Tetralogy of Fallot is a cyanotic heart defect. It is a combination of 4 ("tetra") abnormalities of the heart which were first described by a French physician, Dr. Etienne Fallot, in 1888. There are about 3000 babies born each year in the United States with Tetralogy of Fallot accounting for 10% of all congenital heart defects. It is the most common of all cyanotic heart defects. Most people born with Tetralogy of Fallot will have had some type of surgery on their heart during childhood either to relieve symptoms (palliation) or to repair the malformations (reparative). It is possible to reach adulthood however, without having had any surgery.

The four defects of Tetralogy of Fallot:

1) A Ventricular Septal Defect or V.S.D.. This is a hole (defect) in the septum (wall) between the right and left ventricles. The V.S.D. allows blue blood on the right side of the heart to mix with oxygen rich red blood on the left side of the heart. This mixing of blue blood into the circulation causes cyanosis and decreases oxygen levels in the blood. A V.S.D. is also a passageway for a blood clot to enter the circulation which could cause a stroke.

2) Infundibular Stenosis. The infundibulum is an area in the outflow of the right ventricle just beneath the pulmonary artery and valve. It forms a passageway for blue blood to flow to the lungs. Infundibular "stenosis" or narrowing means that there is a there is a narrowing in this passageway that obstructs the flow of blood to the lungs. The degree of obstruction can range from mild to severe. This will cause the pressure inside the right ventricle to go up as it tries to push the blood through the area of obstruction. An added problem is that the pulmonary valve itself and the pulmonary artery may also be narrowed which further adds to the obstruction.

3) An overriding aorta. The aorta is the artery that carries oxygen rich red blood to the rest of the body and is normally open only to the left ventricle. In Tetralogy of Fallot the aorta is shifted toward the right ventricle and "straddles" the V.S.D. The aorta is then open to both the right and left ventricle. This allows mixing of blue and red blood as it flows out of the heart to the rest of the body. Thus the amount of oxygen in the circulation is lower than normal causing symptoms of shortness of breath, fatigue and a decreased activity level.

4) Right ventricular hypertrophy. Hypertrophy is a term used to describe a muscle that has become thickened. In Tetralogy of Fallot the right ventricular muscle wall is thickened because the right ventricle must work harder to push blood through the narrowed pulmonary artery. The muscle becomes thickened in much the same way a body builder increases arm size by pushing against resistance. Unfortunently an increase in heart muscle thickness may precipitate irregular heart rhythms or cause patchy areas of scar tissue making the heart very "irritable". This makes the heart prone to irregular heart rhythms and reduced performance.

The V.S.D. and the infundibular stenosis are the major defects of Tetralogy of Fallot. These defects cause:


a) oxygen poor blood to bypass the lungs and circulate to the body

b) increase the pressure inside the right side of the heart

c) decrease blood flow to the lungs

References
1. Warnes CA: Tetralogy of Fallot and pulmonary atresia/ventricular septal defect. Cardiology Clinics 11(4):643-650, 1993.

2. Warnes CA, Driscoll DJ, Fuster V, Puga FJ: Tetralogy of Fallot. In: Mayo Clinic Practice of Cardiology, 3rd edition, E. R. Giuliani, B. J. Gersh, M. D. McGoon, D. L. Hayes, H. V. Schaff (eds.), Mosby, St. Louis, 1996.

3. Dodds GA, Warnes CA, Danielson GK: Aortic valve replacement after repair of pulmonary atresia and ventricular septal defect or tetralogy of Fallot. J Thorac Cardiovasc Surg 113:736-741,1997.