What do we see on the ECG of a patient with a left bundle branch block?
The most important changes in the QRS complex are:
1) The specifically widened R in 1, aVL, V5, and V6, which shows an M configuration;
2) The upper transition point in v6 is located more than 55 ms away from the beginning of the R wave; and
3) in the chest wall leads, the latest negative motion of the R wave will be found in V5 or V6.
4) In the leads with M-configured QRS complexes (lead 1, aVL, V5, and V6), discordant T wave inversion is typically present. In the remaining precordial wall leads, in particular V1 and V2, we can see deep S waves.