Interesting case of narrow QRS tachycardia with atrioventricular dissociation.

A 30-year-old man with no structural heart disease has been evaluated for paroxysmal palpitation with documented regular narrow QRS tachycardia that has not responded to intravenous adenosine. Surface electrocardiogram has not shown any pre-excitation. He has been taken for an electrophysiology study after informed consent. Diagnostic catheters were placed at the coronary sinus, His bundle region, and right ventricle. During catheter manipulation a regular narrow QRS tachycardia with incomplete right bundle branch block morphology and normal QRS axis similar to the clinical tachycardia got induced. No other tachycardia was induced. What is the mechanism of tachycardia?

Commentary

The tachycardia showed incomplete right bundle branch block morphology (RBBB) with atrioventricular (AV) dissociation (Fig. 1A) narrowing the differentials to (1) typical AV nodal re-entrant tachycardia with upper common pathway block, (2) focal junctional tachycardia, (3) orthodromic re-entrant tachycardia involving nodofascicular bypass tract with upper common pathway block, and (4) ventricular tachycardia (VT) involving the conduction system. Intracardiac electrograms (Fig. 1B) during the tachycardia show AV dissociation. The third QRS beat (asterisk) in Fig. 1B represents a capture beat suggesting the mechanism as VT. The ladder diagram depicted in Fig. 1C illustrates AV dissociation during VT with capture beat. The spontaneous atrial ectopic beat (A′) has captured the node–His–Purkinje system with subsequent depolarization of the right ventricle culminating as a capture beat. In view of the RBBB morphology, normal QRS axis with no underlying heart disease possibility of upper septal VT was considered and confirmed by entrainment response from the left ventricular upper septum. The tachycardia was successfully ablated at the left ventricular upper–mid septum. Patient remains asymptomatic at 6 months after ablation. VT involving the conduction system and exiting closer to the His bundle presents as relatively narrower QRS tachycardia and poses a diagnostic challenge to the treating physician.

Fig. 1

Narrow QRS tachycardia with A-V dissociation.

VT presents usually as wide QRS tachycardia. However, involvement of the His–Purkinje system in the tachycardia circuit or proximity of the tachycardia focus to the conduction system can result in a narrow QRS tachycardia. Fascicular VT can present as narrow QRS tachycardia due to involvement of the conduction system in the tachycardia circuit. Among the subtypes of fascicular VT, upper septal idiopathic fascicular VT is very rare. This tachycardia uses portions of the posterior fascicle as the anterograde limb (which can be considered as an orthodromic form of posterior fascicular VT) and the septal fascicle as the retrograde limb, with simultaneous passive activation of the right bundle branch and anterior fascicle, which accounts for the relatively narrow QRS that can be very similar to baseline QRS [1].