Microvascular Filtration in High Flow POTS          (Lower limb Neuropathic POTS)

Home ] Up ] Exercise Intolerance- the Exercise Pressor Reflex in POTS ] Skeletal Muscle Pump ] Normal Leg Venous Capacitance ] Postural Neurocognitive ] Splanchnic Pooling in Normal Flow POTS ] Nitric Oxide Dysfunction in Low Flow POTS ] Angiotensin-II in POTS ] Decreased Upright Cerebral Blood Flow and Cerebral Autoregulation in POTS ] Postural Hyperpnea ] Nitric Oxide is Decreased in Angiotensin-II dependent Low flow POTS but increased along with Splanchnic pooling Neuropathic POTS ] Local Vascular Responses in POTS ] [ Microvascular Filtration in High Flow POTS ] POTS as Thoracic Hypovolemia ]

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 blood flow.


A representative supine limb plethysmographic tracing is shown. Blood flow is assessed by sudden venous occlusion shown on the left of the top panel and in expanded detail in the left lower panel. A least squares fit is superimposed to obtain estimates of flow. After measurement of Pv, the limb is pressurized by using 10 mmHg pressure increments beginning at 20mmHg. The change in limb volume for a pressure step from 20 to 30 mmHg is shown in the right lower panel. This has been partitioned into contributions from intravascular capacitance vessel filling and from microvascular filtration.


The 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.


Representative volume changes during 35o upright tilt are shown for a POTS patient (in black) and for a healthy control subject (in blue). A linear fit to the curves starting after approximately 5 minutes tilt is superimposed. The slope of the linear portions is greater in the POTS patient. The increased slope indicates the increased magnitude of filtered fluid. When this portion is subtracted by “curve stripping” from the total curve, the residual curves (shown as dotted lines) represent contributions to leg volume from capacitance vessel filling. Thus standing capacity is similar in the POTS patient and in the control subject. These curves are similar for POTS and control subjects.

All 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.



Exercise Intolerance- the Exercise Pressor Reflex in POTS
Skeletal Muscle Pump
Normal Leg Venous Capacitance
Postural Neurocognitive
Splanchnic Pooling in Normal Flow POTS
Nitric Oxide Dysfunction in Low Flow POTS
Angiotensin-II in POTS
Decreased Upright Cerebral Blood Flow and Cerebral Autoregulation in POTS
Postural Hyperpnea
Nitric Oxide is Decreased in Angiotensin-II dependent Low flow POTS but increased along with Splanchnic pooling Neuropathic POTS
Local Vascular Responses in POTS
Microvascular Filtration in High Flow POTS
POTS as Thoracic Hypovolemia