Orthostatic Intolerance in Women

Abstract

Although women (W) are generally recognized to be less orthostatic ally tolerant than men (M), the sex differences leading to this disparity have not been fully elucidated. We hypothesized that during head-up tilt W would demonstrate less splanchnic vasoconstriction than M, leading to blood pooling in the splanchnic region, lower blood pressure, and lower orthostatic tolerance. PURPOSE: To determine the change in splanchnic vascular conductance (SpVC) between baseline (supine) and 70° head-up tilt in women and men. METHODS: Splanchnic blood flow (SBF), heart rate (HR), blood pressure (SBP DBP, MAP), cardiac output (Q), systemic vascular conductance (SVC), stroke volume (SV), SpVC (SBF/MAP), and non-SpVC (SVC-SpVC) were measured during supine baseline, 70° head-up tilt, and recovery in 14 healthy W (23±6 yrs; mean±SD) and 13 age-matched M (24±5 yrs). To assess changes in SpVC and non-SpVC we examined the difference between baseline and tilt (ASpVC, Anon-SpVC). All other variables were assessed by stage and sex. A Kaplan-Meier survival curve was used to assess differences in tilt time between the sexes. RESULTS: Median tilts time to presyncope tended to be lower in W when compared to M (15.7 vs. 22.5 min; χ2=3.42, p=0.06). ANOVA indicated that W had lower blood pressure than M. MAP was lower during supine rest (77±5 vs. 87±8 mmHg, p <0.0001); this difference continued throughout tilt until the period preceding presyncope (66 ± 11 vs. 78±13 mmHg, p=0.0004). HR was higher in W at baseline (66±6 vs. 63±5 bpm, p=0.013) and during tilt (97±14 vs. 93±10 bpm, p <0.0001). Q was not different between W and M at baseline (6.37±1.51 vs. 7.32±1.46 L/min, p=NS) and decreased similarly during tilt (4.49±1.35 vs. 4.42±0.92 L/min, p=NS). ASpVC was less mW than M(1.7±3.2 vs. 4.8±3.6 mL/min/mmHg, p=0.027). Anon-SpVC was not different between the sexes. CONCLUSIONS: Women demonstrate lower blood pressure during head-up tilt, and consequently lower tilt-table tolerance. The principal reason for this difference appears to be diminished vasoconstriction in the splanchnic vascular bed.