An atrioventricular (AV) canal is a large hole in the center of the heart between the upper chambers and lower chambers. Also, the tricuspid and mitral valves that normally separate the heart's upper and lower chambers aren't formed as individual valves. Instead, a single large valve forms that crosses the defect.
The large opening in the center of the heart lets oxygen-rich (red) blood from the heart's left side (blood that has just gone through the lungs) pass into the heart's right side. There, the oxygen-rich blood, along with venous (bluish) blood from the body, is sent back to the lungs. The heart pumps an extra amount of blood and may enlarge. Most babies with an atrioventricular canal don't grow normally and may become malnourished.
In some babies, the common valve between the upper and lower chambers doesn't close properly; this allows blood to leak backward from the heart's lower chambers to the upper ones. This leak (called regurgitation or insufficiency) can occur on the right side, left side, or both sides of the heart. With a valve leak, the heart pumps an extra amount of blood, becomes overworked and enlarges.
Heart with Atrioventricular Canal