Termination of atrio-ventricular nodal reentrant tachycardia

ECG 17A: Regular tachycardia, with narrow QRS, of 200 bpm; 1:1 atrioventricular conduction; probable atrial activity at the end of the QRS, negative in inferior leads and positive in V1 and aVR;

ECG 17B:

ECG 17B: Termination of the tachycardia after injection of verapamil; the modification of the QRS pattern on the sinus complexes confirms that it was indeed a retrograde atrial conduction;

Comments: This patient presented typical episodes of atrio-ventricular nodal reentrant tachycardia. In some patients, the atrioventricular node may be the site of a two-way functional longitudinal dissociation whose conduction velocities and refractory periods differ even if it is impossible to identify two different specific cellular structures. The atrioventricular node can be divided into a rapid anterior and superior pathway with a long refractory period and a slower posterior and inferior pathway with a shorter refractory period. In the common form of atrio-ventricular nodal reentrant tachycardia (slow-fast), by far the most frequent, reentry is established between the slow pathway used in the anterograde direction and the fast pathway used in the retrograde direction. The onset of the tachycardia is generally characteristic: after an atrial extrasystole, the fast pathway becomes blocked (longer refractory period compared to the slow pathway), the impulse is thus transmitted in anterograde manner by the slow pathway with a jump in conduction, a sudden prolongation of the PR interval and of the AH interval. Conduction through the slow pathway is sufficiently slowed to allow time for the fast pathway to exit its refractory period and regain its excitability. Retrograde penetration can then be achieved in the fast pathway and reciprocating (reentrant) tachycardia can begin if the refractory periods and conduction velocities are compatible for a new anterograde conduction through the slow pathway. In a patient presenting typical atrio-ventricular nodal reentrant tachycardia, atrial pacing can easily trigger an episode, as opposed to being much more difficult using ventricular pacing. During rapid atrial pacing, the AH interval is progressively prolonged then abruptly with a conduction jump (increase in AH interval of more than 50 ms for a 10 ms coupling reduction of the atrial stimulation cycle) after which the tachycardia begins with an AH interval of less than 50 ms.

This type of tachycardia exhibits certain electrocardiographic characteristics:

• the electrocardiogram between tachycardia episodes is usually normal;
• the rate of the tachycardia may vary from one patient to another (between 130 and 250 bpm); it is generally very rapid in children (> 200 bpm) and slower in the elderly (< 150 bpm) when nodal conduction is impaired;
• the QRS are narrow in the majority of cases, even if a conduction aberration is possible; a QRS complex alternans can be observed without being specific;
• 1:1 atrioventricular conduction; the P’ waves are retrograde and can be superimposed on the terminal potion of the QRS; they are thereby difficult to identify; primary atrial depolarization is anterosuperior (at the emergence of the pathway); they can modify the morphology of the QRS (in comparison with complexes of sinus origin) with a characteristic pattern of pseudo S wave in the inferior leads and pseudo r’ wave (discrete right delay) in V1 or in aVR; in rarer instances, atrial activity can be positioned just prior to the QRS; certain electrocardiographic patterns are less typical with an anterograde conduction block and two atrial activities for one ventricular complex;

Take-home message:  The typical electrocardiographic pattern of intranodal tachycardia corresponds to a regular narrow QRS tachycardia, with 1:1 atrioventricular conduction and an indistinguishable atrial activity since superimposed on the QRS complex.