Flecainide-induced arrhythmia in canine ventricular epicardium. Phase 2 reentry?
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.


RESULTS
Standard microelectrode techniques were used. Flecainide (10-20 microM) produced either prolongation or marked abbreviation of APD 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 flecainide-treated 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 of epicardium. 4-Aminopyridine, a transient outward current (Ito) blocker, reversed the flecainide-induced marked abbreviation of APD in epicardium and abolished reentrant activity in all cases. Flecainide failed to induce reentry in preparations pretreated with 4-aminopyridine.


CONCLUSIONS
Our data suggest that the presence of a prominent Ito in epicardium contributes the development of marked electrical heterogeneity in the ventricle after exposure to flecainide. Flecainide-induced dispersion of repolarization, especially when accompanied by prominent conduction delays, results in extrasystolic activity via a mechanism that we have termed "phase 2 reentry." Our results also suggest a role for Ito blockers in the treatment of reentrant arrhythmias.
Krishnan
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SC
Masonic Medical Research Laboratory, Utica, N.Y. 13504.
Antzelevitch
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C
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced* / Q000503:physiopathology; D004285:Dogs; D004699:Endocardium / Q000187:drug effects / Q000503:physiopathology; D005424:Flecainide / Q000494:pharmacology*; D006352:Heart Ventricles; D008297:Male; D008955:Models, Cardiovascular; D009994:Osmolar Concentration; D010496:Pericardium / Q000187:drug effects* / Q000503:physiopathology; D011930:Reaction Time
D015761:4-Aminopyridine; D005424:Flecainide