Sex differences in vasoconstrictor reserve during 70 deg head-up tilt

Women are generally recognized to be less orthostatically tolerant than men. We hypothesized that during head-up tilt (HUT), women would demonstrate less splanchnic vasoconstriction, leading to splanchnic pooling, lower blood pressure and lower orthostatic tolerance. Mean arterial blood pressure (MAP), heart rate (HR), cardiac output ((Q) over dot (c), assessed by C2H2 rebreathing), stroke volume, splanchnic blood flow (SpBF, assessed by Indocyanine Green clearance) and vascular conductance (systemic, SVC = (Q) over dot (c)/MAP; splanchnic, SpVC = SpBF/MAP; non-splanchnic, non-SpVC = SVC - SpVC) were measured during supine baseline conditions, 70 deg HUT and recovery in 14 healthy women (23 +/- 6 years old; mean +/- s.d.) and 16 men (23 +/- 5 years old). The proportion of sexes surviving 45 min of HUT trended towards significance (chi(2) = 2.92, P = 0.09). The MAP was lower in women than in men (supine, 77 +/- 5 versus 86 +/- 9 mmHg, P < 0.01; tilt, 72 +/- 8 versus 83 +/- 10 mmHg, P < 0.01), while HR and cardiac index ((Q) over dot (c)/body surface area) were not different between the sexes (heart rate supine, 66 +/- 6 versus 64 +/- 8 beats min(-1); heart rate tilt, 96 +/- 13 versus 94 +/- 10 beats min(-1); cardiac index supine, 3.8 +/- 0.9 versus 3.7 +/- 0.7 l min-1 m-2; cardiac index tilt, 2.7 +/- 0.8 versus 2.3 +/- 0.5 l min(-1) m(-2)). The SpBF and SpVC were lower in women at rest but not during tilt (SpBF supine, 1174 +/- 243 versus 1670 +/- 391 ml min(-1), P < 0.01; SpVC supine, 14.83 +/- 3.61 versus 19.59 +/- 4.95 ml min(-1) mmHg(-1), P < 0.01; SpBF tilt, 884 +/- 300 versus 1094 +/- 271 ml min(-1); SpVC tilt, 13.14 +/- 4.28 versus 14.82 +/- 4.16 ml min(-1) mmHg(-1)). However, in the women the SpVC did not decrease from baseline to tilt (delta SpVC, in women, -1.70 +/- 3.19 ml min(-1) mmHg(-1), n.s.; in men, -4.81 +/- 3.44 ml min(-1) mmHg(-1), P < 0.01), suggesting a blunted vasoconstrictor response. In conclusion, women tended to have lower tilt-table tolerance associated with a smaller splanchnic vasoconstrictor reserve than men.