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Figure 1 Ectopic focus originating from within a pulmonary vein. (A) A premature atrial contraction originating from the right superior pulmonary vein (RSPV) is noted by the arrow. It precedes all other intracardiac recordings and surface P-wave. Additionally, within the vein there is distal to proximal activation. (B) An ectopic beat from the same vein serves as a trigger for atrial fibrillation. D = distal; DCS = distal coronary sinus; HALO = duodecapolar catheter in right atrium; LSPV = left superior pulmonary vein; MCS = middle coronary sinus; P = proximal; PCS = proximal coronary sinus.