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Clinical Science (2003) 104, (329–340) (Printed in Great Britain)

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Contrasting neurovascular findings in chronic orthostatic intolerance and neurocardiogenic syncope

Julian M. STEWART*† and Amy WELDON*

*Department of Pediatrics, New York Medical College, Valhalla, NY 10595, U.S.A., and †Department of Physiology, New York Medical College, Valhalla, NY 10595, U.S.A.

Key words: fainting, orthostatic intolerance, syncope, vasoconstriction, venous resistance.

Abbreviations: POTS, postural tachycardia syndrome; P_v, venous pressure.

Correspondence: Professor Julian M. Stewart, Ph.D., Professor of Pediatrics, Professor of Physiology, The Center for Pediatric Hypotension, Suite 618, Munger Pavilion, New York Medical College, Valhalla, NY 10595, U.S.A. (e-mail stewart@nymc.edu).

Simple faint (neurocardiogenic syncope) and postural tachycardia syndrome (POTS) characterize acute and chronic orthostatic intolerance respectively. We explored the hypothesis that vascular function is similar in the two conditions. We studied 29 patients with POTS and compared them with 20 patients with neurocardiogenic syncope who were otherwise well, and with 15 healthy control subjects. We measured continuous heart rate, respiration and blood pressure, and used venous occlusion strain gauge plethysmography to measure calf and forearm blood flow, peripheral arterial resistance, peripheral venous resistance and venous pressure (P_v). Upright tilt was performed to 70° for 10min, during which calf blood flow and volume were measured. Calf P_v was increased (to 27.2±2.0mmHg) in a subgroup of POTS patients, who also had increased arterial resistance (57±6mmHg·ml^-1·min^-1·100ml^-1 tissue), increased venous resistance (2.4±0.3mmHg·ml^-1·min^-1·100ml^-1 tissue), and decreased peripheral flow (1.0±0.2ml·min^-1·100ml^-1 tissue) in the calf; other POTS patients with a normal P_v had decreased arterial resistance (18±2mmHg·ml^-1·min^-1·100ml^-1 tissue) and increased blood flow (3.8±0.3ml·min^-1·100ml^-1 tissue). Syncope patients were not different from controls (P_v = 11.4±0.5mmHg; calf flow = 3.1±0.2ml·min^-1·100ml^-1 tissue; arterial resistance = 27±2mmHg·ml^-1·min^-1·100ml^-1 tissue; venous resistance = 1.2±0.3mmHg·ml^-1·min^-1·100ml^-1 tissue). When upright, syncope patients and control subjects had similar increases in heart rate and calf volume, stable blood pressure, and decreases in blood flow. POTS patients had markedly increased heart rate and calf blood flow, unstable blood pressure, and pooling in the lower extremities, regardless of subgroup. We conclude that peripheral vascular physiology in patients with POTS is abnormal, in contrast with normal peripheral vascular physiology in neurocardiogenic syncope.

Received 15 July 2002/22 October 2002; accepted 3 January 2003

The Biochemical Society and the Medical Research Society © 2003