The conduction system of heart is comprised of sinoatrial (SA) node, atrioventricular (AV) node, the bundle of HIS, left and right bundle branches and purkinje fibres. SA is the pacemaker of the heart, located in the right atrium. It starts the electrical impulse which travels throughout the heart on a specified route. First, it travels through the atria, stimulating left and right atrium, then it must pass a group of specialized cells, called AV node, before it can go to the lower ventricle chambers. After passing through AV node, the impulse travels along a track called the "bundle of HIS". This bundle divides into a right bundle and a left bundle. These two bundles go to the right and left lower chambers of the heart (ventricles). Normally, the electrical impulse travels down both the right and left branches at the same speed. Thus, both ventricles contract at the same time. But occasionally there's a block in one of the branches, in this case, left branch. This causes the impulse to take a different route to reach the left ventricle. Consequently, left ventricle contracts a fraction of a second slower than normal. This causes the deflections in the electrocardiographic tracing (QRS) to become wide and bizarre.
There is no specific symptoms related to left bundle branch block (LBBB), rather, the symptoms are associated with the underlying cause.