Venous response to orthostatic stress.
SourceAmerican Journal of Physiology : Heart and Circulatory Physiology, 295, 4, (2008), pp. H1587-93
Article / Letter to editor
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American Journal of Physiology : Heart and Circulatory Physiology
SubjectEBP 2: Effective Hospital Care; IGMD 5: Health aging / healthy living; IGMD 6: Hormonal regulation; NCEBP 12: Human Reproduction; NCEBP 14: Cardiovascular diseases; UMCN 5.2: Endocrinology and reproduction
Head-up tilt (HUT) induces a reduction in preload, which is thought to be restored through sympathetic venoconstriction, reducing unstressed volume (V(u)) and venous compliance (VeC). In this study, we assessed venous inflow and outflow responses and their reproducibility and determined the relation with autonomic function during HUT. Eight healthy non-pregnant women were subjected to 20 degrees head-down tilt to 60 degrees HUT at 20 degrees intervals. At each rotational step, we randomly assessed forearm pressure-volume (P-V) curves (venous occlusion plethysmography) during inflow (VeC(IN)) and outflow [venous emptying rate (VER(OUT))]. VeC(IN) was defined as the ratio of the slope of the volume-time curve and pressure-time curve, with direct intravenous pressure measurement. VER(OUT) was determined using the derivate of a quadratic regression model using cuff pressure. We defined V(u) as the y-intercept of the P-V curve. We calculated, for both methods, the coefficients of reproducibility (CR) and variation (CV). Vascular sympathetic activity was determined by spectral analysis. VeC(IN) decreased at each rotational step compared with the supine position (P<0.05), whereas VER(OUT) increased. CR of VeC(IN) was higher in the supine position than VER(OUT) but lower during HUT. CV varied between 19% and 25% (VeC(IN)) and between 12% and 21% (VER(OUT)). HUT decreased V(u). The change in VeC(IN) and VER(OUT) correlated with the change in vascular sympathetic activity (r= -0.36, P<0.01, and r=0.48, P<0.01). This is the first study in which a reproducible reduction in VeC(IN) and V(u) and a rise in VER(OUT) during HUT are documented. The alterations in venous characteristics relate to changes in vascular sympathetic activity.
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