In this video we review the ECG changes associated with damage to different components of the cardiac conducting system. Damage to the AV node or diffuse damage to multiple components on the right and left sides of the intraventricular conducting system impairs the transmission of atrial depolarisation into the ventricular myocardium manifesting as AV block on the ECG. This may be first second or third degree.

Isolated damage to the main body of fibres in one of the branches of the bundle of His results in left or right bundle branch block on the ECG. The QRS complexes are wide as depolarisation travels slowly through the affected chamber, outside the conducting system.

The left bundle branch divides into three peripheral branches or fascicles. The two major subdivisions of the left bundle, the left anterior fascicle and the left posterior fascicle depolarise the walls of the left ventricle. When one of these major fascicles is damaged, the qrs complex duration remains within the normal range. However, the altered pattern of flow within the left ventricular wall manifests on the ECG as a shift in the cardiac axis.