Tetralogy of Fallot (ToF) is a heart condition present at birth (congenital heart defect). ToF develops early in pregnancy, when the heart is dividing and forming its pumping chambers. The heart defects of ToF prevent some of the blood from your body from going directly to your lungs for oxygen. This causes the blood entering your heart to mix with the blood leaving your heart. ToF makes it difficult for your body to get enough oxygen-rich blood.
There are 4 defects that cause ToF:
A hole between the right and left side of the heart (VSD).
A missing or underdeveloped valve that allows blood to flow from the right side of the heart to the lungs (pulmonary stenosis).
Excess thickness of the muscles