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This Article Full Text Full Text (PDF) All Versions of this Article: 295/4/H1587    most recent 00571.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Krabbendam, I. Articles by Spaanderman, M. E. A. PubMed PubMed Citation Articles by Krabbendam, I. Articles by Spaanderman, M. E. A.

Venous response to orthostatic stress

Department of Obstetrics and Gynecology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands

Submitted 30 May 2008 ; accepted in final form 7 August 2008

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° 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 (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.

head-up tilt; venous compliance; venous emptying rate; venous capacity; autonomic system