Simultaneous appearance of endocardial late potentials and ability to induce sustained ventricular tachycardia after procainamide administration.

We describe a patient in whom procainamide induced the appearance of late potentials during intraoperative sinus rhythm electrogram mapping. Only nonsustained ventricular tachycardia (VT) could be induced while off all antiarrhythmic drugs. After administration of the procainamide, programmed stimulation induced sustained VT coincident with the appearance of late potentials during sinus rhythm. The late potential was recorded from the same site, during normal sinus rhythm, where mid to late diastolic activation during VT was recorded, and where cryotermination occurred during cryomapping. We hypothesize that procainamide slowed conduction, manifested as prolongation or appearance of late potentials in sinus rhythm, and facilitated induction of sustained reentrant ventricular tachycardia.