The signals from the natural pacemaker of the heart is conducted down to the ventricles by a structure known as the bundle of His. The bundle of His has two branches – left and right. The right branch goes to the right ventricle and the left one goes to the left ventricle. When the conduction through the left bundle is delayed or blocked, a characteristic pattern is seen on the ECG, which is known as the left bundle branch block (LBBB). Though LBBB is often associated with structural heart disease unlike RBBB (right bundle branch block), LBBB can also occur in isolation, without any structural heart disease.