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Figure 1 (A) Schematic representation of the circular mapping catheter with 10 electrodes. Pulmonary vein (PV) muscle potentials were recorded at the proximal PV (within the first centimeter from the ostium) in bipolar mode from 10 bipoles (1 to 2, 2 to 3, etc., up to 10 to 1 with the initial electrode as the anode and the next electrode as the cathode). Polarity reversal was defined as a sudden change of the main deflection of pulmonary vein potential (PVP) from positive to negative when analyzing adjacent bipoles in ascending order. (B) Schema of the radial propagation of activation fronts from two different breakthroughs in the left superior PV. The numbers positioned at the ostium of the vein represent the numbers of 10 electrodes of circular mapping catheter. There are two distinct breakthroughs at both the anterior and posterior aspect of the vein. Radial propagation of activation front through one breakthrough (posterior wall) is reflected by an electrogram polarity reversal across adjacent two bipoles (6 to 7 and 7 to 8). In the case of wider breakthrough (anterior wall), an electrogram polarity reversal is observed across three consecutive bipoles (2 to 3, 3 to 4 and 4 to 5) with an intervening bipole (3 to 4) showing relatively isoelectric initial deflection.