POSSIBLE ATRIAL PROARRHYTHMIC EFFECTS OF CLASS-1C ANTIARRHYTHMIC DRUGS

The class IC antiarrhythmic drugs encainide and flecainide have been used to treat ventricular arrhythmias.1,2 Although not currently approved, they are also being used in the treatment of atria1 arrhythmias.3,4 This antiarrhythmic action results from a marked depressant effect on conduction velocity and a moderate effect on prolonging refractoriness in the myocardium.5,6 This marked depression of conduction velocity also results in the highest incidence of ventricular proarrhythmic effects observed with any class of antiarrhythmic drug.7,8 However, less is known regarding their potential to produce atria1 proarrhythmic effects.3,4 Recently we observed 6 patients who were being treated with flecainide or encainide for preexisting ventricular or atria1 arrhythmias, and who developed new or modified symptomatic atria1 arrhythmias with a rapid ventricular response. In 2 cases, life-threatening consequences arose from the arrhythmia. The clinical details of these cases are presented and the potential mechanisms of the atria1 proarrhythmic effects of the 1C drugs are discussed. © 1990.