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1 Pediatrics and Physiology, New York Medical College, Valhalla, NY, USA
2 LDM Associates, San Jose, CA, USA
* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.
Chronic orthostatic intolerance is associated with postural tachycardia syndrome (POTS) in which the diagnosis is made by abnormal upright tachycardia. Some patients are unable to evoke baroreflex mediated vasoconstriction and have increased calf blood flow. Others have low calf blood flow and increased peripheral arterial resistance. We tested the hypothesis that myogenic, venoarteriolar and reactive hyperemic responses are abnormal in low flow POTS. We studied 14 patients aged 13-19 years with POTS and evenly subdivided among low flow and high flow subgroups compared to 9 healthy control subjects. POTS was confirmed by findings of a heart rate increase exceeding 30 beats/min on an initial upright tilt to 70°. We used venous occlusion strain gauge plethysmography to measure calf venous pressure and blood flow, while supine and when the calf was lowered by 40 cm to evoke myogenic and venoarteriolar responses. We remeasured flow and venous pressure during venous hypertension alone produced by occlusion cuff pressure to 40 mmHg to evoke only the venoarteriolar response. We measured reactive hyperemia of the calf using plethysmography and in the skin using laser Doppler flowmetry. Baseline blood flow in low flow POTS was reduced compared to high flow and control subjects (0.8±0.2 vs 4.4±0.5 and 2.7±0.4 ml/min/100ml) but increased during leg lowering (1.2±0.5). Blood flow decreased in the other groups. Baseline peripheral arterial resistance was significantly increased in low flow POTS and decreased in high flow POTS compared to control (39±13 vs 15±3 and 22±5 ml/100ml/min/ mmHg) but decreased to 29±13 in low flow POTS during venous hypertension. Resistance increased in the other groups. Maximum calf hyperemic flow and cutaneous flow were similar in all subjects. The duration of hyperemic blood flow was curtailed in low flow POTS compared with either control or high flow POTS subjects (plethysmographic time constant = 20±2 vs 29±4 and 28±4 sec, cutaneous time constant = = 60±25 vs 149±53 sec in controls). Thus, local blood flow regulation in low flow POTS patients is impaired.
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