During the normal development of the heart, a wall (septum) and two valves grow to separate the heart into four chambers. These atrioventricular valves (AV valves) regulate blood flow from the atria (upper chambers) to the ventricles (lower chambers). The AV valve on the right side is called the tricuspid valve; on the left is the mitral valve.

When the septum and valves do not grow together properly, a large hole occurs in the middle of the heart. This is called an atrioventricular (AV) septal defect (also called AV Canal or Endocardial Cushion Defect). There are several types of AV septal defects depending on how abnormal the septum and valves are. An example of the most severe type, a "complete" AV septal defect, is shown above. Instead of two valves, there is only one "common" AV valve over a large hole between all four chambers.

In complete AV septal defect, blood can mix in all four chambers. "Red blood" which has already been through the lungs, passes from the left to right side, going back to the lungs again. Some "blue blood" also goes to the left side. Both sides of the heart work hard. With time, blood vessels in the lungs can be damaged from all the extra blood. Infants with this defect grow poorly, tire easily, and have frequent respiratory infections. The age and type of repair vary according to the severity of the defect and symptoms. In general, surgery is needed at an early age.

Children with AV septal defects require penicillin (also called SBE prophylaxis) prior to any dental work or surgery on the mouth, bowel, or bladder. This helps prevent the uncommon, but possible, occurrence of bacterial infection affecting the heart near the defect or heart valves.