Left anterior fascicular block

**Definition:** Disorder of the atrioventricular conduction system characterised by left axis deviation for age (frontal plane axis between -45° and -90°), qR pattern in lead aVL, R-peak time in lead aVL of 45 ms or more, and a QRS duration that does not meet age dependent criteria for complete bundle branch block (less than 120 ms in adults, less than 100 ms in children 4 to 16 years of age, and less than 90 ms in children less than 4 years of age)