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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

This article has been cited by other articles:

				K. D. Monahan Letter to the editor: "Does baroreflex unloading decrease limb venous compliance in women?" Am J Physiol Heart Circ Physiol, February 1, 2009; 296(2): H534 - H534. [Full Text] [PDF]

				I. Krabbendam, R. Scholten, and M. E. A. Spaanderman Reply to "Letter to the Editor: 'Does baroreflex unloading decrease limb venous compliance in women?'" Am J Physiol Heart Circ Physiol, February 1, 2009; 296(2): H535 - H535. [Full Text] [PDF]