AssessmentsCyanotic congenital heart defects: Pathology review
USMLE® Step 1 style questions USMLE
A 1-day-old female newborn is being evaluated in the neonatal intensive care unit for severe respiratory distress and cyanosis. The patient was born at 38 weeks gestation via vaginal delivery to a 30-year-old woman who had minimal prenatal care. Temperature is 36.4°C (97.5°F), pulse is 150/min, blood pressure is 87/55 mm Hg, and respiration rate is 60/min. An electrocardiogram reveals left axis deviation with a superior axis and chest x-ray shows decreased pulmonary markings and a hypoplastic right ventricle. Which of the following is the most likely diagnosis?
Content Reviewers:Rishi Desai, MD, MPH
At the pediatric cardiology clinic, two mothers were chatting about their kids. One mom spoke about a 5 year old boy named Blake, who was a bluish color at birth and had a continuous machine-like heart murmur between the scapulas.
Another mom spoke about her 12 year old son, Paul, who was healthy at birth, but when he was breastfeeding or crying, his skin turned pale, and then blue. As a child, Paul got out of breath easily and needed to squat down to recover. And during his school physical, he was found to have a heart murmur.
Both Blake and Paul have cyanotic congenital heart defects, or CHDs, which usually start causing problems within the first 3-8 weeks of life. They can be broadly grouped into life-threatening cyanotic heart defects, or the less dangerous acyanotic heart defects.
Let’s go over 5 of the life-threatening cyanotic congenital heart defects: persistent truncus arteriosus, transposition of the great vessels, tetralogy of fallot, total anomalous pulmonary venous return, and tricuspid atresia.
Now the first 3 are caused by outflow tract defects that develop during the formation of the aorta and pulmonary artery. In fetal development the heart looks like a long tube; the top part is the truncus arteriosus and the part inferior to that is the bulbus cordis. Neural crest cells migrate into the bulbus cordis and trigger the formation of the aorticopulmonary septum. This structure is formed when two endocardial cushions appear on the right-superior and left-inferior walls. These grow like a spiral - imagine a corkscrew - and they wrap around each other forming a single septum that divides the truncus into the roots of the aorta. One root connects to the primitive left ventricle, and the other connects to the pulmonary artery and primitive right ventricle. That’s how blood gets routed to the right place!
Okay, so if the aorticopulmonary septum doesn’t form, or forms incompletely, the result is a persistent truncus arteriosus. For your exams, it’s important to know that this is caused by the failure of neural crest cells to properly migrate to the bulbus cordis. So we end up with a single vessel that’s connected to both the left and right ventricle, allowing oxygenated blood and deoxygenated blood to mix. This large common trunk eventually divides into the aorta and the pulmonary artery, and both carry partially oxygenated blood. When the partially oxygenated blood goes out to the body, it causes cyanosis.
Okay, moving on. If the spiraling of the aorticopulmonary septum doesn’t occur at all, we get transposition of the great vessels, where the aorta connects to the right ventricle and the pulmonary artery connects to the left ventricle. Here, deoxygenated blood from the systemic circulation goes to the right side of the heart, and gets pumped out of the aorta again. Meanwhile, oxygenated blood from the lungs goes to the left side of the heart, and gets pumped back to the lungs. So, for the test, remember that we end up with 2 seperate closed systems. Another high yield fact is that the only way that a newborn can survive is with a Patent Ductus Arteriosus or PDA, an opening between the aorta and pulmonary artery that allows some of the oxygenated and deoxygenated blood to mix. But since the PDA normally closes soon after birth it has to be kept open to keep a newborn alive.
Now, there’s tetralogy of Fallot, which is the most common cyanotic congenital heart defect. And “tetralogy” refers to four main features that you absolutely have to remember! First, a part of the right ventricle wall under the outflow tract, called the infundibular septum, is displaced anteriorly which narrows the right ventricular outflow tract, leading to pulmonary stenosis. Second, the narrowing of the right ventricular outflow tract increases resistance to blood flow, so the myocardium of the right ventricle hypertrophies to overcome that resistance. On x-ray the enlarged heart looks like a boot. Third, there’s a VSD, a tiny hole, between the ventricles that allows blood to shunt across. Initially, the left sided pressures are higher so blood flows to the right, but over time right sided pressures get so high that blood flows to the left - this is called Eisenmenger’s syndrome, which is very important to remember. In other words, some of the deoxygenated blood bypasses the lungs and goes to the left vent ricle. Fourth, there’s a displaced aorta that sits right above the ventricular septal defect, and this is called an overriding aorta.
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