A somatostatin analog improves tilt table tolerance by decreasing splanchnic vascular conductance.

Splanchnic hemodynamics and tilt table tolerance were assessed after an infusion of placebo or octreotide acetate, a somatostatin analog whose vascular effects are largely confined to the splanchnic circulation. We hypothesized that reductions in splanchnic blood flow (SpBF) and splanchnic vascular conductance (SpVC) would be related to improvements in tilt table tolerance. In randomized, double-blind, crossover trials, hemodynamic variables were collected in 14 women and 16 men during baseline, 70° head-up tilt (HUT), and recovery. A repeated-measures analysis of variance was used to compare changes from baseline with respect to sex and condition. HUT elicited an increase in heart rate and decreases in mean arterial pressure, cardiac index, stroke index, and systemic vascular conductance. Additionally, SpVC and non-SpVC were lower during HUT. Octreotide reduced SpBF and SpVC and increased systemic vascular conductance and non-SpVC. Changes in SpBF and SpVC between supine and HUT were smaller in women (P < 0.05). Tilt table tolerance was increased after administration of octreotide [median tilt time: 15.7 vs. 37.0 min (P < 0.05) and 21.8 vs. 45.0 min (P < 0.05) for women and men, respectively]. A significant relationship existed between change (Δ) in SpBF (placebo-octreotide) and Δtilt time in women (Δtilt time = 2.5-0.0083 ΔSpBF, P < 0.01), but not men (Δtilt time = 3.41-0.0008 ΔSpBF, P = 0.59). In conclusion, administration of octreotide acetate improved tilt table tolerance, which was associated with a decrease in SpVC. In women, but not men, the magnitude of reduction in SpBF was positively associated with improvements in tilt tolerance.

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(c), assessed by C2H2 rebreathing), stroke volume, splanchnic blood flow (SpBF, assessed by Indocyanine Green clearance) and vascular conductance (systemic, SVC = (.)Qc/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 ( /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 (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.