Electrocardiographic masking of a myocardial infarction by a fasciculoventricular Mahaim fiber, atrioventricular reentry with anterograde conduction over a fasciculoventricular Mahaim fiber and retrograde conduction over a concealed Kent bundle.

An intracardiac electrophysiologic study was performed in a patient with paroxysmal tachycardia. The patient had two previous episodes of myocardial infarction, but abnormal Q waves were absent in previously obtained electrocardiograms. Electrocardiographic diagnosis of inferior and anterior infarction was made because the electrocardiogram taken on admission temporarily showed abnormal Q waves. Atrioventricular reentry with anterograde conduction over a fasciculoventricular Mahaim fiber and retrograde conduction over a concealed Kent bundle was observed during the tachycardia. It was also demonstrated that electrocardiographic masking of the Q waves was caused by conduction over a fasciculoventricular Mahaim fiber.