Paroxysmal short runs of irregular narrow QRS tachycardia: What is the mechanism?

The present case that showed the frequent transition between left atrial tachycardia and AV nodal re-entrant tachycardia has never been reported. This instructive case highlights the need for careful interpretation of intracardiac electrocardiogram and suggests that differential pacing maneuvers are not feasible in transitional tachycardia with atrial fusion.