Left bundle branch block - Conduction disturbances

Left bundle branch block

Definition

Block of conduction in the left branch of the bundle of His, which normally facilitates transmission of impulses to the left ventricle

Aetiology/pathophysiology

The most common cause is ischaemic heart disease. The block may be complete or partial.

Clinical features

Most patients are asymptomatic but reversed splitting of the second heart sound may be observed. Normally, the second heart sound splits during inspiration because the increased flow of blood into the right side of the heart delays the pulmonary component. In left bundle branch block the second heart sound is split on expiration, because left ventricular conduction delay causes the aortic valve to close after the pulmonary valve. On inspiration it occurs together with the pulmonary component because the pulmonary component is also delayed.

Investigations

ECG shows a characteristic RsR’ M shape in seen in lead V5 or V6. There is a deep ‘S’ wave in leads V1– V3. Left bundle branch block results in a QRS complex of greater than 0.12 seconds. There is left axis deviation and the morphology of the QRS complex is abnormal (see Fig. 2.13). Acute left bundle branch block may be a sign of acute myocardial infarction.

Complications

Syncope due to intermittent complete heart block.

Management

Treatment is not necessary.