If the QRS complex is wider than 0.12 seconds this is mostly caused by a delay in the conduction tissue of one of the bundle branches:
*[[#Left_Bundle_Branch_Block_.28LBBB.29| Left Bundle Branch Block (LBBB))]]
RBBB| Right Bundle Branch Block(RBBB)]]
*Interventricular conduction delay
A right or left axis rotation can be caused by a:
Criteria for LAFB| left anterior fascicular block (LAFB)]]*[[# Criteria for LPFB| left posterior fascicular block (LPFB)]]
Sometimes this conduction delay is '''frequency-dependent ''': the bundle branch block occurs only at higher heart rates and disappears at slower heart rates.