Causes

Tetralogy of Fallot occurs as the baby's heart is developing during pregnancy. Usually, the cause is unknown.

Tetralogy of Fallot includes four defects:

• Narrowing of the lung valve (pulmonary valve stenosis). Narrowing of the valve that separates the lower right chamber of the heart (right ventricle) from the main blood vessel leading to the lungs (pulmonary artery) reduces blood flow to the lungs. The narrowing might also affect the muscle beneath the pulmonary valve. Sometimes, the pulmonary valve doesn't form properly (pulmonary atresia).
• A hole between the bottom heart chambers (ventricular septal defect). A ventricular septal defect is a hole in the wall (septum) that separates the two lower chambers of the heart (left and right ventricles). The hole causes oxygen-poor blood in the right ventricle to mix with oxygen-rich blood in the left ventricle. This causes inefficient blood flow and reduces the supply of oxygen-rich blood to the body. The defect eventually can weaken the heart.
• Shifting of the body's main artery (aorta). Normally the aorta branches off the left ventricle. In tetralogy of Fallot, the aorta is in the wrong position. It's shifted to the right and lies directly above the hole in the heart wall (ventricular septal defect). As a result, the aorta receives a mix of oxygen-rich and oxygen-poor blood from both the right and left ventricles.
• Thickening of the right lower heart chamber (right ventricular hypertrophy). When the heart's pumping action is overworked, the muscular wall of the right ventricle becomes thick. Over time this might cause the heart to stiffen, become weak and eventually fail.

Some children or adults who have tetralogy of Fallot may have other heart defects such as a hole between the heart's upper chambers (atrial septal defect), a right aortic arch or problems with the coronary arteries.