Orthostasis fails to produce active limb venoconstriction in adolescents.
Stewart, Julian M., Jean Lavin, and Amy Weldon.
Departments of 1Pediatrics and 2Physiology, The Center for Pediatric Hypotension, New York Medical College, Valhalla, New York 10595
APStracts 8:0330A, 2001.
Orthostasis is characterized by translocation of blood from the upper body and thorax into dependent venous structures. Although active splanchnic venoconstriction is known to occur, active limb venoconstriction remains controversial. Based on prior work, we initially hypothesized that active venoconstriction does occur in the extremities during orthostasis in response to baroreflex activation. We investigated this hypothesis in the arms and legs of 11 healthy volunteers, aged 13-19 yr, using venous occlusion strain gauge plethysmography to obtain the forearm and calf blood flows and to compute the capacitance vessel volume-pressure compliance relation. Subjects were studied supine and at "minus"10, +20, and +35° to load the baroreflexes. With +20° of tilt, blood flow decreased and limb arterial resistance increased significantly (P < 0.05) compared with supine. With +35° of tilt, blood flow decreased, limb arterial resistance increased, and heart rate increased, indicating parasympathetic withdrawal and sympathetic activation with arterial vasoconstriction. The volume-pressure relation was unchanged by orthostatic maneuvers. The results suggest that active venoconstriction in the limbs is not important to mild orthostatic response.
Received 14 February 2001; accepted in final form 17 May 2001
APS Manuscript Number A159-1.
Article publication pending J Appl Physiol
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 29 June 2001