A spontaneous or pacing‐induced transition from slow‐fast atrioventricular nodal reentrant tachycardia to fast‐slow type using a superior slow pathway

Wenckebach-periodic VA prolongation and abrupt shortening of HH interval during tachycardia indicate (i) a retrograde block at the upper common pathway that manifested a retrograde atrial activation via the superior slow pathway, and (ii) an antegrade return of a retrograde atrial activation to His bundle via the fast pathway.