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 (Pv>20 mm Hg) and normal Pv on the basis of resting supine Pv obtained in previous work. Subjects were studied using strain gauge plethysmography to measure blood flow, Pv, and the venous compliance volume-pressure relation while supine and during incremental head-up tilt testing at −10°, 0°, 20°, and 35°. Volume-pressure relations of controls and POTS patients with normal Pv and high Pv were not different and were unchanged by orthostasis. Supine leg peripheral resistance was greater than control resistance in patients with high Pv (54±9 versus 30±6 mm Hg · mL−1 · 100 mL−1 · min−1) and less than control resistance in patients with normal Pv (17±2 mm Hg · mL−1 · 100 mL−1 · min−1). On upright tilt, resistance decreased in high Pv to approximate resistance in normal Pv. Resistance in controls increased throughout tilt. Leg Pv increased in patients with normal Pv and in controls but remained unchanged in the high Pv 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.
Received January 8, 2002; revision received March 8, 2002; accepted March 10, 2002.