Effects of Class IA Antiarrhythmic Drugs on the Reentrant Circuit in Common Human Atrial Flutter.

In 9 patients with the common type of atrial flutter (AF), a zone of slow conduction was observed proximal to the coronary sinus orifice. Class IA antiarrhythmic drugs were administrated intravenously in all 9 patients. AF was converted to sinus rhythm in only 1 patient. In the remaining 8 patients, although IA drugs suppressed conduction velocity, this effect was not preferential with regard to the zone of slow conduction. Conversion of AF to sinus rhythm by rapid pacing was achieved in these 8 patients. At a critical pacing cycle length, AF was interrupted just after conduction block developed in the zone of slow conduction. A progressive decrease in conduction velocity was observed in the zone of slow conduction for several beats just before the onset of conduction block. In conclusion, class IA drugs facilitate the interruption of AF by rapid atrial pacing by inducing conduction block in the zone of slow conduction.