What is atrioventricular canal defect?

Atrioventricular canal defect (AV canal) is a congenital (present at birth) heart defect. Other terms used to describe this defect are endocardial cushion defect and atrioventricular septal defect (AVSD). As the fetus is growing, something occurs to affect heart development during the first eight weeks of pregnancy, and certain areas of the heart do not form properly. AV canal is a complex heart problem that involves several abnormalities of structures inside the heart, including the following:

  • atrial septal defect - an opening in the interatrial septum, or dividing wall between the two upper chambers of the heart known as the right and left atria.
  • ventricular septal defect - an opening in the interventricular septum, or dividing wall between the two lower chambers of the heart known as the right and left ventricles.
  • improperly formed mitral and/or tricuspid valves - the valves that separate the upper heart chambers (atria) from the lower heart chambers (ventricles) are improperly formed. Specifically, there is an abnormality in the left-sided valve (the mitral valve): it has three cusps rather than the two cusps that normally form the valve. One of the normal cusps is divided into two cusps. This division between these two cusps is called the cleft (or a cut in the mitral valve).

Normally, oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, and then is pumped out to the body through the aorta.

An atrial septal defect allows oxygen-rich (red) blood to pass from the left atrium, through the opening in the septum (the wall) between the two atria, and then mix with oxygen-poor (blue) blood in the right atrium.

A ventricular septal defect allows oxygen-rich (red) blood to pass from the left ventricle, through the opening in the septum (the wall) between the two ventricles, and then mix with oxygen-poor (blue) blood in the right ventricle.

Abnormalities of the mitral or tricuspid valves allow blood that should be moving forward from the ventricle into either the pulmonary artery or the aorta to instead flow backward into the atria. This results in leakage of the mitral or tricuspid valves, knows as regurgitation or insufficiency.

Atrioventricular canal defects occur in about 5 percent of all congenital heart disease cases and are more common in infants with Down syndrome.