Microvascular Filtration Is Increased in Postural Tachycardia Syndrome

Abstract

Background— Postural tachycardia syndrome (POTS) is related to defective peripheral vasoconstriction of dependent extremities with redistributive hypovolemia. Methods and Results— To test whether enhanced microvascular filtration produces leg enlargement, we studied 12 patients 13 to 19 years of age with POTS and defective leg vasoconstriction and 13 age-matched healthy control subjects, with strain-gauge plethysmography used to measure venous pressure (P v ), forearm and calf blood flow, vascular capacitance, and the microvascular filtration coefficient (K f ). Measurements were made while the patient was supine and at steady state during upright tilt to 35°. Supine P v was not different in POTS, but upright leg P v tended to be increased above control. Arm and leg peripheral arterial resistance was decreased in the supine and upright positions in patients with POTS compared with control subjects ( P =0.01, upright legs). Supine K f was not significantly increased in the forearm in patients with POTS but was increased in the calf (9.3±2.2 versus 5.7±2.4 [10 −3 ] mL/100 mL per minute per mm Hg, P =0.04), correlating with calf blood flow (r s =0.84, P =0.002). K f was invariant with orthostasis. The hydraulic contribution to upright filtered flow at 35° tilt, the product of K f and P v , was approximately twice that of control (0.41±0.09 versus 0.19±0.04 mL/100 mL per minute, P =0.04). Conclusions— Increased microvascular filtration accounts for enhanced leg swelling in patients with POTS with increased arterial blood flow.

Excessive heart rate response to orthostatic stress in postural tachycardia syndrome is not caused by anxiety

“…As expected, the HR responses to LBNP in POTS patients were enhanced. This excessive tachycardia may be caused by central hypovolemia, because LBNP induces venous pooling and also accentuates extravasation within the interstitium of the leg, thereby reducing plasma volume (13), which may be exaggerated in POTS patients (34,36,37). Additionally, hypovolemia (8,26) and cardiac atrophy due to deconditioning (Refs.…”

Decreased capillary filtration but maintained venous compliance in the lower limb of aging women

“…Furthermore, age-related changes might be sex specific, in accordance with age-related changes of arteries, with women having a slower decrease in arterial compliance with age compared with men, an effect probably attributed to estrogen (9, 48). Estrogen has been shown to affect cellular transcription of elastin and collagen, and estrogen-receptors are known to exist in vascular smooth muscle cells (21,22,32).There is a marked capillary fluid filtration in the lower limb during quiet standing or lower body negative pressure (LBNP), increasing the hypovolemic stimulus over time independently from venous compliance and capacitance, and its hypovolemic importance is indicated by the increased fluid filtration in lower limbs of subjects with postural tachycardia syndrome (25,26,46). We recently described an increased capillary fluid filtration and capillary filtration coefficient (CFC) in the calf of young women compared with men, possibly caused by higher levels of estrogen in women, since estrogen enhances capillary filtration (26,45,47).…”

“…capillary filtration coefficient; lower body negative pressure; age A TRANSITION FROM the supine to standing posture leads to a progressive pooling of blood in the compliant veins in the lower part of the body, which produces an immediate circulatory challenge due to the concomitant decrease in thoracic blood volume (5,6,39). Subjects with increased limb venous compliance have been shown to have greater orthostatic intolerance, probably due to an increased venous capacitance (24,36,46), although this relationship has recently been challenged by Hernandez and Franke (18). Venous compliance in the lower limbs decreases with age in men (33,38,39,49), but there is no reason to believe that venous compliance and capacitance are similar in men and women, since a sex difference has been established in young adults (26,30,34).…”

“…There is a marked capillary fluid filtration in the lower limb during quiet standing or lower body negative pressure (LBNP), increasing the hypovolemic stimulus over time independently from venous compliance and capacitance, and its hypovolemic importance is indicated by the increased fluid filtration in lower limbs of subjects with postural tachycardia syndrome (25,26,46). We recently described an increased capillary fluid filtration and capillary filtration coefficient (CFC) in the calf of young women compared with men, possibly caused by higher levels of estrogen in women, since estrogen enhances capillary filtration (26,45,47).…”

Relation of Postural Vasovagal Syncope to Splanchnic Hypervolemia in Adolescents

“…16 This involved elevating the limb above heart level until no further decrease in volume was obtained and then using 4-minute-long, 10-mm Hg pressure steps to a maximum of 60 mm Hg. At lower pressures, limb size reaches a plateau representing venous filling, whereas at higher pressure, a linear component representing microvascular filtration 17 and a vascular component are superimposed. These are extracted by least-squares methods, 18 and capacitance is calculated from the sum of supine venous volume and vascular increments.…”