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Figure 3 (A) Intracardiac recording during tachycardia with the ablator catheter at the mitral annulus showing the earliest activation in the left atrium. (B) Ablation performed at this site leads to prolongation of cycle length and termination of tachycardia with dissociation of coronary sinus (CS) potential (arrows). The interval between the last two atrial complexes during tachycardia is identical to the interval between the recorded CS potential. (C) Intracardiac recording during sinus rhythm showing CS potential (arrows) is now dissociated from CS atrial electrogram and identical to the tachycardia cycle length. This suggests that the CS potential is the site of origin of the tachycardia.