Atypical atrio-ventricular nodal reentrant tachycardia

ECG 18A: The initial tracing is normal (sinus, normal PR, narrow QRS); ventricular extrasystole couplet; first tachycardia cycle with narrow QRS; wide QRS tachycardia (left delay) with probable retrograde atrial conduction (negative P’ waves in inferior leads and positive in V1) with long RP’ (P’ wave at the peak of the T wave);

ECG 18B:

ECG 18B: Tachycardia termination after a sino-carotid massage;

Comments: Electrophysiological study confirmed that this patient presented episodes of atypical atrio-ventricular nodal reentrant tachycardia with a conduction aberration of the left bundle branch block type. This atypical form, known as fast-slow, corresponds to approximately 5% of intranodal reentry induced in the electrophysiology laboratory; the fast nodal pathway is used in the anterograde direction and the slow pathway in the retrograde direction. This type of tachycardia is preferably triggered by a ventricular extrasystole or by rapid ventricular pacing. Contrary to the typical shape, a retrograde conduction jump revealing a nodal duality and a P’ wave located remotely from the QRS complex can be observed on the first tachycardia cycle, the ascent to the atrium being achieved through the slow pathway (blocked retrograde pathway).

During the tachycardia, there is usually:

1. narrow QRS complexes even if, as in this patient, the presence of a bundle branch block via a conduction aberration is possible;
2. a long RP’ interval;
3. P’ waves most often negative in the inferior leads and positive in V1. The late P’ wave superimposes or follows the T wave of the preceding cycle and can be identified much more easily than in the typical form. The primary atrial depolarization site is located at the base of the Koch triangle or in the first centimeters of the coronary sinus explaining the negative deflection in the inferior leads.

This particular electrocardiographic pattern (narrow QRS tachycardia, 1:1 ratio between atria and ventricles, long RP’ interval and negative P’ waves inferiorly) may sometimes hamper the differential diagnosis with atrial tachycardia or orthodromic tachycardia using a septal accessory pathway with slow retrograde conduction (Coumel tachycardia). The clinical setting (more or less permanent tachycardias for PJRT versus tachycardia episodes with sudden onset and termination) can direct the diagnosis. The recording of the beginning and the end of the tachycardia can also facilitate the diagnosis (atrial tachycardia generally starting on an atrial extrasystole and ending on a ventricular cycle, an atypical intranodal reentry generally beginning on a ventricular extrasystole and being terminated by vagal maneuvers). Electrophysiological study generally provides a definitive answer.

Take home message: In an atypical atrio-ventricular nodal reentrant tachycardia (fast-slow form), the circuit is reversed relative to the typical form thus explaining the electrocardiographic pattern: narrow QRS tachycardia (but possible conduction aberration), long RP’ interval and negative P’ waves in the inferior leads.