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.