Venous response to orthostatic stress

Abstract

Head-up tilt (HUT) induces a reduction in preload, which is thought to be restored through sympathetic venoconstriction, reducing unstressed volume (Vu) 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° head-down tilt to 60° HUT at 20° intervals. At each rotational step, we randomly assessed forearm pressure-volume (P-V) curves (venous occlusion plethysmography) during inflow (VeCIN) and outflow [venous emptying rate (VEROUT)]. VeCIN was defined as the ratio of the slope of the volume-time curve and pressure-time curve, with direct intravenous pressure measurement. VEROUT was determined using the derivate of a quadratic regression model using cuff pressure. We defined Vu 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. VeCIN decreased at each rotational step compared with the supine position ( P < 0.05), whereas VEROUT increased. CR of VeCIN was higher in the supine position than VEROUT but lower during HUT. CV varied between 19% and 25% (VeCIN) and between 12% and 21% (VEROUT). HUT decreased Vu. The change in VeCIN and VEROUT 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 VeCIN and Vu and a rise in VEROUT during HUT are documented. The alterations in venous characteristics relate to changes in vascular sympathetic activity.