Atrial septal defect

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Introduction0:00–0:20

With atrial septal defects, atrial refers to the heart's two upper chambers called the atria. Septal refers to the septum, which is the wall that separates them, while defect means there's an opening that shouldn't be there.
So an atrial septal defect is a congenital heart defect characterized by an abnormal gap in the wall separating the atria.

Embryology0:20–1:47

To understand how this opening forms, we need to review how the atrial septum forms during fetal development. Early during heart development, the atria start as a single chamber called the common atrium.
Around the 4th week of development, a thin wall called the septum primum begins to grow downward from the roof of the common atrium, gradually dividing it into right and left atria.
As the septum primum grows, it leaves a small opening at the bottom called the ostium primum or first opening. But as the septum primum continues to grow, the ostium primum becomes narrower, and before the septum primum reaches the endocardial cushions and completely closes the ostetium primum, another opening called the ostium secundum or second opening appears in the upper area of the septum.
Then, just next to the septum primum, the heart grows a thicker wall called the septum secundum, that grows downward and covers the ostium secundum like a curtain.
This leaves a small gap called the foramino Valley. You can think of the septum primum and septum secundum as a pair of double doors that don't quite line up perfectly, allowing blood to slip through the space between them.
This arrangement creates a one-way passage, with the septum primum acting like a flap valve, allowing blood to flow from the right atrium to the left atrium, but not the other way around.

Causes1:47–2:37

This atrial communication is essential during fetal development, because the fetus isn't using its lungs yet. In the fetus, the lungs are collapsed and filled with fluid, so the pulmonary vessels are squeezed tight.
Because of this high resistance in the pulmonary vessels, the right ventricle is unable to pump blood into the lungs. Instead, the blood takes a detour through two fetal shunts.
One of those is the foramino Valley, while the other one is the ductus arteriosis, which is a vessel that connects the pulmonary artery to the aorta.
These shunts allow the blood from the right ventricle to bypass pulmonary circulation and enter systemic circulation. At the same time, the placenta provides a low resistance pathway for blood leaving the left side of the heart, so the left ventricle doesn't need to generate much more pressure than the right.

Pathophysiology2:37–3:19

However, everything changes with the baby's first breaths. As the lungs expand, the pulmonary vessels open, causing pulmonary resistance to drop.
At the same time, clamping the umbilical cord takes the placenta out of the circulation, removing its low resistance pathway and raising systemic resistance.
At this point, the left ventricle starts pumping against a much higher pressure than the right ventricle. This pressure difference between the two sides of the heart pushes the septum primum against the septum secundum, sealing off the foraminal valley.

Signs and Symptoms3:19–3:44

Now when the walls that separate the right and left atria don't form or fuse properly, a gap is left behind. This opening is what we call an atrial septal defect, or ASD.

Complications3:44–4:30

Based on the part of the septum that doesn't close properly, ASDs are classified into several different types. One of the most common types is secundum ASD, which accounts for around 90% of all cases.
This type occurs when the septum secundum doesn't cover the ostium secundum. Or when the ostium secundum itself is too large.
A less common type is primum ASD, which happens when the septum premum fails to fully grow downward and fuse with the endocardial cushions, leaving a persistent gap between the right and left atria.
Although they are rare, ostium primum defects are found in around 25% of people with Down syndrome, and are commonly associated with other congenital cardiac anomalies.

Treatment4:30–4:47

Finally, an ASD can also develop near the superior or inferior vena cava, or close to the coronary sinus, but these two types account for only 10% of all atrial septal defects.

Review4:47–9:40

No matter the type, these openings let blood flow between the atria in a way that isn't supposed to happen, and since the left atrial pressure is higher than the right, the oxygenated blood flows from the left atrium through the ASD into the right atrium.
As a result, the blood goes back to the pulmonary circulation, essentially taking an extra trip to the lungs. This is why atrial septal defects are classified as cyanotic heart defects, meaning the body still gets plenty of oxygen, and there is no cyanosis.
However, it's important to know that in rare cases, untreated large ASDs can lead to severe pulmonary hypertension and reversal of the shunt, called Eisenmanger syndrome, which causes cyanosis.
Usually atrial septal defects are well tolerated, and individuals may remain asymptomatic for decades. Over the years though, the chronic left to right shunt can overload the right side of the heart, leading to signs of heart failure like fatigue, shortness of breath, and exercise intolerance.
The volume overload can also cause the right atrium to dilate, making it easier for abnormal rhythms like atrial fibrillation to develop.
Another important complication of ASDs is the development of paradoxical emboli from the venous circulation. Say someone develops a deep vein thrombosis in their legs.
Usually if a thrombus breaks off and a thromboembolus travels up to the heart, it'll go from the right atrium to the lungs, eventually causing pulmonary embolism.
But if pressure in the right atrium temporarily exceeds the pressure in the left atrium, like during straining or coughing, the clot can cross through the ASD into the left atrium.
From here, the thromboembolus will reach the left ventricle and enter the systemic circulation, which can result in stroke or ischemia in other organs.
Most atrial septal defects are diagnosed in early childhood, often during a routine checkup, when a pediatrician hears a heart murmur.
However, since many small ASDs are asymptomatic, they can go undetected until adulthood. In fact, ASDs end up being the most common congenital cardiac defect seen in adults.
Upon auscultation, the classic finding is a fixed splitting of S2 due to a delay in the closure of the pulmonic valve. Normally when you take a deep breath, the pressure within the chest drops, so more blood flows into the right side of the heart, and it takes a little longer for the pulmonic valve to close.
This can be heard as a physiologic splitting of S2 during inspiration, where the pulmonary valve closes a bit later than the aortic valve.
But with an ASD, the right side is always getting extra blood from that left to right shunt, so the pulmonic valve always closes a bit later, not just during inspiration.
As a result, splitting of S2 is fixed, meaning that it's split to the same degree during inspiration and expiration. You might also hear a systolic murmur at the upper left sternal border, not because of turbulent flow across the ASD but because of the extra blood flow across the pulmonic valve.
Many small ASDs close on their own during childhood, but if the defect is large or causes symptoms, it might require surgical closure.
A catheter-based device can be used to plug the opening, kind of like patching a hole in a wall. For defects that are too large or have unusual anatomy, open heart repair might be needed.
All right, as a quick recap. An atrial septal defect is a congenital heart defect where the septum between the right and left atrium doesn't close up all the way and remains open after birth.
And since the left atrial pressure is higher than the right, the oxygenated blood flows from the left atrium into the right atrium.
As a result, the blood goes back to the pulmonary circulation, essentially taking an extra trip to the lungs. This is why atrial septal defects are classified as cyanotic heart defects, meaning the body still gets plenty of oxygen, and there's no cyanosis.