Syncopes in a patient with right bundle branch block and left anterior fascicular block

ECG 8A: On arrival at the emergency room, normal PR interval (180 ms), complete right bundle branch block pattern (QRS duration 120 ms, rSR’ pattern in V1) with left anterior fascicular block (left axis, q wave in lead I and wide S wave in lead II);

ECG 8B: The patient is admitted with telemetry monitoring; new syncope episode with evidence on the tracing of a prolonged asystole with numerous blocked P waves; rare ventricular escape rhythms;

ECG 8C: A dual-chamber pacemaker is implanted; the electrocardiogram shows a rhythm predominantly detected in the atrium (a single paced atrial cycle) and paced in the ventricle (low right ventricular septal pacing lead explaining QRS negativity in lead I and in inferior leads); DDD pacing mode (synchronization of the ventricles on the paced or sensed atria);

Comments: This patient initially presented a bifascicular block. The left axis deviation adding to a typical pattern of right bundle branch block should evoke the associated diagnosis of left anterior fascicular block.

This patient visited the emergency room following the occurrence of several syncopes within a brief time span (two days). Clinical (several convulsive syncopes with no obvious trigger) and electrocardiographic evaluation (conduction disorder) is strongly suggestive of a cardiac origin which justifies an immediate hospitalization with telemetry monitoring. The criteria for emergency hospitalization following syncope include (1) the presence of an underlying cardiac disease, (2) the presence of an electrocardiographic abnormality pointing to a conduction or rhythm disorder, 3) exertional syncope, 4) severe trauma, 5) family history of sudden death or inherited Brugada-type disease.

The association of right bundle branch block and left anterior fascicular block is relatively common. The fascicular block deviates the very first QRS vectors downward and to the right, the middle QRS vectors upward and to the left. The right bundle branch block adds terminal QRS vectors directed to the right (right ventricular potentials).

Take-home message: A left axis deviation in a patient with right bundle branch block points to the presence of an associated left anterior fascicular block.