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Departments of 1 Pediatrics and 2 Physiology, The Center for Pediatric Neurovascular Disease, New York Medical College, Valhalla, New York 10595
Dependent pooling occurs in postural orthostatic
tachycardia syndrome (POTS) related to defective
vasoconstriction. Increased venous pressure (Pv) >20 mmHg
occurs in some patients (high Pv) but not others (normal Pv). We
compared 22 patients, aged 12-18 yr, with 13 normal controls.
Continuous blood pressure and strain-gauge plethysmography were used to
measure supine forearm and calf blood flow, resistance, venous
compliance, and microvascular filtration, and blood flow and swelling
during 70° head-up tilt. Supine, high Pv had normal resistance in
arms (26 ± 2 mmHg · ml
1 · 100 ml · min) and legs (34 ± 3 mmHg · ml1 · 100 ml · min) but
low leg blood flow (1.5 ± 0.4 ml · 100 ml1 · min1). Supine leg Pv
(30 ± 2 vs. 13 ± 1 mmHg in control) exceeded the threshold
for edema (isovolumetric pressure = 19 ± 3 mmHg). Supine,
normal Pv had high blood flow in arms (4.1 ± 0.2 vs. 3.5 ± 0.2 ml · 100 ml1 · min1 in
control) and legs (3.8 ± 0.4 vs. 2.7 ± 0.3 ml · 100 ml1 · min1 in control) with low
resistance. With tilt, calf blood flow increased steadily in POTS with
high Pv and transiently increased in normal Pv. Calf volume increased
in all POTS patients. Arm blood flow increased in normal Pv only with
forearm maintained at heart level. These data suggest that there are
(at least) two subgroups of POTS characterized by high Pv and low flow
or normal Pv and high flow. These may correspond to abnormalities in
local or baroreceptor-mediated vasoconstriction, respectively.
vasodilation; venoarteriolar reflex; orthostatic intolerance; head-up tilt
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