A medical device for thermally treating cardiac tissue includes a catheterhaving aproximal end, a distal end, and a thermally conductive region. A firstelectrode on the catheterproximal to the thermally conductive region is included. A second electrode onthe catheterproximal to the first electrode is included. The first and second electrodesare operable tomeasure an impedance value across at least a portion of a thoracic cavity.