The venoarterial reflex

Abstract

The venoarterial reflex, or postural vasoconstriction reflex, is the decline in limb blood flow in the dependent position due to an increase in pre-capillary vascular resistance.1 Impairment of the venoarterial reflex may be a cause of unexplained leg swelling. The venoarterial reflex prevents loss of fluid to the extravascular space when the legs are dependent. Testing of this reflex evaluates the ability of the cutaneous pre-capillary sphincters to constrict in response to dependency or elevated venous pressure and modulate the hydrostatic pressure.1 The mechanism of this autoregulation is threefold: a local sympathetic axonal reflex, an arteriolar-mediated myogenic response, and a minor central sympathetic efferent nerve component.1 The impaired venoarterial reflex may explain swelling in patients receiving calcium channel blockers.2 It is a cause of swelling in diabetics or patients with spinal degeneration that have neuropathy.3 It may explain edema in women during the luteal phase of their menstrual cycle.4 Attenuation of the reflex with higher skin temperatures may contribute to swelling experienced in hot climates.5 With the patient in a supine position, photoplethysmography (PPG) probes are applied to the great toe (Panel A) to monitor capillary flow at baseline showing normal amplitude pulsatile flow. One leg is then lowered 50 cm off the side of the exam table (Panel B). If the reflex is intact, there is an immediate reduction in the amplitude of the PPG pulsations (Panel C). If the reflex is absent or impaired, there will be no appreciable reduction in the pulsations (Panel D) compared to the supine leg position. Treatment of leg swelling caused by an impaired venoarterial reflex is application of prescription knee length compression hose (20–30 mmHg) daily. Use of diuretics is minimally helpful. Evaluation of the venoarterial reflex is an inexpensive, simple test for excluding this abnormality as a cause of otherwise unexplained leg swelling. Panel A Panel B