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Figure 2 (A) An example of the typical atrial activation sequence with the ablation signal at the successful site. The first beat is a sinus beat showing earliest activation at the high crista terminalis (CT). The second beat is a spontaneous atrial ectopic originating from the tachycardia focus. Earliest activity on the right atrial and coronary sinus (CS) catheters is at the proximal His bundle pair. However, the mapping and ablation (MAP) catheter located at the superior mitral annulus (MA) reveals a much earlier signal, which was 40 ms ahead of P-wave onset. Note the atrial-to-ventricular ratio is indicative of an annular location. (B) Graphic representation of the mean activation times at each of the recorded endocardial sites for the seven patients. Earliest endocardial activation on these standard catheters occurred at the His bundle region in all seven patients. HBE = His bundle. Numbers listed in catheter location refer to bipolar pair.