Tetralogy of Fallot

Tetralogy of Fallot or TOF is a congenital heart condition where patients have four heart abnormalities.

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.