EFO_0004138 (bundle branch block) can be described as follows. A bundle branch block refers to a defect of the heart's electrical conduction system in which the bundle branch may cease to conduct electrical impulses appropriately. This results in altered pathways for ventricular depolarization. Since the electrical impulse can no longer use the preferred pathway across the bundle branch, it may move instead through muscle fibers in a way that both slows the electrical movement and changes the directional propagation of the impulses. As a result, there is a loss of ventricular synchrony, ventricular depolarization is prolonged, and there may be a corresponding drop in cardiac output. When heart failure is present, a specialized pacemaker may be used to resynchronize the ventricles. Also known as: bundle-branch block.