Regional cutaneous microvascular flow responses during gravitational and LBNP stresses

The most significant cardiovascular event during the transition to microgravity is the redistribution of vascular transmural pressures that results from the loss of hydrostatic gradients along the length of the body. The well-documented effects of this redistribution include facial venous engorgement, headache, and a significant decrease in leg volume. These effects predominantly represent bulk fluid volume shifts, especially in the venous macro- and microcirculation, where volume is a direct function of pressure, related by the mechanical compliance of the vascular compartment. When considering the effect of gravitational pressure alterations on microcirculatory blood flow and volume, however, this direct monotonic relationship no longer applies. Regional microvascular perfusion is largely a function of local arteriolar tone, which is subject to a variety of central and local controls. Lower body venous pooling during application of footward gravitational stress unloads arterial and cardiopulmonary baroreceptors, increasing sympathetic arteriolar tone to elicit vasoconstriction and a general decrease in microvascular perfusion. The same stimulus also triggers an increase in the levels of circulating vasoactive hormones, such as norepinephrine and angiotensin II, further augmenting arteriolar tone. Vasomotor tone is also mediated by local mechanisms such as myogenic autoregulation and veno-arteriolar reflexes, which enhance microvascular tone in response to elevated local arteriolar and venular pressure, respectively. Due to the regional variability of local hydrostatic pressures, microvascular flow responses to gravitational stress probably vary along the length of the body. Although these differences in local autoregulation have been observed previously during whole-body tilting, they have not been investigated during application of artificial gravitational stresses, such as lower body negative pressure (LBNP) or +Gz centrifugation. Although these stresses can create equivalent G-levels at the feet, they result in distinct distributions of vascular transmural pressure along the length of the body, and should consequently elicit different magnitudes and distributions of microvascular response. In the present study, the effects of whole-body tilting and LBNP on the level and distribution of microvascular flows within skin along the length of the body were compared.