| Orthostatic
intolerance is often characterized by “pooling” in the lower extremities
comprising acrocyanosis and dependent edema when upright. We tested the
hypothesis that pooling results from increased venous compliance in POTS
patients. Fifteen patients aged 13-19 years were compared with 11 healthy age
matched controls. We divided the POTS group into patients with high venous
pressure (>20 mm Hg) and normal venous pressure (Pv) on the basis
of resting supine Pv obtained in previous work. Subjects were studied
using strain gauge plethysmography to measure blood flow, Pv, and the
venous compliance volume-pressure relation while supine and during incremental
head-up tilt testing at -10o, 0o, +20o
and +35o. Volume-pressure
relations of controls, normal Pv, and high Pv POTS
patients were not different and were unchanged by orthostasis. Supine leg
peripheral resistance (R) was greater than control in high Pv POTS
(54±9 vs 30±6 mmHg/ml/100ml/min) and less than control in normal Pv
POTS (17±2). On upright tilt, R decreased in high Pv to approximate
R in normal Pv. R in controls increased throughout tilt. Leg Pv
increased in normal Pv and controls but remained unchanged in the
high Pv group. :
The findings suggest that pooling in POTS is due to blunted arterial
vasoconstriction producing passive redistribution of blood within peripheral
venous capacitance beds. Venous compliance in POTS is similar to control.
|
Normal Leg Venous Capacitance
