An Increased Endothelial-Independent Vasodilation Is the Hallmark of the Neurally Mediated Syncope

Background: The neurally mediated syncope (NMS) is sustained by complex cardiac and vascular reflexes, acting on and amplified by central autonomic loops, resulting in bradycardia and hypotension. Methods: We evaluated by ultrasound the flow-mediated vasodilation (FMD) and the nitrate-mediated dilation (NMD) in 17 patients with NMS, induced by drug-free tilt test in 6 subjects and by nitrate-potentiated tilt test in the other 11 cases; the syncope was classified as vasodepressive (VD) in 8 cases, cardioinhibitory (CI) in 7, and mixed in 2. Results: The FMD was not different from controls (10.2 +/- 4.5 vs 11.4 +/- 3.9, P = ns), with normal recovery times; the NMD was greater in fainting subjects than in controls (26.7 +/- 7.3 vs 19.0 +/- 3.6, P < 0.05), with higher values in VD than in CI syncope (31.1 +/- 7.0 vs 23.1 +/- 5.0, P = ns); compared to controls, subjects with NMS showed normal recovery times after FMD but longer recovery times after nitrate administration (13.0 +/- 5.6 vs 6.3 +/- 0.7 minutes, P < 0.05). Conclusions: The evaluation of endothelial function supports evidence that NMS is characterized by a marked and sustained endothelial-independent vasodilation, in the presence of a normal FMD; vascular hyperreactivity in response to nitrate administration is particularly overt in vasodepressive syncope and can explain the high rate of responses to nitrate administration during tilt test.