- Criteria for left bundle branch block (LBBB) 
- QRS >0,12 sec
- Broad monomorphic R waves in I and V6 with no Q waves
- Broad monomorphic S waves in V1, may have a small r wave
In left bundle branch block (LBBB) the conduction in the left bundle is slow. This results in delayed depolarisation of the left ventricle, especially the left lateral wall. The electrical activity in the left lateral wall is unopposed by the usual right ventricular electrical activity. The last activity on the ECG thus goes to the left or away from V1. Once you remember this, LBBB is easy to understand.
Diagnosis of myocardial infarction in LBBB is difficult as the ST segment is allways abnormal.