Left bundle branch block

**Definition:** Disorder of the atrioventricular conduction system in which the QRS duration is greater than or equal to 120 ms in adults, greater than 100 ms in children 4 to 16 years of age, and greater than 90 ms in children less than 4 years of age; there is a QS or rS pattern in lead V1 and a wide slurred R wave in leads I and V6.