A rare cause of narrow QRS complex tachycardia: the tortoise and the hare

The patient underwent an electrophysiology study. At baseline, there was sinus rhythm with normal AH and HV intervals. Dual atrioventricular (AV) nodal physiology with manifest 1:2 AV conduction (‘double fire’) was observed during atrial extrastimuli (Fig. 1). During sinus acceleration, periods of sustained 1:2 AV conduction were observed. After administration of isoproterenol, a fast-slow AV nodal re-entrant tachycardia (AVNRT) could be induced with a tachycardia cycle length of 290 ms. After radiofrequency ablation of the slow pathway at the right inferoseptal area, no AH jump was present and no tachycardia could be induced. The presenting ECG was, most likely, a dual AV nodal non-re-entrant tachycardia (DAVNNT) (Fig. 2).

DAVNNT is caused by simultaneous antegrade conduction over the fast and slow pathways. DAVNNT is a rare arrhythmia and a systematic review in 2016 identified only 68 cases in the literature [1]. It is often misclassified as atrial fibrillation or premature beats. The combination of AVNRT and DAVNNT is even rarer [2]. DAVNNT may cause tachycardiomyopathy [3]. The ECG provides the clues for the diagnosis, demonstrating one normal P wave followed by two QRS complexes. These QRS complexes are usually narrow, but aberrancy is possible. Aberrant conducted QRS complexes are often misclassified as premature ventricular complexes. The differential diagnosis of AVNRT with 2:1 VA conduction is refuted by the absence of retrograde P waves (positive P‑wave morphology in inferior leads) and the slightly irregular RR intervals. Ablation of the slow pathway eliminates dual AV nodal conduction and is the cornerstone of invasive treatment.