Pooling in chronic orthostatic intolerance: arterial vasoconstrictive but not venous compliance defects.

BACKGROUND Orthostatic intolerance is characterized by postural tachycardia syndrome (POTS) with exaggerated tachycardia, orthostatic symptoms, and "pooling" (which comprises acrocyanosis and dependent edema when upright). My colleagues and I tested the hypothesis that pooling results from increased venous compliance in POTS patients. METHODS AND RESULTS Fifteen patients aged 13 to 19 years were compared with 11 healthy, age-matched controls. The POTS group was divided into patients with high venous pressure (P(v)>20 mm Hg) and normal P(v) on the basis of resting supine P(v) obtained in previous work. Subjects were studied using strain gauge plethysmography to measure blood flow, P(v), and the venous compliance volume-pressure relation while supine and during incremental head-up tilt testing at -10 degrees, 0 degrees, 20 degrees, and 35 degrees. Volume-pressure relations of controls and POTS patients with normal P(v) and high P(v) were not different and were unchanged by orthostasis. Supine leg peripheral resistance was greater than control resistance in patients with high P(v) (54+/-9 versus 30+/-6 mm Hg x mL(-1) x 100 mL(-1) x min(-1)) and less than control resistance in patients with normal P(v) (17+/-2 mm Hg x mL(-1) x 100 mL(-1) x min(-1)). On upright tilt, resistance decreased in high P(v) to approximate resistance in normal P(v). Resistance in controls increased throughout tilt. Leg P(v) increased in patients with normal P(v) and in controls but remained unchanged in the high P(v) group. CONCLUSIONS The findings suggest that pooling in POTS is due to blunted arterial vasoconstriction, which produces passive redistribution of blood within peripheral venous capacitance beds. Venous compliance in POTS is similar to that in control subjects.