Syncope due to complete atrioventricular block with junctional escape rhythm

ECG: Third-degree AV block pattern: sinus tachycardia (105 beats/minute) with ventricular bradycardia (40 beats/minute); atrioventricular dissociation; probable junctional escape rhythm with right bundle branch block and left anterior fascicular block;

Comments: The diagnosis of complete atrioventricular block is defined by the complete interruption of atrioventricular conduction and is based on the presence of an atrioventricular dissociation, the atria and the ventricles being under the control of independent pacemakers, with a ventricular rate slower than the atrial rate. There is no relationship between P waves and QRS complexes, PR intervals are variable without any repetitive sequence.

The rate of ventricular escape rhythm is generally regular and not, or minimally, influenced by exercise or vagal maneuvers. The rate and duration of the QRS complexes vary according to the site of the escape rhythm:

• Junctional escape rhythm: the origin of the escape being located high above the bifurcation of the bundle of His, the duration of QRS complexes is normal (less than 100 ms) unless there is a preexisting branch block (as in this patient); ventricular rate is generally moderately slowed (between 40 and 50 beats per minute);
•  Escape rhythm originating in the bundle of His or of in one of the branches: the origin of the escape being located either at the level of the bundle of His or below the bifurcation on one of the branches, the QRS may be narrow (His escape rhythm) or broad with a bundle branch block pattern (His + bundle branch block or bundle branch escape rhythm); ventricular rate is generally slowed (between 30 and 40 beats per minute);
• Ventricular escape rhythm: the origin of the escape being located low and corresponding to an idioventricular focus, the duration of the QRS complexes is very broad (between 140 and 200 ms) does not meet bundle branch block criteria; ventricular rate is very slow (sometimes less than 20 beats per minute).

Atrial rhythm is most often sinus even if atrial fibrillation or atrial flutter can be observed. The sinus rhythm is frequently accelerated, a physiological reaction of the sinus node to the decreased output caused by the ventricular bradycardia.

Take-home message: The diagnosis of complete AV block is defined by the complete interruption of atrioventricular conduction following a conduction block located at the level of the junction, the bundle of His or its branches. The rate and duration of the QRS vary according to the site of the escape rhythm. There is often a reflex sinus tachycardia.