SYSTEM WITH AN ENDOCARDIAL DISPERSIVE ELECTRODE AND A MONOPOLAR RF ABLATION PEN

Methods and devices for forming a lesion in a target tissue having a cavity within. A first RF electrode and a second RF electrode can be coupled to opposite poles of an RF current source. The second electrode can be inserted into the tissue cavity and expanded to contact the target tissue from within. The first electrode can be externally disposed against the target tissue while applying RF current between the first and second electrodes to ablate the target tissue. Some methods are directed to ablating tribiculated atrial wall tissue to treat atrial fibrillation. The second electrode can contact the tribiculated tissue directly from within to provide a direct path between the two electrodes. In some methods, the second electrode is inserted through an incision made to remove an atrial appendage. The methods can provide deeper, narrower lesions relative to those made using remote, indifferent electrodes. Atrial fibrillation ablation procedures can be performed using the invention, requiring fewer incisions than conventional methods.