Flecainide - Induced Arrhythmia in Canine Ventricular Epicardium Phase 2 Reentry ? Subramaniam

Background. We recently reported that sodium channel block can produce opposite effects on action potential duration (APD) and refractoriness in epicardial versus endocardial tissues of the canine ventricle. In addition, strong sodium channel current inhibition was found to cause loss of the action potential dome in epicardium but not endocardium, thus inducing a marked dispersion of repolarization and refractoriness between epicardium and endocardium as well as among neighboring epicardial sites. The marked heterogeneity that evolves under these conditions provides a substrate for the development of arrhythmias. Flecainide was found to induce extrasystolic activity more readily than other sodium blockers. The present study contrasts the electrophysiological actions of flecainide in canine ventricular epicardium and endocardium and examines the characteristics of flecainide-induced arrhythmias in epicardial sheets of canine ventricle. Methods and Results. Standard microelectrode techniques were used. Flecainide (10-20 jgM) produced either prolongation or marked abbreviation ofAPD in epicardium but only minor changes in the APD of endocardium. Marked abbreviation of APD in epicardium was due to loss of the action potential dome (plateau phase). Arrhythmias displaying characteristics of reentry could be readily induced in flecainidetreated preparations either by increasing the stimulation rate or by introduction of extrastimuli. Flecainide-induced slowing of conduction, more accentuated at the faster stimulation rates, appeared to act synergistically with the drug-induced dispersion of repolarization to generate reentry in these relatively small sheets ofepicardium. 4-Aminopyridine, a transient outward current (It.) blocker, reversed the flecainide-induced marked abbreviation ofAPD in epicardium and abolished reentrant activity in all