Atrial flutter with a large bystander segment without double potentials in the cavotricuspid isthmus

We present two cases of common-type atrial flutter with a large bystander segment without double potentials in the cavotricuspid isthmus. In both cases, right atrial angiography demonstrated a prominent pouch at the center of the isthmus. When radiofrequency energy was applied to the tricuspid side of the isthmus, delayed potentials abruptly appeared on the local electrograms. When radiofrequency energy was applied on the inferior vena cava side of the isthmus, the tachycardia was terminated. Although ablation was not applied to the bottom of the pouch, bidirectional isthmus block was achieved. These outcomes indicate that the pouch represented a bystander segment.