Abstract: A method and apparatus are disclosed for determining if a previously carried out heart ablation has been effective in isolating tissue. The method involves inserting an esophageal device, e.g. a catheter, into the esophagus so that an electrode is adjacent the heart, pacing the electrode with an electrical pacing signal, and determining if the heart adopts the pacing signal. Adoption of the pacing signal is indicative of the tissue not being electrically isolated by the ablation, and so may signal that a re-ablation procedure is necessary. Non adoption of the pacing signal is indicative of the tissue being electrically isolated by the ablation. Also disclosed is a processing unit for comparing a measured heart signal with a predetermined pacing signal to determine heart adoption of the pacing signal. The device and processing unit may be used to measure natural heart signals in the esophagus to assist in correct catheter positioning.