[Intermittent atrioventricular block in an accessory pathway associated with complete infrahissian block].

The appearance of syncope in a patient with an accessory pathway is often a sign of high conduction capacity along the pathway, and calls for urgent ablation. Serious complications may ensue if it is assumed that these patients have an underlying mechanism of tachyarrhythmia, and ablation of the accessory pathway performed hastily without careful electrophysiological evaluation may lead to serious complications. The case described here, despite the patient's unusual presenting features, illustrates that the causal mechanism of syncope is not always what it appears to be. Although our patient had an obvious accessory pathway that was clearly evident in the electrocardiogram, the actual cause of the problem was complete infrahissian atrioventricular block.