Vagal and Sympathetic Mechanisms in Patients With Orthostatic Vasovagal Syncope

Background Autonomic and particularly sympathetic mechanisms play a central role in the pathophysiology of vasovagal syncope. We report direct measurements of muscle sympathetic nerve activity in patients with orthostatic vasovagal syncope. Methods and Results We studied 53 otherwise healthy patients with orthostatic syncope. We measured RR intervals and finger arterial pressures and in 15 patients, peroneal nerve muscle sympathetic activity before and during passive 60 degree head-up tilt, with low-dose intravenous isoproterenol if presyncope did not develop by 15 minutes. We measured baroreflex gain before tilt with regression of RR intervals or sympathetic bursts on systolic or diastolic pressures after sequential injections of nitroprusside and phenylephrine. Orthostatic vasovagal reactions occurred in 21 patients, including 7 microneurography patients. Presyncopal and nonsyncopal patients had similar baseline RR intervals, arterial pressure, and muscle sympathetic nerve activity. Vagal baroreflex res...