Left bundle branch block

In left bundle branch block, activation of the intraventricular septum is reversed and electrical impulses to the left ventricle are delayed. These altered electrical forces produce a wide QRS complex (greater than 0.12 seconds in duration) with an abnormal morphology. In leads I and V6, abnormal initial forces fail to produce any Q wave or S wave, and the resultant R wave is steep and often notched (circled below). Furthermore, a deep rapid S wave is generated in lead V1 (arrow below). The ST segment is slightly elevated in multiple leads and the T wave polarity is diffusely opposite to the ventricular complex.