Tetralogy of Fallot (ToF) is a combination of Ventricular Septal Defect (VSD) with Pulmonary Stenosis, with the Aorta "Overriding" the VSD and with an abnormal enlargement or pathologic increase in muscle mass of the right ventricle. This combination is termed "Tetralogy of Fallot".
The abnormal flow of blood from the heart into the lungs leads to blood being diverted through the VSD to the aorta. Blood flow and lung circulation is reduced causing the child to appear "Blue".
Tetralogy of Fallot (ToF) is made up of the following four defects of the heart and its blood vessels:
A hole in the wall between the two lower chambers―or ventricles―of the heart. This condition also is called a ventricular septal defect.
A narrowing of the pulmonary valve and main pulmonary artery. This condition also is called pulmonary stenosis.
The aortic valve, which opens to the aorta, is enlarged and seems to open from both ventricles, rather than from the left ventricle only, as in a normal heart. In this defect, the aortic valve sits directly on top of the ventricular septal defect.
The muscular wall of the lower right chamber of the heart (right ventricle) is thicker than normal. This also is called ventricular hypertrophy.
Because a baby with tetralogy of Fallot may need surgery or other procedures soon after birth, this birth defect is considered a critical congenital heart defect.