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(Circulation. 2003;107:2816.)
© 2003 American Heart Association, Inc.

Clinical Investigation and Reports

Microvascular Filtration Is Increased in Postural Tachycardia Syndrome

From the Department of Pediatrics and Physiology, New York Medical College, Valhalla, NY.

Correspondence to Julian M. Stewart, MD, PhD, Professor of Pediatrics and Physiology, Suite 618, Munger Pavilion, New York Medical College, Valhalla, NY 10595. E-mail stewart{at}nymc.edu

Background— Postural tachycardia syndrome (POTS) is related to defective peripheral vasoconstriction of dependent extremities with redistributive hypovolemia.

Methods and Results— To test whether enhanced microvascular filtration produces leg enlargement, we studied 12 patients 13 to 19 years of age with POTS and defective leg vasoconstriction and 13 age-matched healthy control subjects, with strain-gauge plethysmography used to measure venous pressure (P_v), forearm and calf blood flow, vascular capacitance, and the microvascular filtration coefficient (K_f). Measurements were made while the patient was supine and at steady state during upright tilt to 35°. Supine P_v was not different in POTS, but upright leg P_v tended to be increased above control. Arm and leg peripheral arterial resistance was decreased in the supine and upright positions in patients with POTS compared with control subjects (P=0.01, upright legs). Supine K_f was not significantly increased in the forearm in patients with POTS but was increased in the calf (9.3±2.2 versus 5.7±2.4 [10^-3] mL/100 mL per minute per mm Hg, P=0.04), correlating with calf blood flow (r_s=0.84, P=0.002). K_f was invariant with orthostasis. The hydraulic contribution to upright filtered flow at 35° tilt, the product of K_f and P_v, was approximately twice that of control (0.41±0.09 versus 0.19±0.04 mL/100 mL per minute, P=0.04).

Conclusions— Increased microvascular filtration accounts for enhanced leg swelling in patients with POTS with increased arterial blood flow.

Key Words: vasodilation • blood flow • tachycardia

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