Exercise induced atrioventricular block with gap phenomenon in atrioventricular conduction.

A 54-year-old man with normal atrioventricular (AV) conduction at rest gave a 4-year history of presyncope during exercise. Treadmill testing showed exercise induced AV block. Electrophysiological study demonstrated rate dependent infranodal AV block and abnormal refractory period of the His-Purkinje system. The gap phenomenon in AV conduction occurred during the programmed stimulation. Supernormal conduction could be considered as the mechanism of the gap phenomenon in this patient.