VAGAL AND SYMPATHETIC MECHANISMS IN PATIENTS WITH ORTHOSTATIC VASOVAGAL SYNCOPE

Background Autonomic and particularly sympathetic mechanisms play a ce ntral role in the pathophysiology of vasovagal syncope. We report dire ct measurements of muscle sympathetic nerve activity in patients with orthostatic vasovagal syncope. Methods and Results We studied 53 other wise healthy patients with orthostatic syncope. We measured RR interva ls and finger arterial pressures and in 15 patients, peroneal nerve mu scle sympathetic activity before and during passive 60 degree head-up tilt, with low-dose intravenous isoproterenol if presyncope did not de velop by 15 minutes. We measured baroreflex gain before tilt with regr ession of RR intervals or sympathetic bursts on systolic or diastolic pressures after sequential injections of nitroprusside and phenylephri ne. Orthostatic vasovagal reactions occurred in 21 patients, including 7 microneurography patients. Presyncopal and nonsyncopal patients had similar baseline RR intervals, arterial pressure and muscle sympathet ic nerve activity. Vagal baroreflex responses were significantly impai red at arterial pressures below (but not above) baseline levels in pre syncopal patients. Initial responses to tilt were comparable; however, during the final 200 seconds of tilt, presyncopal patients had lower RR intervals and diastolic pressures than nonsyncopal patients and gra dual reduction of arterial pressure and sympathetic activity. Frank pr esyncope began abruptly with precipitous reduction of arterial pressur e, disappearance of muscle sympathetic nerve activity, and RR interval lengthening. Conclusions Patients with orthostatic vasovagal reaction s have impaired vagal baroreflex responses to arterial pressure change s below resting levels but normal initial responses to upright tilt. S ubtle vasovagal physiology begins before overt presyncope. The final t rigger of human orthostatic vasovagal reactions appears to be the abru pt disappearance of muscle sympathetic nerve activity.