Left bundle branch block (LBBB) is a frequent electrocardiographic (ECG) condition reported in patients who have impaired cardiac conduction along both the anterior and posterior left fascicles of the His-Purkinje system. It is often associated with other heart conditions, such as coronary artery disease, cardiomyopathy, and valvular disease. Patients with LBBB have prolonged QRS complexes without R waves in V1 through V6. The QRS complex is usually wider than 120 ms and sometimes has a slurred ventricular impulse or "sick sinus syndrome-like" appearance. Electrocardiography (ECG) shows a wide, flat QRS complex with no identifiable P waves and no reciprocal activity between leads V5 and V6. There may be some indication of left ventricular dysfunction from one or more of the following: enlargement of the ventricles, reduced contractility, increased internal resistance, or abnormal wall motion. The presence of LBBB should alert the physician to search for other causes of an abnormal ECG, especially coronary artery disease. Diagnosis requires finding evidence of delayed activation of the left ventricle on an ECG recording.