In left bundle branch block, the IV septum and left ventricle are not activated normally. Rather, the impulse crosses the septum slowly from right to left and then engages the distal portion of the left ventricular Purkinje system. As a result, the spread of excitation through the left ventricle is delayed by a minimum of 40 msec, the time required for trans septal activation of the left venricle, the zequence of left ventricular activation is abnormal, the QRS complex is prolonged by at least 40 msec and the morphology of the QRS complex s altered.