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Figure 4 Example of scar-related intra-atrial re-entrant tachycardia (IART) from patient 11, who had undergone atrioventricular septal repair. There was a zone of slow conduction as evidenced by the fractionated potentials. There was also a line of conduction block (double potentials) causing the activation to proceed from Halo 4 down to Halo 10 and then back up to Halo 1. Pacing was performed in the lateral right atrium through the distal ablation catheter (ABLATE-D). A zone with low amplitude and fractionated potentials, suggestive of right atrial scar tissue, demonstrated acceleration of the tachycardia with concealed fusion. The postpacing interval was almost identical to the tachycardia cycle length. The surface electrocardiographic leads are I, L, F and V₁. Intracardiac electrograms are as in Figure 3.