A process for preventing atrial premature contractions originating within a pulmonary vein from being conducted into the left atrium of a human heart. More specifically, an ablation lesion is formed which electrically isolates the located source of the atrial premature contraction in the pulmonary vein from connection with the left atrium and blocks passage of the atrial premature contraction originating from the located source. In one embodiment, the steps of the process include advancing a medical device into the left atrium of a human heart, introducing the advanced medical device into the pulmonary vein from the left atrium of a human heart, sensing electrical activity within the pulmonary vein using the introduced medical device, locating a source of the atrial premature contraction within the pulmonary vein using the sensed electrical activity, and forming an ablation lesion in tissue of the pulmonary vein at a location proximal from the located source of the atrial premature contraction within the pulmonary vein. Conductive media and various medical devices may be used with the process.