Diagnostic criteria for left bundle branch block

Complete left bundle branch block has widening of the QRS complex to 120 ms or more. If LBBB pattern is seen with QRS width less than 120 ms, it may be called incomplete left bundle branch block, though that term is rarely used in practice.

Broad, notched or slurred R waves in lateral leads I, aVL, V5, V6.

Small or absent initial r waves in V1 and V2 followed by deep S wave. If the initial r is absent, then it is a QS complex without any subsequent R wave after the Q.

No septal q waves in I, V5, V6. That is due to the absence of the initial left to right activation of the interventricular septum which occurs in the presence of normal conduction in the left bundle branch.

Prolonged time to peak R wave of >60 ms in V5, V6, indicating delayed completion of activation of left ventricle.