Prolongation of QRS complex in ECG represents:
Intrinsic impairment of conduction in either the right or the left bundle system (intraventricular conduction disturbances) leads to prolongation of the QRS interval.

With complete bundle branch blocks, the QRS interval is >120 ms in duration.
With incomplete blocks, the QRS interval is between 100 and 120 ms.
Acute cor pulmonale
Chronic cor pulmonale
Left ventricular hypertrophy
Bundle branch block