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Figure 1 Surface electrocardiogram P waves in one patient with atrial fibrillation (45-year-old man) during pacing at six sites in four pulmonary veins (PVs) are shown. Pacing in left PVs produced P waves with characteristic features of low amplitude in lead I, negativity in lead aVL, similar amplitude in both leads II and III, notched shape in lead II (most evident during left inferior [LI] pacing in this case), and long positive phase in lead V1. In contrast, P waves during right PV pacing were clearly positive in lead I, relatively flat in lead aVL, and had a low amplitude ratio of lead III/II (<0.8). The P waves during superior PV pacing are taller than those of inferior PV pacing in inferior limb leads. A positive P-wave in lead aVL, seen during RI pacing in this case, is a specific marker of right PV origin of pacing. Bottom-LS = bottom of the left superior PV; Bottom-RS = bottom of the right superior PV; RI = right inferior PV; Top-LS = top of the left superior PV; Top-RS = top of the right superior PV.
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