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Figure 1 (A) Tracings showing atrial activation sequence during ventricular stimulation (left side) and during atrial tachycardia (right side). Tachycardia originated from the right side of the interatrial septum. (B) Tracings showing atrial activation sequence during ventricular stimulation (left side) and during atrial tachycardia (right side). Tachycardia originated from the left side of the interatrial septum. HB = His bundle region; d = distal; m = middle; p = proximal; RAA = right atrial appendage; RV = right ventricle; TA = inferoseptal aspect of the tricuspid annulus.