High flow POTS is related to
defective peripheral vasoconstriction of dependent extremities with
redistributive hypovolemia. We tested whether enhanced microvascular
filtration produces leg enlargement we studied 12 patients aged 13-19 years
with POTS and defective leg vasoconstriction and 13 age-matched healthy
control subjects using strain gauge plethysmography to measure venous
pressure, Pv, forearm and calf blood flow, vascular capacitance,
and the microvascular filtration coefficient, Kf. Measurements
were made while supine, and at steady state during upright tilt to 35o.
Supine Pv was not
different in POTS but upright leg Pv tended to be increased above
control. Arm and leg peripheral arterial resistance was decreased supine and
upright in POTS compared to controls (p=. 01 upright legs). Supine Кf,
was not significantly increased in POTS patients in the forearm but was
increased in the calf (9.3±2.2 vs 5.7±2.4 (10-3)
ml/100ml/min/mmHg, p=.04) correlating with calf blood flow (rs=0.84,
p=.002). Кf was invariant with orthostasis. The hydraulic
contribution to upright filtered flow at 35o tilt, the product of
Кf and Pv, was approximately twice control (0.41±.09
vs 0.19±.04 ml/100ml/min, p=.04).
Thus, increased microvascular filtration
accounts for enhanced leg swelling in POTS patients with increased arterial
figure shows the microvascular filtration relation – the fitted linear
relation between limb filtration flow and occlusion cuff pressure.
Filtration occurs only above a critical occlusion pressure, Pi. The slope is
Κf, the microvascular filtration coefficient. By extrapolation
the y-intercept or the normalized filtered flow at zero hydraulic pressure
may be obtained which is related to interstitial pressures, oncotic pressure
and lymphatic drainage.
supine filtration coefficients, Κf, for all supine subjects are
shown. Arm data are shown in the upper panels while leg data are shown in
the lower panels. There is a significant correlation between filtration
coefficients in the legs of POTS patients compared to control subjects.
There is no significant arm correlation.