Atrioventricular canal defect occurs before birth when a baby's heart is developing. Some factors, such as Down syndrome, might increase the risk of atrioventricular canal defect. But the cause is generally unknown.

The normal-functioning heart

The heart is divided into four chambers, two on the right and two on the left.

The right side of your heart moves blood into vessels that lead to the lungs. There, oxygen enriches the blood. The oxygen-rich blood flows back to your heart's left side and is pumped into a large vessel (aorta) that circulates blood to the rest of your body.

Valves control the flow of blood into and out of the chambers of your heart. These valves open to allow blood to move to the next chamber or to one of the arteries, and close to keep blood from flowing backward.

What happens in atrioventricular canal defect

In partial atrioventricular canal defect:

  • There's a hole in the wall (septum) that separates the upper chambers (atria) of the heart
  • Often the valve between the upper and lower left chambers (mitral valve) also has a defect that causes it to leak (mitral valve regurgitation)

In complete atrioventricular canal defect:

  • There's a large hole in the center of the heart where the walls between the atria and the lower chambers (ventricles) meet. Oxygen-rich and oxygen-poor blood mix through that hole.
  • Instead of separate valves on the right and left, there's one large valve between the upper and lower chambers.
  • The abnormal valve leaks blood into the ventricles.
  • The heart is forced to work harder and enlarges.
Aug. 14, 2015