Antiarrhythmic and electrophysiologic effects of ibutilide in a chronic canine model of atrial flutter.

We studied the effects of orally administered ibutilide, a class III antiarrhythmic agent, in a model of reentrant atrial flutter in conscious dogs. After baseline determination of atrial effective refractory period (AERP) and demonstration of reproducible induction of atrial flutter by rapid atrial pacing, 8 dogs received either placebo or one of six doses of ibutilide ranging from 0.1 to 5 mg/kg. Refractory periods and the ability to induce atrial flutter were then assessed at periodic intervals for 8 hours. Ibutilide produced dose-related increases in AERP which were well correlated with prevention of initiation of atrial flutter after doses > or = 0.25 mg/kg. Placebo and 0.1 mg/kg ibutilide had no effect on AERP or the ability to induce atrial flutter. Doses of 0.25 to 1.0 mg/kg ibutilide significantly increased AERP and prevented induction of atrial flutter for 4-6 h. After treatment with 2.5 or 5 mg/kg ibutilide, significant increases in AERP and prevention of induction of atrial flutter persisted throughout the 8-h study period. The cycle length of inducible atrial flutter was significantly increased after administration of 5 mg/kg ibutilide. The results demonstrate oral efficacy of ibutilide with rapid onset of action (in 30-60 min), resulting in increased AERP and prevention of induced atrial flutter in this model.