What is Tetralogy of Fallot?

Primary four malformations

As classically described, tetralogy of Fallot involves four heart malformations which present together:

1. A ventricular septal defect (VSD): a hole between the two bottom chambers (ventricles) of the heart. The defect is centered around the 'outlet septum', the most superior aspect of the septum, and in the majority of cases is single and large. In some cases septal hypertrophy can narrow the margins of the defect.
2. Pulmonic stenosis: Right ventricular outflow tract obstruction, a narrowing at (valvular stenosis) or just below (infundibular stenosis) the pulmonary valve. The stenosis is mostly the result of hypertrophy of the septoparietal trabeculae, however the deviated outlet septum is believed to play a role. The stenosis is the major cause of the malformations, with the other associated malformations acting as compensatory mechanisms to the pulmonic stenosis. The degree of stenosis varies between individuals with TOF, and is the primary determinant of symptoms and severity. This malformation is infrequently described as sub-pulmonary stenosis or subpulmonary obstruction.
3. Overriding aorta: defined as when the aortic valve is not restricted to the left ventricle, thus having biventricular connections. The aortic root can be moved anteriorly or override the septal defect, but it is still to the right of the root of the pulmonary artery. The degree of override is quite variable, being between 5-95% of the valve being connected to the right ventricle.
4. Right ventricular hypertrophy: The right ventricle is more muscular than normal, causing a characteristic coeur-en-sabot (boot-shaped) appearance as seen by chest X-ray. Due to the misarrangement of the external ventricular septum, the right ventricular wall increase in size to deal with the increased obstruction to the right outflow tract. This feature is now generally agreed to be a secondary anomaly, as the level of hypertrophy generally increases with age.