Left bundle branch block is a problem with the heart’s electrical wiring (conduction) system.
Your heart has 4 chambers. The 2 upper chambers are called atria, and the 2 lower chambers are called ventricles. In a healthy heart, the signal to start your heartbeat begins in the upper right chamber of the heart (right atrium). From there, the signal activates the left atrium and travels to the lower chambers (right and left ventricles) of the heart. As the signal travels along the heart’s conduction system, it triggers nearby parts of the heart to contract in a coordinated manner.
Two bundle branches carry the electrical signal through the ventricles to the bottom of the heart and cause the ventricles to beat. These are termed the right bundle and left bundle. In left bundle branch block, there is a problem with the left branch of the electrical conduction system. The electrical signal can’t travel down this path the way it normally would. The signal still gets to the left ventricle, but it is slowed down. That’s because the signal has to spread from the right bundle branch through the heart muscle and slowly activate the left ventricle. So the left ventricle contracts a little later than it normally would. This can cause an uncoordinated contraction of the heart. As a result, the heart may eject blood less efficiently. For most people, this is not a big problem. But if you have underlying heart failure, left bundle branch block can make it worse.
Some people may have left bundle branch block for many years without any problems. But a newly diagnosed left bundle branch block may mean there is some underlying heart condition that requires prompt treatment. An aggressive evaluation may be necessary if you have new onset of a left bundle branch block.
Some people with left bundle branch block may need a permanent pacemaker. A pacemaker helps keep the heart beating at the correct rate. This is usually only needed if you are having symptoms or have another conduction problem along with left bundle branch block.