Atrial septal defect

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Atrial septal defect



Atrial septal defect


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USMLE® Step 1 questions

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High Yield Notes

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Atrial septal defect

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USMLE® Step 1 style questions USMLE

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A 25-year-old man is referred to a cardiologist for evaluation for intermittent cyanosis and exercise intolerance. Symptoms began around half-a-year ago, and they have progressively worsened. Past medical history is notable for a congenital heart defect, which was diagnosed shortly after birth. However, he was subsequently lost to follow-up and was not treated for this condition. The patient smokes half-a-pack of cigarettes per day. Physical exam is notable for cyanosis of the lips and clubbing of the fingers. Cardiac catheterization is performed, and oxygen saturation in the cardiac chambers and outflow tracts are shown below:  

 Which of the following is the most likely explanation of these findings? 

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Atrial septal defect (ASD) p. 305

congenital rubella p. 306

diastolic murmur in p. 297

Down syndrome p. 306

fetal alcohol syndrome p. 306


atrial septal defect p. 305

patent foramen ovale p. 286

Foramen ovale

atrial septal defect p. 305

Heart failure p. 318

atrial septal defect p. 305

Patent foramen ovale

atrial septal defect vs p. 305

septal fusion failure p. 286


The atrial septum separates the right and left atrium of the heart. An atrial septal defect describes when there’s an opening in a baby’s heart between these two upper chambers. But how does this opening form? Well, let’s run through how the septum develops in the first place.

When the heart is first developing, a tissue called the septum primum between the left and right atria grows downward, slowly creating two separate chambers by closing this gap, or ostium primum, which means “first opening.” The septum primum then fuses with the endocardial cushion and closes the gap completely. Meanwhile, a hole appears in the upper area, called the ostium secundum, or “second opening.” Now, we also have the septum secundum which grows downward, just to the right of the septum primum, and covers the ostium secundum like a curtain, leaving a small opening called the foramen ovale, thus essentially creating a makeshift valve that allows blood to go from right atrium to left atrium, but not the other way.

The developing newborn gets oxygenated blood from the placenta, which goes from the umbilical vein over to the right atrium, which is different because after development, only deoxygenated blood goes to the right atrium, and then gets sent to the lungs to pick up some oxygen. In the developing fetus, though, it’s already oxygenated, so instead of going to the right ventricle and to the lungs and back to the left atrium, it just bypasses the right ventricle and lungs through the foramen ovale and goes into the left atrium. The oxygenated blood then goes to the left ventricle to be pumped to the body.


  1. "Atrial septal defect" undefined (undefined)
  2. "Heart embryology video" undefined (2011)
  3. "Fetal circulation." undefined (2016)
  4. "Robbins Basic Pathology (10 edition)" Elsevier (2017)
  5. "Classification of atrial septal defects (ASDs), and clinical features and diagnosis of isolated ASDs in children" undefined (2017)

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