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Hemodynamic consequences of rapid changes in posture in humans

¹Department of Integrative Physiology, The University of Iowa, Iowa City, Iowa; and ²The Department of Physiology, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway

Submitted 20 October 2006 ; accepted in final form 23 April 2007

Tolerance to +G_z gravitational stress is reduced when +G_z stress is preceded by exposure to hypogravity (fraction, 0, or negative G_z). For example, there is an exaggerated fall in eye-level arterial pressure (ELAP) early on during +G_z stress (head-up tilt; HUT) when this stress is immediately preceded by –G_z stress (head-down tilt; HDT). The aims of the present study were to characterize the hemodynamic consequences of brief HDT on subsequent HUT and to test the hypothesis that an elevation in leg vascular conductance induced by –G_z stress contributes to the exaggerated fall in ELAP. Young healthy subjects (n = 3 men and 4 women) were subjected to 30 s of 30° HUT from a horizontal position and to 30 s of 30° HUT when HUT was immediately preceded by 20 s of –15° HDT. Four bouts of HDT-HUT were alternated between five bouts of HUT in a counterbalanced designed to minimize possible time effects of repeated exposure to gravitational stress. One minute was allowed for recovery between tilts. Brief exposure to HDT elicited an exaggerated fall in ELAP during the first seconds of the subsequent HUT (–17.9 ± 1.4 mmHg) compared with HUT alone (–12.4 ± 1.2 mmHg, P <0.05) despite a greater rise in stroke volume (Doppler ultrasound) and cardiac output over this brief time period in the HDT-HUT trials compared with the HUT trials (thereafter stroke volume fell under both conditions). The greater fall in ELAP was associated with an exaggerated increase in leg blood flow (femoral artery Doppler ultrasound) and was therefore largely (70%) attributable to an exaggerated rise in estimated leg vascular conductance, confirming our hypotheses. Thus brief exposure to –G_z stress leads to an exaggerated fall in ELAP during subsequent HUT, owing to an exaggerated increase in estimated leg vascular conductance.

orthostatic hypotension; orthostatic intolerance; tilt; hypogravity; microgravity; stroke volume; leg blood flow; cerebral perfusion

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