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Departments of 1 Pediatrics and 2 Physiology, Center for Pediatric Hypotension, New York Medical College, Valhalla, New York 10595
Chronic orthostatic
intolerance is often related to the postural orthostatic tachycardia
syndrome (POTS). POTS is characterized by upright tachycardia.
Understanding of its pathophysiology remains incomplete, but edema and
acrocyanosis of the lower extremities occur frequently. To determine
how arterial and venous vascular properties account for these findings,
we compared 13 patients aged 13-18 yr with 10 normal
controls. Heart rate and blood pressure were continuously
recorded, and strain-gauge plethysmography was used to measure forearm
and calf blood flow, venous compliance, and microvascular filtration
while the subject was supine and to measure calf blood flow and calf
size change during head-up tilt. Resting venous pressure was higher in
POTS compared with control (16 vs. 10 mmHg), which gave the appearance
of decreased compliance in these patients. The threshold for
edema formation decreased in POTS patients compared with controls (8.3 vs. 16.3 mmHg). With tilt, early calf blood flow increased in POTS
patients (from 3.4 ± 0.9 to 12.6 ± 2.3 ml · 100 ml
1 · min1) but did not increase in
controls. Calf volume increased twice as much in POTS patients compared
with controls over a shorter time of orthostasis. The data suggest that
resting venous pressure is higher and the threshold for edema is lower
in POTS patients compared with controls. Such findings make the POTS
patients particularly vulnerable for edema fluid collection. This may
signify a redistribution of blood to the lower extremities even while
supine, accounting for tachycardia through vagal withdrawal.
vasomotion; autonomic; head-up tilt; chronic fatigue
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