Augmented fragmentation of atrial activity upon premature electrical stimuli by verapamil.

The significance of fragmented activity obtained from the atrium during electrophysiologic study (EP study) was confirmed since it was always observed just at the onset of fibrillation. It was preceded by extra stimuli or atrial flutter, and the initial site of fragmentation varied from case to case. The premature-stimulus-induced widening of the atrial wave was observed in some cases who had normal size of atrium and no heart failure. The widening was augmented by administration of verapamil but not by procainamide, suggesting that for such widening or fragmented activity, slow-fiber-mediated conduction seems to be responsible. One case with intraatrial reentrant tachycardia developed fragmentation lasting for 1.0 second after verapamil. Therefore, some apparently normal atria may have a subclinical electrophysiologic abnormality that can be disclosed by premature stimulations or verapamil.