Mobitz type II second-degree atrioventricular block with narrow QRS and junctional extrasystoles: what is the mechanism?

A 74-year-old man underwent an electrophysiological study because of Mobitz type II second-degree atrioventricular (AV) block with narrow QRS and frequent junctional extrasystoles. During the study, there were very frequent single His bundle depolarizations with multiple coupling intervals that reproduce the ECG findings. In this case, some His bundle extrasystoles result in retrograde concealed conduction and prolonged local refractoriness in the AV node that manifest as block of the next atrial impulse.