The heart beats by using electric circuits. These circuits allow the electricity to follow a specific route through the heart. These electric routes or pathways are sometimes grouped together into specialized areas called nodes and bundles.
Bundles send out little fibers that go into the muscle of the heart.
The nodes, bundles, and fibers are responsible for the unified beating of the heart and the rate at which it beats. A defect along any of these pathways can cause a heart block. There are many kinds of heart block. Each type depends on where the damage has occurred in the electrical pathway. The worst form of heart block is called complete heart block.