Syncope, left bundle branch block and second-degree atrioventricular block type 1

ECG 7A: Sinus rhythm, left atrial enlargement (widened and fragmented P wave); left bundle branch block: QRS width > 120 ms; absence of q wave along with wide and notched R wave in leads I, aVL, V5 and V6 with delayed intrinsicoid deflection > 60 ms; QS and rS pattern in V1 and V2; ST segment depression and negative T wave in leads I, aVL, V5 and V6;

ECG 7B: Left bundle branch block; bradycardia associated with a Mobitz 1 second-degree AV block pattern; gradual prolongation of the PR interval followed by blocked P wave (Wenckebach periods);

Comments: This patient presented with a syncope which resulted in a fall with bleeding scalp wound. The electrocardiogram revealed a characteristic pattern of complete left bundle branch block. During an episode of lightheadedness, the ECG showed a bradycardia in conjunction with a second-degree (Mobitz 1) AV block pattern.

When there is an associated branch block, a second-degree AV block with Wenckebach periodicity may result from a slowing of conduction in the atrioventricular node, but should also raise suspicion and warrant investigation for a delayed activation on the branch opposite to the blocked branch (trifascicular block). The infra-Hisian Wenckebach periods in these instances are associated with a constant AH interval and a gradual prolongation of the HV interval, the blocked P wave being followed by a His potential not conducted to the ventricle.

The indications for implantation in the setting of second-degree AV block type 1 are controversial. The causal relationship between conduction disorder and the onset of symptoms is difficult to establish, especially when the symptoms are modest and unspecific. There is a class IIA indication in the European guidelines when a second-degree AV block type 1 is responsible for symptoms or when the conduction disorder is of intra- or infra-Hisian localization (major risk of progression to a complete atrioventricular block) upon electrophysiological study (EPS). This patient underwent an EPS which confirmed the presence of a trifascicular block and underwent pacemaker implantation.

Take-home message: When associated with a bundle branch block, a second-degree AV block Wenckebach type pattern may result from a delayed conduction in the atrioventricular node but should raise suspicion and warrant exploring for a delayed activation on the opposite branch of the blocked branch (trifascicular block). Electrophysiological study may reveal a prolonged HV interval.