A person with a third degree heart block has a delay in the electrical impulse that travels through the heart. This delay results in an incomplete heart beat.

An electrical impulse stimulates the muscle fibers in the heart to contract. The impulse spreads through the heart in a very organized manner, so that the atria contract first, followed by the ventricles. In third degree heart block, an organized heart beat is lost: the ventricles beat independently of the atria.