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1 Pediatrics, New York Medical College, Valhalla, NY, USA; Physiology, New York Medical College, Valhalla, NY, USA
2 Pediatrics, New York Medical College, Valhalla, NY, USA
3 Pediatrics, New York Medical College, Valhalla, NY, USA; NASA Ames Research Center, Moffet, California, USA
* To whom correspondence should be addressed. E-mail: stewart{at}nymc.edu.
Previous investigations have allowed for stratification of POTS patients based on peripheral blood flow. One such subset, comprising normal flow POTS patients, is characterized by normal peripheral resistance and blood volume while supine but thoracic hypovolemia and splanchnic blood pooling when upright. We studied 32 consecutive POTS patients aged 14-22 years comprising 13 with low flow POTS, 14 with normal flow POTS, and 5 with high flow POTS compared to 12 comparably aged healthy volunteers. We measured changes in 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 incremental tilt to 20°, 35°, and 70°. We validated IPG measures of thoracic and splanchnic blood flow against indocyanine green dye dilution measurements. We validated IPG leg blood flow against venous occlusion plethysmography. Control subjects developed progressive vasoconstriction with incremental tilt. In comparison splanchnic blood flow was increased supine in normal flow POTS despite marked peripheral vasoconstriction and did not change during incremental tilt producing progressive splanchnic hypervolemia. Absolute hypovolemia was present in low flow POTS; all supine flows and volumes were reduced; there was absence of vasoconstriction with tilt in all segments, and segmental volumes tended to increase uniformly throughout tilt. Lower body (pelvic and leg) flows were increased in high flow POTS at all angles with consequent lower body hypervolemia during tilt. Our main finding is selective and maintained orthostatic splanchnic vasodilation in normal flow POTS despite marked peripheral vasoconstriction in these same patients. Local splanchnic vasoregulatory factors may counteract vasoconstriction and venoconstriction in these patients. In addition there was abnormal lower body vasoconstriction in high flow POTS and unchanged vasoconstriction in low flow POTS which was sustained at initially elevated supine levels.
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