Pseudo sinus rhythm originating from the left superior pulmonary vein in a patient with paroxysmal atrial fibrillation.

We report the case of a patient with paroxysmal atrial fibrillation in whom the background cardiac rhythm falsely mimicked sinus rhythm but actually originated from the left superior pulmonary vein. P waves during the ectopic rhythm were flat in lead I, negative in lead aVL, and without a typical "dome-and-dart" feature in precordial leads. Radiofrequency applications inside the left superior pulmonary vein eliminated both atrial fibrillation and the ectopic pacemaker.