Two Different Mechanisms for the Transition from Fast/Slow to Slow/Slow Atrioventricular Nodal Reentrant Tachycardia

The reentry circuit of Fast/Slow atrioventricular nodal reentrant tachycardia (AVNRT) is not well understood. We experienced two cases of AVNRT where Fast/Slow and Slow/Slow AVNRT shifted to each other. In the first case, anterograde conduction over the fast pathway and retrograde conduction over one of the two slow pathways was considered as the reentrant circuit of Fast/Slow AVNRT. The transition from Fast/Slow to Slow/Slow AVNRT was caused by the shift of the anterograde pathway from the fast pathway to the other slow pathway. In the second case, anterograde conduction over the one slow pathway and retrograde conduction over the other slow pathway was considered as the circuit of Fast/Slow AVNRT. Fast pathway activated His bundle but did not participate in the reentrant circuit. The transition from Fast/Slow to Slow/Slow AVNRT was caused by the conduction block of the fast pathway. These two cases with the transition from Fast/Slow to Slow/Slow AVNRT show that two different circuits may underlie Fast/Slow AVNRT.