Case PresentationThe sequential ablation strategy for persistent atrial fibrillation involves a series of predetermined ablation steps while monitoring of the impact after each step by measuring AF cycle length at "sentinel" electrodes, principally the right and left atrial appendages. 1,2 We describe a 69-year-old woman undergoing ablation for persistent AF in whom atrial fibrillation reverted to atrial tachycardia during the procedure. The left ventricular ejection fraction was 68% and the left atrial Figure 1. Transition from organized AF to AT1 during ablation. There was progressive organization of the AF until this point with increments on the CL, but irregularity is still noticed (left panel). Finally, there is the transition to atrial tachycardia (right panel). I, II, and V 1 = surface ECG leads CS 1-2 to CS 9-10 = coronary sinus electrode pairs from distal to proximal.diameter on the parasternal axis was 47 mm. During ablation of the tachycardia, transition directly into another atrial tachycardia was observed.The ablation procedure was done as previously described, 3 using a steerable decapolar catheter (5-mm electrode spacing, Xtrem, ELA Medical) positioned within the coronary sinus (CS), a circumferential mapping catheter (Lasso, Biosense-Webster) introduced by transseptal access and positioned at the ostia of the pulmonary veins (PV), and a 3.5-mm irrigated-tip ablation catheter (Celsius Thermocool, Biosense-Webster). After the transseptal procedure, a single bolus of 50 IU/kg heparin was administered.The CL measured at the left atrial appendage (LAA) increased from 130 ms to 150 after all PVs were isolated. There was no change in CL after a roof line. 3 Ablation of areas with extensive fractionation 2 at the inferior left atrium, left septum, and lateral left atrium increased LAA CL to 160 ms. Finally, ablation of a fractionated area between the LAA and left inferior pulmonary vein caused reversion of atrial fibrillation to a regular atrial tachycardia of CL 190 ms (Fig. 1).Mapping of this tachycardia suggested tricuspidcaval isthmus-dependent tachycardia with counterclockwise