Left Bundle Branch Block (LBBB) ECG Criteria
The ECG criteria for a left bundle branch block include:
- QRS duration greater than 120 milliseconds
- Absence of Q wave in leads I, V5 and V6
- Monomorphic R wave in I, V5 and V6
- ST and T wave displacement opposite to the major deflection of the QRS complex
A simple way to diagnose a left bundle branch in an ECG with a widened QRS complex (> 120 ms) would be to look at lead V1. If the QRS complex is widened and downwardly deflected in lead V1, a left bundle branch block is present. If the QRS complex is widened and upwardly deflected in lead V1, a right bundle branch block is present. The image below shows the typical findings of a left bundle branch block in the precordial ECG leads.