760 Syncope and the Autonomic Nervous System KEITH G. LURIE, M.D., and DAVID BENDITT, M.D. From the Cardiac Arrhythmia Center at the Universiiy of Minnesota. Minneapolis. Minnesota Syncope and the Autonomic Nervous System. The autonomic nervous system plays a centriil role in the maintenance of hemodynamic stability. Dysfunction of Ihis complex regula- tory system can lead to the development of loss of consciousness. This article summarizes our current understanding of the role of the autonomic nervous system in maintaining a stable blood pressure and heart rate under normal and abnormal physiologic conditions. The role ctf baroreceptors. mecbanoreeeptors, chemoreceptors, vascular reactivity, and tbe interaction of tbese sensor systems witb tbe central nervous system as a whole are reviewed. Current con- cepts related to tbe mechanisms of unexplained syncope and tbe "state-of-tbe-art" diagnostic and treatment options are also discussed (J Cardiovasc Electrophysiol, Vol. 7, pp. 760-776, Au- gtist 1996) vasovagal syncope, heart, central nervous system, tilt table testing, baroreceptors, mechatioreceptors Introduction In evolutionary terms, tbe ability of vertebrates to maintain a stable blood pressure during a wide range of diverse activities is testimony to the re- markably complex regulatory processes that have developed over the millions of years since crea- tures first crawled out of the sea. It is no sur- prise, therefore, that we find ourselves with only limited com pre hen.si on of these exquisitely con- trolled regulatoiy schemes. Stimuli from a myriad of intemiU and external sources bombaid the au- tonotiiic nervous system on a daily basis. It is this system that is responsible for axjrdinating the com- plex interplay that involves, to some extent, nearly every major organ system in the body. In humans, we have grown to appreciate the need to better understand the processes that gov- eni hemodynamic stability because of the poten- tial for tbis regulatory system to fail. Failure to .sense, process, and respond appropriately to the constant barrage of stimuli can result in loss of consciousness and, in more severe instances, loss Addre:ss for correspotitJencc: Keith Lurie. M.D., Box 508 U.M.H.C.. 420 Delaware Street. S.E.. Minneapolis. MN 5.'545.'5. Fax:612-626-4411. Manuscript received 31 March 1995; Accepted for publication 20 January 1996. of life. Given tbe complexity of the control mech- anisms, it is extraordinary tbat failure does not oc- cur more frequently. In this review article, we describe some ol' the key features of autonomic nervous system regulation of bUxxl pressure and focus, more specifically, on the pathopbysiology underlying an abaipt loss of blood pressure and tbe resultant loss of consciousness. Although it is difficult to estimate, it is likely that up to 50'^ of all individuals may develop syn- cope at least once during their lifetime.' Nearly all individuals experience tnuisient dizziness secondiiry to positional change, especially after going from squatting to standing or after bending over. True syncope accounts for as many as 6% of ail hos- pital admissions.-' As such, it represents a signifi- cant cost to the health care system and to society at large. Given true syncope's potential economic significance, it is .surprising how little we still know about bow to diagnose and treat the different syn- copal disorders. Under normal conditions, "autoregulatory" fea- tures of the cerebral vascular bed maintain tight control of total cerebral blood How.-*^ Normal cere- bral bkKxJ flow can be maintained witb mean ar- terial pressures > 70 mmHg.-' When reduced levels of cerebral blood flow persist for more tban 10 seconds, Uie condition known as "syncope" de- velops. For the purposes of tbis article, syncope is