The interaction of antiarrhythmic drugs and the energy for cardioversion of chronic atrial fibrillation.

Antiarrhythmic drugs may alter the energy for cardioversion of ventricular arrhythmias. This study compares the energy necessary for cardioverting chronic atrial fibrillation in 57 patients taking type Ia, Ic, or type III antiarrhythmic drugs. Patients taking Ia (n = 22) or III (n = 14) drugs had a median energy for cardioversion of 100 joules, while the patients taking Ic (n = 17) drugs had a median energy of 200 joules (P = 0.03). There were no differences in the frequency of unsuccessful cardioversion. There were no serious adverse events in any of the three groups, although three patients in the Ic group had greater than 3 second pauses after the shock. The data suggest that the use of Ic antiarrhythmic drugs results in a higher energy for cardioversion of atrial fibrillation. However with higher energies, conversion is as successful as for type Ia and type III.