Bundle branch block
Electrical impulses that cause your heart to beat (contract) start in the heart's upper right chamber (right atrium) and travel to the lower chambers (ventricles). In bundle branch block, the pathway these impulses follow is delayed or blocked. The pathway includes two branches — the left and the right bundles. If one bundle is damaged, the ventricles rely on the other bundle to receive and respond to signals from the right atrium. If both bundles are blocked, the heart may beat very slowly, which can require a pacemaker.