First-degree atrioventricular block and trifascicular block

ECG: Sinus rhythm (positive P waves in leads I, II, V5, V6, negative in aVR); left atrial enlargement (wide P wave, bifid in leads II, III, V4-V6, with wide negative component in V1); fixed and prolonged PR interval (240 ms): first-degree AV block; wide QRS (160 ms) with right bundle branch block pattern (large amplitude R wave in V1, wide S wave in V6) and left anterior fascicular block (left axis, rS pattern in inferior leads);

Comments: This electrocardiogram shows a bifascicular block pattern (right bundle branch block + left anterior fascicular block) and a moderately prolonged PR. This combination is sometimes erroneously considered as a trifascicular block. This electrocardiogram alone does not confirm the presence of a trifascicular block, since the long PR may be the result of a slowing of conduction in the atrioventricular node (bifascicular block) instead of the left posterior hemibranch, the remaining fascicule. It is not solely an issue of semantics, since the prognosis differs according to the degree of progression of the conduction disorders. An electrophysiological study allows defining the site of the slowing explaining the prolongation of the PR interval by differentiating between a delay in the atrium or atrioventricular node (prolonged AH interval), a delay in the bundle of His (prolonged or split His potential) and a distal conduction delay in the branches and Purkinje network (prolonged HV interval). In this patient, there was a pathological HV interval at 90 ms, indicative of conduction slowing in the “functional” hemibranch confirming the presence of a trifascicular block.

Certain electrocardiographic abnormalities reveal the presence of a trifascicular block: alternating left and right bundle branch block, alternating right bundle branch block/left anterior fascicular block and right bundle branch block/left posterior fascicular block.

In this patient, the probability of a complete atrioventricular block responsible for syncope was very high, justifying the implantation of a pacemaker.

Take-home message: Most often, the electrocardiographic pattern can only suggest the location of the block or the slowing of atrioventricular conduction. For example, an association on the electrocardiogram between right bundle branch block, left anterior fascicular block and prolonged PR interval does not necessarily reflect the presence of a trifascicular block. Demonstration of a long HV interval during an electrophysiological study confirms the diagnosis of trifascicular block with conduction delay in the remaining fascicule (left posterior hemibranch).