Acute cutaneous microvascular flow responses to whole-body tilting in humans.

The transition from upright to head-down tilt (HDT) posture in humans increases blood pressure superior to the heart and decreases pressure inferior to the heart. Consequently, above heart level, myogenic arteriolar tone probably increases with HDT, in opposition to the withdrawal of baroreceptor-mediated sympathetic tone. We hypothesized that due to antagonism between central and local controls, the response of the facial cutaneous microcirculation to acute postural change will be weaker than that in the leg, where these two mechanisms reinforce each other. Cutaneous microvascular flow was measured by laser Doppler flowmetry simultaneously at the shin and the neck of 7 male and 3 female subjects. Subjects underwent a stepwise tilt protocol from standing control to 54 degrees head-up tilt (HUT), 30 degrees, 12 degrees, 0 degrees, -6 degrees (HDT), -12 degrees, -6 degrees, 0 degrees, 12 degrees, 30 degrees, 54 degrees, and standing, for 30-sec periods with 10-sec transitions between postures. Flows at the shin and the neck increased significantly (P < 0.05) from standing baseline to 12 degrees HUT (252 +/- 55 and 126 +/- 9% (means +/- SE) of baseline, respectively). From 12 degrees to -12 degrees tilt, flows continued to increase at the shin (509 +/- 71% of baseline) but decreased at the neck to baseline levels (100 +/- 15% of baseline). Cutaneous microvascular flow recovered at both sites during the return to standing posture with significant hysteresis. Flow increases from standing to near-supine posture are attributed at both sites to baroreceptor-mediated vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)