Blood distribution adaptations in paraplegics during posture changes: peripheral and central reflex responses

The veno-arteriolar reflex (VAR) in spinal-cord-injured subjects (SCI) has been attributed little interest, although it might contribute substantially to their blood redistribution. This peripheral reflex response, which is based on an axon reflex, consists of a reduction in limb blood flow following an increase in venal transmural pressure. The purpose of the present investigation was to assess the peripheral and central cardiovascular adaptations of paraplegics with high (HP) and low (LP) spinal lesions to subsequent, passive posture changes involving leg dependency and upright sitting, and to compare them to able-bodied (AB) subjects. Lower-limb cutaneous vascular conductance (CVC) was evaluated from skin blood flow measurements (laser Doppler flowmetry) taken from the dorsal foot, and from ankle blood pressure. Cardiac stroke volume, heart rate and myocardial performance were assessed using impedance cardiography and brachial blood pressure. During leg dependency, a significant vasoconstriction was noted in all three groups. The initial decrease in CVC was higher in HP (-76.82%) than in AB (-45.82%), the values for LP (-67.08%) lying in between these two (significant group x time interaction: F = 2.832; P = 0.042). There were no differences for parameters of central hemodynamics. No between-group differences were noted in any parameter tested during upright sitting. CVC remained at a similar low level as compared to leg dependency, stroke volume decreased, heart rate and blood pressure increased, and myocardial performance remained constant. The present results suggest that paraplegics have a peripheral VAR in their paralyzed lower limbs, and that this contributes to their cardiovascular stability.