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Figure 1 A, Simultaneous recordings of the surface ECG, leads I and aVF, and 56 bipolar electrograms from the BC in a patient with focal AT. The first beat is a sinus beat. The next three beats are tachycardia beats. His bundle potential is recorded in electrode pairs F_2–3 and F_3–4. The asterisks show the earliest spot of activation during sinus rhythm (SR) and AT. Spline A was located in the anterolateral RA, splines B and C in the lateral region, splines D and E in the posterior region and splines G and H across the tricuspid valve. The activation times are marked with red bars. B, Animated maps of the SR (upper panel) and AT (lower panel) beats. Planar and three-dimensional options are shown. During SR the impulse emerged in the high lateral area (spline B_1–2) and propagated rapidly down the lateral wall. The complete activation of the RA took 85 ms. During focal AT, the earliest activity emerged in the mid-posterior wall (spline E_4–5). The activation sequence of the RA was entirely different from that of SR. The complete activation of the RA took 95 ms.