A 52-year-old woman without a history of cardiac surgery underwent electrophysiological study for an atrial tachycardia (AT). Echocardiography revealed no evidence of structural heart disease or anatomic abnormalities. At baseline, the AT was incessant and the P waves were upright in lead I and the inferior leads and biphasic (±) P waves in lead V1 (Figure 1). During mapping at the lower posterior right atrium (RA), an unexpected movement of the mapping catheter was observed. A right atriogram performed with a pigtail catheter revealed an aneurysmal structure between the RA and the diaphragm (Figure 2). This structure exhibited a passive expansion and shrinkage with each cardiac cycle. Far-field electrograms were recorded at the sites along the junction between the RA and this structure, and no electrograms were recorded at any other sites within this structure (Figure 3). Activation mapping revealed a centrifugal activation pattern from …