Spontaneous Localized Persistent Atrial Fibrillation with an Exit Block Mimicking Atrial Tachycardia at the Left Posterior Wall

We describe a 37-year-old man with spontaneous localized atrial fibrillation (AF) with an exit block at the posterior wall of the left atrium (LA). The 12-lead ECG exhibited an atrial tachycardia-like pattern, with distinctive P waves and an isoelectric baseline between the P waves. The cycle length of the P waves ranged from 320 to 500 msec. While the fractionated and rapid deflections were recorded from the posterior wall of the LA, the rest of the atria and the coronary sinus exhibited discrete atrial potentials with irregular intervals. Radiofrequency energy applications to the surrounding tissue created complete isolation of the localized AF area, and the AF was terminated. Fibrillatory activation in the posterior wall of the LA can act as a driver as well as an initiator of atrial fibrillation.