Modulation of an idioventricular rhythm by vagal tone

Department of Medicine, University of Toronto, Ontario, Canada.

A 28 year old man with a stable permanent idioventricular rhythm of 80 to 85 beats/min, with all the characteristics of a pacemaker, is described. This pacemaker was slowed by maneuvers that enhanced vagal tone, including carotid sinus massage, the postrelease phase of the Valsalva maneuver and phenylephrine. The pacemaker was also slowed by a cholinesterase inhibitor (edrophonium hydrochloride) and accelerated by a muscarinic receptor blocking drug (hyoscine butylbromide). The actions of these maneuvers and agents were independent of sympathetic tone as propranolol pretreatment did not alter their effects. Similarly, propranolol did not affect the pacemaker rate. The pacemaker was not dependent on a slow inward current because verapamil did not affect its rate. The pacemaker accelerated in response to increased sympathetic tone induced by exercise and upright tilting and to the adrenergic agonist isoproterenol. The pacemaker was localized to the high posterior septal region of the left ventricle underneath the mitral valve. This report describes in a man an idioventicular pacemaker that is innervated by sympathetic and vagal fibers and responsive to alterations in tone of both limbs of the autonomic nervous system. It offers the first clear proof that a ventricular pacemaker can be innervated and controlled by the vagus nerve and provides its location.