An old name for this defect is an atrioventricular canal defect, or AV canal for short. More complex forms of AVSD also include the opening between the two ventricles, along with the lack of separation of the AV valve into separate mitral and tricuspid valves. The reason this defect may be so complex is that it probably occurs during the very first stages of fetal development. It then goes back to the lungs even though it has already been oxygenated. Because of the large amount of extra blood going to the lungs, high blood pressure may occur there and over time can damage the blood vessels.
Blood then leaks backward from the ventricular chambers to the atrial chambers.
This leak is referred to as an insufficiency of the valve. When the valve leaks, the heart has to pump more blood. This can lead to many other problems, including enlargement of the heart. This defect is usually corrected in infancy with open heart surgery to restore normal blood circulation through the heart. The operation represents a surgical separation of the valves, ventricles and atria, so it tends to be a big operation.