An unbalanced AVSD (the left ventricle is very small) and Fontan physiology. The Fontan procedure connects the inferior vena cava (IVC) to the right pulmonary artery. Now all the deoxygenated blood from the body bypasses the heart and directly enters the lungs. There is also a small hole between the Fontan conduit and the right atrium which is called a fenestration. The fenestration allows a small amount of the blue deoxygenated blood from the Fontan conduit to enter the heart where the red oxygenated blood is located. This fenestration is meant to relieve some pressure by acting as a "pop-off valve" until the lungs adjust to the new circulation.