Concealed his bundle extrasystoles in congenital heart disease.

His bundle electrograms were recorded in two children with cardiac conduction disturbances. In the surface electrocardiograms of one patient His bundle extrasystoles mimicked atrial nonconducted bigeminal rhythms; in the other, they simulated second degree atrioventricular (A-V) block and conducted junctional extrasystoles. In both instances the conduction disturbance was a result of concealed conduction of the His bundle extrasystole into the A-V junction. We conclude that the surface electrocardiogram is suggestive but not definitive in diagnosing His bundle extrasystoles and that His bundle recordings are indispensable for correct diagnosis and treatment.