerance in women is associated with decreased cardiac filling rather than reduced responsiveness of vascular resistance during orthostatic challenges. Thus cardiac mechanics and Frank-Starling relationship may be important mechanisms underlying the gender difference in orthostatic tolerance. orthostatic hypotension; stroke volume; vascular resistance; pulmonary capillary wedge pressure; lower body negative pressure IN HUMANS, TOLERANCE TO ORTHOSTATIC stress requires the successful integration of multiple elements of cardiovascular control. Syncope may occur when there is an abnormally large postural decrease in central blood volume, cardiac filling pressure, stroke volume (SV), and cardiac output (CO), or inadequate neurohumoral reflex responses to orthostasis, primarily tachycardia and vasoconstriction (2). It has been noted that women have lower orthostatic tolerance than men (7,15,23,35,50). Specific mechanisms underlying this difference have not been totally elucidated and are probably multifactorial.One recent study (50) on astronauts suggested that the incidence of orthostatic presyncope after spaceflight was greatest in women, and this was ascribed to a combination of inherently lower resistance and relative hypoadrenergic responses. However, evidence for this conclusion was not definitive, because the results were obtained from a very small number of subjects analyzed retrospectively. Frey et al. (13,14) proposed that women might respond to orthostatic challenges with more prominent vagal withdrawal, whereas men might respond with greater sympathetic stimulation to the peripheral vasculature. This notion was supported by the observation that women demonstrated greater elevations in ...

Epidemiology of reflex syncope

“…3). A peak around 15 years with predominance in females was also observed in the Framingham cohort [52].A recent study suggests that this gender difference is due to a decreased cardiac filling in women during orthostatic challenges [24]. Reflex syncope was rare before age 10, but there was a steady increase in first occurrence between ages 10 and 20.…”

WISE-2005: effect of aerobic and resistive exercises on orthostatic tolerance during 60 days bed rest in women

“…Increased duration of bed rest or spaceXight is associated with higher incidence of orthostatic intolerance , and women are more susceptible to orthostatic intolerance than men (Convertino 1998;Fu et al 2004).…”

“…Several factors contribute to orthostatic intolerance: moderate hypovolemia (Buckey et al 1996;Custaud et al 2002), decreased stroke volume (Buckey et al 1996), myocardial atrophy (Levine et al 1997), reduced baroreXex sensitivity (Sigaudo-Roussel et al 2002) and increased distensibility of lower extremity blood vessels (Convertino et al 1989;Belin de Chantemele et al 2004b). Furthermore, females have a lower orthostatic tolerance than males (Convertino 1998;Fu et al 2004). Lower orthostatic tolerance is observed after short-duration space Xights (5-16 days) with a four times higher incidence of presyncopal symptoms during a 10 min head-up tilt test in female astronauts compared with males (28 and 7%, respectively) (Harm et al 2001).…”

Hemodynamics and brain blood flow during posture change in younger women and postmenopausal women compared with age-matched men

“…cerebrovascular; sex; age; orthostatic ORTHOSTATIC HYPOTENSION (a condition that sometimes leads to fainting) is an important and sometimes dangerous condition that particularly affects young women during daily life, as reflected by admission to emergency rooms (51). It has been speculated that female sex hormones might modulate reflex neural or vascular properties, resulting in the greater incidence of orthostatic hypotension in young women (5,14,15,52,69). An additional indication of impaired orthostatic responses in women includes the observation that women have a greater propensity towards postural orthostatic tachycardia syndrome (18, 60).…”

“…It has been speculated that female sex hormones might modulate reflex neural or vascular properties, resulting in the greater incidence of orthostatic hypotension in young women (5,14,15,52,69). An additional indication of impaired orthostatic responses in women includes the observation that women have a greater propensity towards postural orthostatic tachycardia syndrome (18,60).…”