The initiating trigger in vasovagal syncope (VVS) is unclear but impaired venous return has been considered as a central factor. A greater leg venous compliance (Vc) may augment the central hypovolemia during standing. The aim of the study was to evaluate Vc in VVS patients and controls.

15 VVS females (24.8±4.9 years) and 15 age-matched female controls (22.9±3.2 years) were studied. Venous occlusion plethysmography was used to measure calf volume changes. A thigh cuff was inflated to 60 mmHg for 8 min with a subsequent linear decrease of 1 mmHg/s in cuff pressure (P). Vc was determined using the first derivate of a quadratic regression equation describing the volume-pressure relationship [Compliance=β1+2β2(P)]. The capillary filtration was subtracted from the volume curve to correct for the effect on Vc.

Vc was reduced in VVS females (P<0.05). No differences were found in the venous capacitance response or capillary filtration. Resting arterial blood flow was lower and the peripheral resistance higher in VVS females (P<0.05). The time for 50% of the capacitance response to be developed was increased in VVS females (P<0.05), and the rate of the capacitance response correlated to the reduced arterial inflow of blood (r=0.64, P<0.01).

The study shows defects both in venous wall properties and arteriolar regulation in patients with VVS. A reduced blood flow to the lower limb with a concomitant reduction in filling rate of the capacitance vessels during standing might change the central baroreceptor response with a defect hemodynamic adjustment as a consequence.