Holter strip of modified leads V1, V5, and aVF showing atrial fibrillation initiated by the second atrial premature beat (arrows) with left and right aberrant conduction in a patient treated with sotalol for paroxysmal atrial fibrillation. Note the long–short RR sequence at initiation of aberrant conduction. Sotalol may have enhanced onset of aberrancy in this case due to prolongation of refractoriness in the Purkinje system.