Endothelial function and peripheral vasomotion in the brachial artery in neurally mediated syncope

Background: Paradoxical peripheral vasodilation is one of the suspected mec hanisms of neurally mediated syncope. Parasympathetic stimulation following sympathetic activation during orthostatic stress mainly contributes to thi s vasodilation. Hypothesis: Since endothelial function modulates peripheral vascular tone, this study aimed to determine whether endothelial function and inappropriat e peripheral vasomotion has a significant role in the pathogenesis of neura lly mediated syncope. Methods: To investigate whether endothelial function is augmented or whethe r abnormal peripheral vasomotion exits, flow-mediated dilation (FMD, endoth elium-dependent vasodilation) and sublingual glyceryl trinitrate-induced di lation (0.3 mg, GTN-D, endothelium-independent vasodilation) were measured in the brachial artery in 16 patients with neurally mediated syncope, aged 33 +/- 10 years, by using high-resolution ultrasound. All patients underwen t positive head-up tilt testing. These measures were compared with those in 16 control subjects matched with the patients by age, gender, and coronary risk factors. For FMD, percent diameter changes were obtained from baselin e to hyperemic conditions (1 min after 5 min occlusion of the forearm arter y). There were five smokers in both the patient and the control groups, but there was no structural heart disease in either group. Results: Baseline brachial artery diameters were comparable (3.8 +/- 0.6 vs . 3.8 +/- 0.7 mm, NS). Flow-mediated dilation in patients with neurally med iated syncope had a normal value of 9.8 +/- 5.0% despite the inclusion of f ive smokers. Flow-mediated dilation and GTN-D in patients with neurally med iated syncope were significantly greater than those in controls (9.0 +/- 5. 0 vs. 3.0 +/- 3.5%, p<0.05; 18.4 +/- 5.5 vs. 14.1 +/- 4.4%, p<0.05). Conclusions: Augmented endothelial function and/or abnormal peripheral vaso motion in peripheral arteries are important in patients with neurally media ted syncope in selected populations.