Wenckebach pattern in right bundle branch block - benign or not?

A Mobitz type I block (Wenckebach phenomenon) with narrow QRS complex is almost always due to a lesion in the AV node. In a type I block with wide QRS complex (>0.12 sec), the block can be in the His-Purkinje system in 60-70% of the cases. Even though the progressive PR prolongation with every conducted beat suggests Wenckebach phenomenon, one needs to pay attention to the accompanying QRS complex. In the setting of persistent right bundle branch block, axis change of subsequent conducted beat before an unconducted p suggests alternating fascicular block, giving clue to unreliable infra-Hisian conduction; and in the setting of symptoms, a pacemaker should be implanted. Infra-Hisian Wenckebach block is rare with only sparse literature reports. The present case report adds to these, suggesting that wide QRS with Wenckebach block on surface ECG may indicate infra-Hisian conduction abnormalities.