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1 Pediatrics, New York Medical College, Valhalla, New York, United States; Physiology, New York Medical College, Valhalla, New York, United States
2 Neuroscience, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, New Jersey, United States; Medicine, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, New Jersey, United States
* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.
Previous investigations have demonstrated a subset of POTS patients characterized by normal peripheral resistance and blood volume while supine, but thoracic hypovolemia and splanchnic blood pooling while upright secondary to splanchnic hyperemia. Such "normal flow" POTS patients often demonstrate hypocapnia during orthostatic stress. We studied 20 POTS patients aged 14-23 years and compared them to 10 comparably aged healthy volunteers. We measured changes in heart rate, blood pressure, heart rate and blood pressure variability, arm and leg strain gauge occlusion plethysmography (SPG), respiratory impedance plethysmography calibrated against pneumotachography, end tidal carbon dioxide (PETCO2), and impedance plethysmographic (IPG) indices of blood volume and blood flow within the thoracic, splanchnic, pelvic (upper leg), and lower leg regional circulations while supine and during upright tilt to 70°. Ten POTS patients demonstrated significant hyperventilation and hypocapnia (POTSHC) while 10 were normocapneic (POTSNCwith minimal increase in postural ventilation comparable to control. While relative splanchnic hypervolemia and hyperemia occurred in both POTS groups compared to controls, marked enhancement in peripheral vasoconstriction occurred only in POTSHC and was related to thoracic blood flow. Variability indices suggested enhanced sympathetic activation in POTSHC compared to other subjects. The data suggest enhanced cardiac and peripheral sympathetic excitation in POTSHC
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