Marked conduction time prolongation observed during atrial tachycardia originating from a giant left atrial appendage.

We experienced a rare case of atrial flutter originating from the giant left appendage (LAA). The local potential of the ablation catheter presented with a rare finding, appearing up to 185 ms earlier than the surface P-wave in the distal LAA. With thoracoscopic LAA clipping, tachycardia was successfully controlled.