(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.