Wikipedia:Osmosis/Tetralogy of Fallot
Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
If you’ve ever played Tetris, you probably know that you use pieces made of groups of four squares. Tetralogy of Fallot or TOF is a congenital heart condition where patients have have four heart abnormalities.
Here’s the normal heart, with upper chambers, the left and the right atria, and lower chambers, the left and the right ventricles, as well as the aorta and the pulmonary artery.
Instead of using this anatomical heart though, I’m going to use actually use this simplified version of the heart instead.
Okay so now let’s go through these four findings in TOF one-by-one. The first abnormality is stenosis, or narrowing of the right ventricular outflow tract into the pulmonary artery. And this could either be narrowing of the valve itself, or narrowing of the infundibulum, the area right below the valve. Either way, this makes it harder for deoxygenated blood to get to the pulmonary circulation.
In response—and this also happens to be the second feature—the myocardium of the right ventricle hypertrophies, or gets really thick, in order to contract harder and push blood past the stenosis.
This causes their heart to look “boot-shaped” on an x-ray.
The third feature is that patients have a large ventricular septal defect, this gap between the ventricles that allows shunting of blood between them.
Now, in a patient with an isolated ventricular septal defect (meaning those that don’t have TOF), oxygenated blood is shunted from the left side to the right side because the pressure on the left is higher than the pressure on the right. For patients with TOF, though, the right ventricular outflow obstruction might block the normal blood flow so much that the pressure in the right ventricle has to be really high to get past it. Well the high right-sided pressures means that the left side of the heart actually becomes the path of least resistance and deoxygenated blood shunts from the right side to the left side.
The fourth and final feature, is that the aorta overrides the ventricular septal defect. This one is super variable, sometimes the aorta’s way over here sitting on top of the septal defect and sometimes it’s more on the left ventricular side. Either way, if deoxygenated blood is shunted from right-to-left, then it flows over to the left ventricle and immediately out to the body. When you think about it—the critical feature of these four is the degree of right ventricular outflow obstruction. With less obstruction, oxygenated blood might be shunted from left‐to‐right and get into the pulmonary circulation, where it essentially takes another run through the lungs. With more obstruction, deoxygenated blood is shunted from right-to-left and enters the systemic circulation, and more deoxygenated blood going to the body essentially means less oxygenation of the tissues, right?
In fact, shunting can be severe enough to let the oxygen saturation fall below 80%, at which point patients’ skin takes on a blueish or purple discoloration, called cyanosis. Tetralogy of Fallot is actually the most common cause of cyanotic congenital heart defects, accounting for about 50-70%, as well as about 10% of all congenital heart defects in general. Now, it’s not quite clear why some babies develop TOF but it is associated with chromosome 22 deletions and DiGeorge syndrome.
Babies with TOF often have cyanosis around their lips and fingernail beds at birth, and can have clubbing of their fingers and toes within a few months. On the other hand, if a baby doesn’t have severe right ventricular outflow obstruction the baby might not be cyanotic. Nevertheless, having any decrease in the normal oxygenation of blood can affect the baby in a lot of ways, and those born with TOF can have a range of symptoms including feeding difficulty and failure to gain weight and develop normally.
Babies with TOF will often experience symptoms and cyanosis in spells, which is referred to as “tet spells”. So say the baby’s cruising around, and therefore increasing oxygen demand, their heart will try to pump more blood, leading to a sudden decrease in oxygen saturation, which causes them to be cyanotic. When this happens, they’ll squat down to reduce the cyanosis. Why does this help, though? Well squatting down or getting into a knee/chest position slightly kinks the femoral arteries in the legs, which increases vascular resistance in the peripheral arteries, and therefore increases pressure in the systemic circulation, which increases pressure in the left ventricle enough such that the pressure in the left side is now greater than the right side, and that shunt temporarily reverses, forcing blood to take the path to the lungs to be oxygenated, therefore reducing cyanosis. When patients do have a serious hypercyanotic episode or “tet spell”, they can be treated by keeping them calm and giving them oxygen and IV fluids along with medications that are ultimately aimed at improving pulmonary blood flow.
A diagnosis of TOF is usually made with echocardiography, which can even be done prenatally. Most patients with tetralogy of Fallot will have cardiac repair surgery in the first year of life, where the ventricular septal defect is closed with a patch, and the right ventricular outflow tract is enlarged. Fixing these two defects resolves problems caused by the other two defects. When the right ventricle no longer has to work so hard to pump blood to the lungs, it can return to a normal thickness, and fixing the ventricular septal defect means that only oxygen-rich blood will flow out of the left ventricle into the aorta.