(b)
Figure 1. Activation mapping of the left atrium and coronary sinus in a patient who presented with clockwise mitral annular flutter. (a) Activation mapping prior to endocardial ablation showed clockwise mitral annular flutter with passive activation of the coronary sinus (CS). (b) Following endocardial ablation with creation of posterolateral mitral annular (MA) line, epicardial conduction involving the CS was demonstrated, which maintains tachycardia. Ablation from within the CS across the endocardial posterolateral MA line slowed and terminated the flutter, and bidirectional block across the line was confirmed.