Paradoxical increase of stimulus to atrium interval despite His-bundle capture during para-Hisian pacing.

Para-Hisian pacing at high output simultaneously captures the His bundle or proximal right bundle, as well as the adjacent ventricular myocardium. At lower output, direct His-bundle, or right-bundle, capture is lost which causes an increase in the stimulus to atrium interval. We describe a case with an increment of 68 ms with capture of the His bundle. This seems to be a paradoxical response, which however can be explained by the presence of retrograde dual AV-node physiology. Continuation of this phenomenon could be related to concealed anterograde invasion of the fast pathway thereby maintaining the retrograde activation during para-Hisian pacing on the slow pathway despite paced cycle lengths (His to His intervals) where retrograde fast pathway conduction proved to be possible.