Left bundle branch block overview

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Left bundle branch block is a cardiac conduction abnormality seen on the electrocardiogram (ECG) whereby there is an impairment of transmission of the cardiac electrical impulse along the fibers of the left main bundle branch, or both the left anterior fascicle and left posterior fascicle. This conduction disturbance is characterized by wide (greater than or equal to 0.12 seconds) QRS complexes. In this condition, activation of the left ventricle is delayed, which results in the left ventricle contracting later than the right ventricle.

LBBB is uncommon among patients under 50 years of age (<0.5%). It occurs in 6% to 8% of patients over the age of 50.

On examination of the cardiovascular system, a paradoxical split of the second heart sound may be heard.

Asymptomatic patients with isolated left bundle branch block and no underlying heart disease require no treatment. Routine follow-up is required in such patients. In symptomatic patients treatment is directed at the underlying cause of left bundle branch block, such as ST elevation myocardial infarction. Patients withsyncope and LBBB may have a rhythm disturbance that requires a pacemaker. Given the dys-ynchrony that occurs with left ventricular contractility,cardiac resynchronization therapy in heart failure patients may be of benefit.

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