Regular Atrial Tachyarrhythmia with Double Coronary Sinus Potentials: What Is the Diagnosis?

Case PresentationA 63-year-old woman underwent electrophysiological studies and radiofrequency catheter ablation of a persistent, drug-refractory atrial tachyarrhythmia (ATA).She had no history of left atrial (LA) surgery or catheter ablation.Positive waves were present in leads II, III, aVF, and V1-V6 of the 12-lead electrocardiogram, inconsistent with typical atrial flutter (AFL).A 7F Inquiry TM eicosapolar electrode catheter (St.Jude Medical, AF Division, Minnetonka, MN, USA) with 2-mm interelectrode spacing was advanced into the coronary sinus (CS) from the right subclavian vein, with its proximal electrodes (19-20) placed at the ostium.Double potentials with dissimilar activation sequences were recorded during ongoing ATA, sequentially from the left lateral region (CS5-CS6) to the CS ostium (CS19-CS20), with the first, low-amplitude, low-frequency potential (DP1) in a distal-to-proximal and the second, higher-amplitude, higher-frequency potential (DP2) in a proximal-to-distal direction, and fusion of the 2 potentials at the CS ostium (Fig. 1A).Constant and progressive fusion of the atrial wave was observed on the surface electrocardiogram during entrainment pacing from the right atrium, consistent with a reentrant mechanism of ATA.The postpacing interval (PPI) at any site in the right atrium was >30 ms longer than the tachycardia cycle length (TCL).During entrainment pacing from CS1-CS2, located at 3 o'clock on the mitral annulus, the PPI between the last pacing stimulus (S) and the first DP2 was >30 ms longer than the TCL (Fig. 1B), whereas during entrainment pacing from CS9-CS10, located at 6 o'clock on the mitral annulus, the interval was nearly equal to the TCL (Fig. 1C).What is (a) the