Left bundle branch block (LBBB) is common after age 60 and is due to a block of the conduction in the bundle that receives impulses from the atrium and conducts the electrical current through the left ventricle (Fig. 1). The ECG is typical, and the physician makes the diagnosis only after looking at the ECG.
A. ECG Diagnostic Criteria
In LBBB the QRS complex is wide and bizarre looking with a duration of greater than 120 ms (see Fig. 4). Other criteria include notched R wave in lead I, aVL, V5, or V6; leads V1 and V2 have small or no R waves; and the ST segment in V1 and V2 is elevated.
B. Causes of Left Bundle Branch Block
RBBB is often seen in normal individuals. LBBB, on the other hand, is more often due to subtle diseases of the heart, including coronary artery disease, valvular heart disease, muscle scars, cardiomyopathies, fibrosis, degen¬erative disease (Lev and Lenegre disease), and all causes of left ventricular hypertrophy. Approximately 15% of patients with aortic stenosis that causes severe left ven¬tricular hypertrophy exhibit LBBB.
Although LBBB is a serious conduction disturbance, the electrical current gets to the left ventricular myocardium via the right ventricular conduction system. This activates the left ventricle with a time delay that is observed as an increased duration of the QRS complex of the ECG. It does not cause disturbance of the heart rate and virtually never requires an artificial pacemaker.

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