The AV node is a group of specialized conducting cells found in the middle of the heart. The AV node is responsible for transmitting electrical pulses from the top part of the heart to the bottom part of the heart. Complete heart block (complete AV block, or third-degree AV block) refers to a condition where the transmission of impulses through the AV node is completely interrupted. In other words, no impulses are able to get through whatsoever.
Complete heart block is usually a very serious situation. Some individuals with complete heart block may be completely asymptomatic and unaware that there is a problem. The abnormality may be diagnosed in this setting as an incidental finding on an ECG or other heart rhythm recording (such as holter monitoring). Most individuals with complete heart block, however, develop symptoms. These may include chronic fatigue because of a lower than normal heart rate, syncope, palpitation, or even nightmares.
Complete heart block can be found in a number of different situations in pediatric patients. Some patients who undergo open heart surgery may develop complete heart block as a result of the surgery. The AV node may be damaged directly by the surgery, or in other instances inflammation or swelling of the heart tissue may damage the AV node. Less commonly, complete heart block may present in someone who has never had heart surgery as a result of a more diffuse electrical conduction system disease. Finally, complete heart block may be the result of transmission of antibodies in the womb from the mother to the fetus. This usually happens in women who have underlying connective tissue disorders such as systemic lupus erythematosus. In this situation, antibodies from the mother cross into the fetus and attack the AV node. There may be evidence of a very slow heart rate in the fetus with development of complete heart block. In some instances of congenital complete heart block the problem may not be detected until the child is several years old.
Treatment of complete heart block usually requires placement of a permanent pacemaker. In cases of young children with congenital complete heart block who are asymptomatic, this may be postponed for several years. However, inevitably almost all patients with complete heart block require a pacemaker to protect the function of the heart and prevent the development of symptoms.