Mechanisms of hypotensive effects of a posture change from seated to supine in humans

The hypothesis tested was that the hydrostatic stimulation of carotid baror eceptors is pivotal to decrease mean arterial pressure at heart level durin g a posture change from seated to supine. In eight males, the cardiovascula r responses to a 15-min posture change from seated to supine were compared with those of water immersion to the xiphoid process and to the neck, respe ctively. Left atrial diameter and cardiac output (rebreathing) increased si milarly during the posture change and water immersion to the xiphoid proces s and further so during neck immersion. Mean arterial pressure decreased by 12 +/- 2 mmHg during the posture change, by 5 +/- 1 mmHg during xiphoid im mersion, and was unchanged during neck immersion. Arterial pulse pressure i ncreased by 12 +/- 3 mmHg during the posture change (P < 0.05) and less dur ing xiphoid and neck immersion by 7 +/- 3 mmHg (P < 0.05). Total peripheral vascular resistance decreased similarly during the posture change and neck immersion and slightly less during xiphoid immersion ( < 0.05). In conclus ion, the hydrostatic stimulation of carotid baroreceptors combined with som e additional increase in arterial pulse pressure, which also stimulates aor tic baroreceptors, accounts for more than hall of the hypotensive response at heart level to a posture change from seated to supine.