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1 Department of Pediatrics, New York Medical College, Valhalla, NY, USA; Department of Physiology, New York Medical College, Valhalla, NY, USA
2 LDM Associates, San Jose, CA, USA
3 Department of Bioengineering, SUNY Binghamton, Binghamton, NY, USA
* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.
Standing translocates thoracic blood volume into the dependent body. The skeletal muscle pump participates in preventing orthostatic intolerance by enhancing venous return. We investigated the hypothesis that skeletal muscle pump function is impaired in postural tachycardia (POTS) associated with low calf blood flow ("low flow POTS") and depends in general on muscle blood flow. Twelve low flow POTS subjects were compared with 10 controls, and with 7 POTS patients with normal calf blood flow using strain gauge plethysmography to measure peripheral blood flow, venous capacitance, and calf muscle pump function. Blood volume was estimated by dye dilution. We found that calf circumference was reduced in low flow POTS (32±1 vs 39±3, 43±3 cm) and, compared to control and POTS patients with normal blood flow, is related to reduced fraction of calf venous capacity emptied during voluntary muscle contraction (ejection fraction, 0.52±.07 vs 0.76±.07, 0.80±.06). We found that blood flow was linearly correlated (rp=0.69) with calf circumference (used as a surrogate for muscle mass). Blood volume was 2.2±0.3 in low flow POTS vs 2.6±0.5 in controls (p=0.17) and 2.4±0.7 in normal flow POTS patients. Decreased calf blood flow may reduce calf size in POTS, impairing the upright ejectile ability of the skeletal muscle pump and further contributing to overall reduced blood flow and orthostatic intolerance in these patients.
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