Four things contribute to create tetralogy of Fallot:
- Ventricular septal defect (VSD) – there is a large hole in the wall between the two lower pumping chambers
- Overriding aorta – the aorta sits over the hole between the bottom pumping chambers
- Narrowing in the outflow to the lungs – the pathway from the heart to the lungs (infundibulum) is smaller than normal, as are the pulmonary valve and main pulmonary artery
- Right ventricular hypertrophy – like any muscle, if the heart has to work harder, it becomes thicker.
Because of the large VSD and outflow obstruction, the muscle of the right ventricle becomes thicker. The degree of obstruction of blood to the lungs can vary from minimal to severe. It is relatively common for the course of the main body artery to be rightward than leftward in tetralogy of Fallot. This is called right aortic arch.
Once blood comes into the right ventricle, it has two choices for where to go:
either to the lungs or though the hole to the aorta and body. If the right ventricle’s outlet is narrow, some of the blood will be forced across the VSD to the aorta. This oxygen-poor blood mixes with blood that is rich in oxygen (red in color) from the other ventricle and goes out to the body. When blue and red blood mix, the child’s skin coloration may turn bluish, evident mostly in the lips, tongue and nail beds. In addition to the bluish discoloration, the heart muscles of the right ventricle will become thicker as it works hard to push blood through the narrow outlet.
According to the American Heart Association, of the thousands of babies born each year with a cardiovascular defect, 9 to 14 percent have tetralogy of Fallot.
Progression and Possible Complications
Because the narrowing of the right ventricle outlet may not be severe initially, the skin of children born with tetralogy of Fallot typically does not appear bluish right away. However, as the outlet becomes increasingly narrowed over time, more “blue” blood may pass through the ventricular septal defect and out to the body, increasing the bluish color.
Occasionally, the narrowing of the outlet and obstruction of blood flow to the lungs become so severe that the amount of blood flowing to the lungs is suddenly significantly reduced. As a result, since very little blood is able to get to the lungs, very little blood carrying oxygen returns to the heart. Not only may this lack of oxygen cause the child to faint, it also intensifies the discoloration (referred to as hypercyanotic spell, tetralogy spell or “tet” spell). This is can be a life-threatening situation and is a medical emergency.
If left unrepaired, tetralogy of Fallot will eventually cause the hard-working right ventricle to enlarge, resulting in right heart failure. In addition, clots that normally form in veins and are filtered in the lungs may cross the ventricular septal defect from the right to the left ventricle and into the body, possibly resulting in a stroke. These clots may lodge and block blood flow, causing damage to organs, such as the kidneys, or leading to stroke. Strokes are rarely seen in the first year of life, however.
Children with this condition, particularly those who undergo treatment for it after one year of age, may develop abnormal heart rhythms. This is due to damage to the right heart muscles resulting from their added workload. Also, heart surgery to repair the defect may cause scarring of the heart muscles. Scarring may contribute to abnormal heart rhythm later in life.
Traditionally, surgery is performed at about 6 to 9 months of age, although it may be performed earlier if necessary.
During the open-heart surgery, the surgeon closes the hole between the two ventricles and widens the right ventricle outlet by removing thickened muscles. A patch may be inserted to enlarge the outflow pathway of the right ventricle to the main pulmonary artery. In doing this, the valve is typically disrupted so that no functional valve tissue remains. This allows there to be no narrowing, but also allows leakage of blood from the lungs to the heart. However, this is generally well tolerated for many years.
Abnormalities with the branching pattern of the coronary arteries may result in the need to place a valved tube (conduit) rather than a patch to allow flow from the heart to the lungs during surgical repair. These conduits may become narrow over time and may leak. Children can outgrow these conduits, as well, as they do not grow with the child. With a patch or conduit, significant leakage may develop over time, which will require placement of a new valve or conduit to prevent blood from leaking back from the lung arteries to the heart. Special metal stents (mesh tubes) with attached tissue valves have been developed to allow less invasive placement of new valves in the hospital’s catheterization lab rather than by open-heart surgery.