cardiac ablation is carried out by introducing a catheter into the left atrium , extending a lasso guide through the lumen of the catheter to engage the wall of a pulmonary vein , and deploying a balloon over the lasso guide . the balloon has an electrode assembly disposed its exterior . the electrode assembly includes a plurality of ablation electrodes circumferentially arranged about the longitudinal axis of the catheter . the inflated balloon is positioned against the pulmonary vein ostium , so that the ablation electrodes are in galvanic contact with the pulmonary vein , and electrical energy is conducted through the ablation electrodes to produce a circumferential lesion that circumscribes the pulmonary vein .