Unusual ventricular atrial conduction shift from the slow pathway to the fast pathway.

A 63-year-old woman underwent an electrophysiological study for a treatment of narrow QRS tachycardia, which was diagnosed as a slow-fast atrioventricular nodal reentrant tachycardia. She exhibited dual nodal retrograde conduction, with transitions from the fast pathway to the slow pathway and back to the fast pathway. After focal cryoapplications to the mid-septal region of the right atrial septum, both antegrade and retrograde slow pathway conduction were eliminated without affecting the antegrade fast pathway conduction. The mechanism of transition of retrograde conduction between the fast and slow pathways was explained by the following two scenarios: First, the slow pathway was initially concealed by the fast pathway and became evident only when the fast pathway was blocked due to a gap phenomenon. Second, the slow pathway conducted earlier than the fast pathway at a specific point between the ventricle and atrium, thereby masking fast pathway conduction. This could be attributed to the high levels of connexin 43 reported around the lower nodal bundle. Learning objectives: When retrograde conduction transitioned from the fast pathway to the slow pathway and then back to the fast pathway, the mechanism was considered a gap phenomenon. This suggested that the slow pathway was initially masked by the fast pathway and could conduct only when the fast pathway was blocked. However, recent findings on the distribution of connexin 43 suggested another possibility that the slow pathway might also mask the fast pathway.