It is unusual for left bundle branch block (LBBB) to exist in the absence of organic disease. Causes are varied and dilated cardiomyopathy is one of the associations. LBBB is a form of interventricular conduction defect. In normal cardiac conduction, impulses travel equally down the left and right bundles, with the septum activated from left to right. In LBBB, conduction delay means that impulses travel first via the right bundle branch to the right ventricle, and then to the left ventricle via the septum, septal activation is thus reversed.

In echocardiography, regional wall motion abnormalities are often easier to recognize suggesting coronary artery disease or non-coronary scenarios. LBBB is recognized by a septal beaking (septal flash) which means early inward septal motion is seen prior to posterolateral wall motion due to septal-lateral dyssynchrony.