A heart block is a condition in which the electrical system of the heart is impaired as the electrical signal cannot travel down through the special pathways. In a heart block, the electrical impulses that cause the heart to beat are delayed. Heart block is caused by certain medications, surgery, rheumatic fever, and primary disease (an infection). A child can also be born with heart block (congenital heart block). A heart block can be a blockage, or a delay, at any level of the electrical conduction system of the heart. Thus, blocks which occur within the sinoatrial node (SA node) are described as sinoatrial blocks; blocks that occur within the atrioventricular node (AV node) are described as atrioventricular blocks; and blocks which occur below the AV node are known as infra-Hisian blocks.
According to the severity of the delays, this condition can be classified as: 1) first-degree heart block, in which the signal is just slowed down a little as it runs along the defective part of the conduction system so that it arrives late traveling from the atrium to the ventricle; 2) second-degree heart block, in which not every impulse reaches its destination, resulting sometimes in an overall slowing of the heart called bradycardia; 3) third degree hear block, which is the most serious, when no signals can travel through the AV node, forcing the heart to use its backup impulse generator in the lower portion of the heart. This impulse usually keeps the heart from stopping entirely, but it is too slow to be an effective pump.