An irregular narrow complex tachycardia: atrial fibrillation or something else?

The intracardiac recording during tachycardia (Fig. 1) shows a regular atrial rhythm, excluding atrial fibrillation. His bundle activation precedes ventricular activation and there is atrioventricular dissociation. The main differential diagnosis comprises atrioventricular node reentrant tachycardia (AVNRT), which may occur without atrial activation, and focal junctional tachycardia (FJT). Distinction is possible by analysing the response to a sinus beat which occurs before junctional depolarisation when the His bundle is non-refractory. Normal conduction of the sinus beat excludes AVNRT with bidirectional AH block. The sinus beat is conducted down the fast AV nodal pathway, leading to advancement of His bundle and ventricular activation without terminating or resetting the tachycardia. In common type (slow-fast) AVNRT, the subsequent refractoriness of the fast pathway would terminate the tachycardia. In contrast, termination is not expected during FJT, because of its focal nature [1]. Therefore, this tachycardia was diagnosed as FJT