Venous response to orthostatic stress

Abstract

Additional material and information about A J P -H eart a n d Circulatory Physiology can be found at:http://www.the-aps.org/publications/ajpheart This infomation is current as of May 11, 2011.A JP -H eart and Circulatory Physiology publishes original investigations on the physiology of the heart, blood vessels, and lymphatics, including experimental and theoretical studies of cardiovascular function at all levels of organization ranging from the intact animal to the cellular, subcellular, and molecular levels. (HUT) induces a re duction in preload, which is thought to be restored through sym pa thetic 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 rela tion with autonomic function during HUT. Eight healthy non-pregnant women were subjected to 20° head-down tilt to 60° HUT at 20° intervals. A t each rotational step, we random ly assessed forearm pressure-volume (P-V) curves (venous occlusion plethysmography) during inflow (VeCIN) and outflow [venous emptying rate (VERoUT)]. V eCIN was defined as the ratio o f the slope of the volume-time curve and pressure-time curve, with direct intravenous pressure m easure ment. V EROUT was determined using the derivate of a quadratic regression m odel using cuff pressure. W e defined Vu as the y-intercept of the P-V curve. W e calculated, for both methods, the coefficients of reproducibility (CR) and variation (CV). V ascular sympathetic activ ity was determined by spectral analysis. V eCIN 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 V EROUT but low er during HUT. CV varied between 19% and 25% (VeCIN) and between 12% and 21% (VEROUT). HUT decreased Vu. The change in V eCIN and V EROUT 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 V eCIN and V u and a rise in V EROUT during HUT are documented. The alterations in venous characteristics relate to changes in vascular sym pathetic activity.head-up tilt; venous compliance; venous emptying rate; venous ca pacity; autonomic system

Assessment of venous hemodynamics and volume homeostasis during pregnancy: recommendations of the International Working Group on Maternal Hemodynamics

“…Such women also show a reduced static capacitance to hold extra added venous fluid without increasing venous pressure and cardiac preload. These observations support the view that low plasma volume reflects reduced venous reserve capacity.…”

“…As stated above, low plasma volume correlates not only with reduced venous compliance but also with increased sympathetic tone and reduced venous capacitance. Moreover, venous responsiveness to head‐up tilt is decreased in pregnant women with low plasma volume. These observations at least suggest an interrelated role of sympathetic dominance and low plasma volume.…”

“…During the early follicular phase, directly after the menstrual period, when women can be considered to be at baseline hemodynamic influence with regard to sex hormones, women with low plasma volume do not display neurohumoral alterations such as elevated renin, angiotensin and aldosterone levels or higher levels of epinephrine or norepinephrine. In addition to this, venous compliance is reduced in women with low plasma volume, as well as dynamic capacity to increase venous tone during orthostatic challenges. Such women also show a reduced static capacitance to hold extra added venous fluid without increasing venous pressure and cardiac preload.…”

Vestibulo‐Sympathetic Responses

“…In particular, the role of the sympathetic nervous system in regulating compliance of lower body veins during postural changes is a matter of debate (106, 175, 203). Nonetheless, it is universally agreed that sympathetic vasoconstrictor activity increases in the upper and lower body during standing, and that an absence of such increases in sympathetic nervous system activity during postural changes results in a precipitous drop in blood pressure (107, 108, 136, 184, 292).…”

Aerobic Exercise Training in Formerly Preeclamptic Women

“…22 We excluded data of the first minute after postural change because we have shown in previous experiments that a new steady state is reached within 60 s after postural change. 23 Head-down tilt was performed to test the responses with maximized venous return. Post hoc, the recordings were analyzed by calculating mean HR and mean blood pressure over 5 minutes starting 60 s after postural changes at each rotational step.…”