Tetralogy of Fallot (TOF) is a congenital heart defect that causes the heart to pump oxygen-poor blood throughout the body. Learn about TOF, its causes, and common features of the condition.

What is Tetralogy of Fallot?

Tetralogy of Fallot (TOF) is a congenital heart defect (heart defect present at birth) made up of multiple heart defects that cause the blood to be inadequately oxygenated. This means organs and tissues do not receive as much oxygen as they need.

What causes TOF?

In most cases, there is no clear cause of TOF. Sometimes, it can found in people with certain genetic conditions.

What are the features of TOF?

TOF typically has four defects:

  • Ventricular septal defect (VSD, or hole between the lower ventricles, or chambers, of the heart)
  • Narrowed pulmonary valve (the tube that moves blood from the heart to the lungs)
  • Thickened muscle wall of the right ventricle (one of the 4 chambers in the heart)
  • Overriding of the aorta (main artery that directs blood to the body). This is when the aorta is positioned above the VSD instead of in the proper position over the left ventricle of the heart. This abnormal position is what allows oxygen-poor blood to go to the body.


Rev. 6/2018