A 3-year-old boy is brought to the clinic by his mother because she is concerned that he is having frequent episodes of breathing difficulty. His mother reports that these episodes occur especially after running around the house, and he often has to squat down to catch his breath. The patient was born at 38 weeks gestation following an uncomplicated pregnancy, in which the mother received full prenatal care. The patient is at the 35th percentile for length and below the 10th percentile for weight. While being examined, he starts to cry, and cyanosis is noted around the lips. Pulse oximetry on room air shows an oxygen saturation of 87%. Further evaluation will most likely reveal which of the following findings?

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.

Key Takeaways

Tetralogy of Fallot, or TOF, is a congenital disorder characterized by four heart abnormalities. There is stenosis in the right ventricular outflow tract, right ventricular hypertrophy, ventricular septal defect, and aortic override of the septal defect. These four defects cause oxygen-poor blood to flow from the right ventricle into the aorta, rather than being pumped to the lungs, which results in cyanosis. TOF is diagnosed with patient examination and health ultrasound, and treatment involves cardiac surgical repair, usually in the first year of life.

Sources

"Robbins Basic Pathology" Elsevier (2017)

"Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)

"Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)

"Tetralogy of Fallot" The Lancet (2009)

"Genetic Origins of Tetralogy of Fallot" Cardiology in Review (2018)

"Tetralogy of Fallot" Cardiology Clinics (2015)

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