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Departments of 1 Pediatrics and 2 Physiology, The Center for Pediatric Hypotension, New York Medical College, Valhalla, New York 10595
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 
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.
vasoconstriction; heart rate variability; autonomic; head-up tilt
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