Effects of disopyramide on cycle length, effective refractory period and excitable gap of atrial flutter, and relation to arrhythmia termination by overdrive pacing.

The administration of class IA antiarrhythmic drugs facilities termination of atrial flutter by overdrive pacing. To investigate the electrophysiologic determinants of this effect, changes in the cycle length, the effective refractory period and the excitable gap of spontaneous type I atrial flutter were studied in 11 patients given intravenous disopyramide (3 mg/kg in 1 hour). After drug infusion, the cycle length of atrial flutter increased from 238 +/- 26 to 298 +/- 38 ms (+25%; p less than 0.001) and the effective refractory period prolonged from 169 +/- 19 to 192 +/- 25 ms (+14%; p less than 0.01). The excitable gap prolonged from 62 +/- 16 to 96 +/- 27 ms (+55%; p less than 0.001). Atrial flutter was terminated by overdrive pacing (mean cycle 203) in 10 of 11 patients; in 1 patient atrial fibrillation resulted after high rate stimulation. In the setting of an anatomically defined reentry circuit, as in type I atrial flutter, the administration of disopyramide prolongs both cycle length and refractory period. The finding of an increased excitable gap suggests that the drug exerts its prominent effect by depressing conduction velocity. A wider excitable gap allows easier penetration of the stimulus in the reentry circuit and accounts for the beneficial effects of type IA antiarrhythmic drugs on the termination of atrial flutter by overdrive pacing.