Electrophysiology and anatomic characterization of an epicardial accessory pathway.

Pericardial access permitted epicardial catheter mapping and ablation of a rapidly conducting posteroseptal accessory pathway (AP) that had failed repeated ablation attempts. Transient block was achieved at the site of an AP potential. The AP was visible at surgery and resected. Histologic examination revealed cells typical of specialized cardiac conduction tissue. The location, size, and presence of conduction tissue likely account for failure of catheter ablation and resistance to drug therapy.