Acyanotic congenital heart defects: Pathology review

Definitions & Key takeaways

Acyanotic congenital heart defects (ACHDs) are structural abnormalities of the heart that prevent or impair the flow of blood through the heart. Heart defects in ACHDs result in left-to-right shunts and do not normally cause cyanosis, meaning they do not interfere with the amount of oxygen or blood that reaches the tissue. ACHDs may be caused by genetic defects like trisomies, maternal infections such as rubella, maternal alcohol use, and certain drugs during pregnancy.

Most ACHDs are diagnosed during infancy or childhood because they cause problems with breathing, feeding, and growth. Some children with ACHDs may not have any symptoms and their condition is only discovered during a routine physical examination.

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Case Study0:00–0:43

In a pediatric cardiology clinic, 4-year-old Tara is brought in by her parents because she has not been acting herself over the past month.
The mother also mentioned that she can’t keep up with the other children when playing and often gets fatigued or short of breath.
Vital signs include a temperature of 37.0 degrees Celsius or 98.6 degrees Fahrenheit, a heart rate of 100 beats per minute, a blood pressure of 110 over 70 mmHg, and a respiratory rate of 18 breaths per minute.
On examination, her skin is pink, and auscultation of the heart reveals a holosystolic murmur over the left sternal border.
Ok, so Tara has some sort of congenital heart defect. Congenital heart diseases are defects in the embryological development of the heart or its major blood vessels.

Pathology0:43–1:38

When the defect causes blood to move from the right to the left side, it’s called a right-to-left shunt. This is because deoxygenated blood from the right side goes to the left side, and then enters the systemic circulation.
A large amount of deoxygenated blood in the systemic circulation gives the physical appearance of cyanosis, which is a bluish discoloration of the skin.
Therefore, right-to-left defects are called cyanotic heart diseases. Conversely, left-to-right shunts are called acyanotic heart defects, because there is no cyanosis.
In general, individuals with acyanotic congenital heart diseases could be asymptomatic or present with signs of heart failure, such as exercise intolerance, shortness of breath, and in the case of infants and young children; poor feeding and failure to thrive.
Okay, of the acyanotic congenital heart diseases, ventricular septal defect, or VSD, is the most common. The ventricular septum normally separates the left and right ventricles, and is made of a membranous component, which is the upper one-third, and a muscular component, which is the lower two-thirds.

VSD1:38–2:40

The defect most commonly occurs in the membranous portion of the septum. Ventricular septal defects are usually small and they often end up closing on their own.
Individuals are asymptomatic at birth, and if symptoms develop, they usually occur a couple of weeks later or even later in life.
This is a helpful clue on exams. On auscultation, a systolic murmur can be heard along the left sternal border.
Bear in mind though that the smaller the defect, the more audible the murmur. That’s because when blood rushes through a more narrow opening, it produces more turbulence and therefore, more noise.
In addition, because the defect allows oxygenated blood to move from the left to the right side of the heart, the oxygen saturation will be higher than normal in the right ventricle and the pulmonary artery.
Okay, now let’s move up the heart and look at atrial septal defects, or ASD. Remember that an atrial septal defect is different from a patent foramen ovale.

Atrial Septal Defect 2:40–6:10

Normally, when the heart is first developing, a strip of tissue called the septum primum between the left and right atria grows downward, slowly creating two separate chambers by closing a gap or opening known as ostium primum.
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.
Now, we also have the septum secundum which grows downward, just to the right of the septum primum, and covers the ostium secundum, leaving a small opening called the foramen ovale, which allows blood to go from the right atrium to the left atrium.
At birth, the septum secundum and septum primum slap shut, and then fuse and close off the foramen ovale. However, for your exams, it’s important to know that the foramen ovale remains patent in approximately 25% of normal adults.
A high yield fact is that the most common atrial septal defect is a problem with the formation of the septum secundum, and it’s specifically called an ostium secundum defect.
Ostium secundum atrial septal defects are usually isolated, whereas the less common ostium primum defects typically occur in association with other congenital cardiac anomalies.
A high yield association of ostium primum defects is with Down syndrome, or Trisomy 21. Similar to ventricular septal defects, individuals are usually asymptomatic.
On auscultation, the most characteristic feature of an atrial septal defect is the fixed split S2. Normally, during inspiration, the S2 heart sound actually splits into two separate sounds.
That’s because during inspiration, there’s negative pressure in the chest to bring in air. That negative pressure also brings a bit more venous blood back to the right atrium and right ventricle, so it takes a little bit more time for the right ventricle to squeeze this extra blood into the pulmonary artery, and it takes a little bit more time for the pulmonary valve to close.
This can be heard as a physiologic splitting of the S2 during inspiration where the pulmonary valve closes a bit later than the aortic valve.
Now, with an atrial septal defect, there’s extra blood that gets shunted from the left atrium to the right atrium and right ventricle, which passes by the pulmonic valve and causes a delay in the closure of the pulmonic valve relative to the aortic valve closure, producing a split S2.
But since the atria communicate via the defect, inspiration produces no net pressure difference between them, and has no effect on the splitting of S2.
So, S2 is fixed, meaning that it’s split to the same degree during inspiration and expiration. Now, similar to ventricular septal defects, the oxygen saturation will be increased in the right ventricle and pulmonary artery.
Saturation will also be increased in the right atrium and this is what distinguishes atrial from ventricular septal defects.An important complication of both a patent foramen ovale and an atrial septal defect is the development of paradoxical emboli from the venous circulation.
Say someone develops a deep vein thrombosis in their legs, from which an embolus breaks off and travels up to the right heart, but instead of going from the right atrium to the right ventricle and to the lungs, it passes through this atrial septal defect to the left atrium, then to the left ventricle and the systemic circulation.
This may lead to the development of a stroke once the embolus travels up the carotid arteries and gets lodged somewhere, and this is specifically called a cryptogenic stroke.

Patent Ductus Arteriosus6:10–9:05

Alright, now we go even higher up in the heart to look at patent ductus arteriosus, or PDA. Now in utero, the ductus arteriosus is the normal communication between the pulmonary artery and the aorta.
The structure serves to shunt blood to the baby’s systemic circulation from the pulmonary artery to the aorta, so from right to left.
This is important because the fetus’ lungs aren’t working yet and blood doesn’t really go there. This is all normal and it’s maintained by high levels of the vasodilator, prostaglandin E2, which is produced by the placenta and the ductus arteriosus.
At birth, a bunch of things change. First, oxygen levels in the blood go up dramatically and the lungs become the main source of oxygenated blood.
Soon after birth, the foramen ovale closes and prostaglandin E2 levels fall, causing the ductus arteriosus to close off and eventually become the ligamentum arteriosum.
If it fails to close, it’s called a PDA. This might happen if for any reason oxygen tension is low.
For example, if a baby is born prematurely, their lungs haven’t fully developed yet, and so they can’t maintain normal levels of oxygen in the blood and the ductus fails to close.
Another high yield association is congenital rubella infection. Okay, now in a PDA, blood moves from the aorta to the pulmonary artery, so left to right.
This happens because the pulmonary vascular resistance decreases after birth, switching the direction of this shunt. So, all that oxygenated blood is going back to the lungs instead of to the systemic circulation.
Now, for your test, keep in mind that in 90% of cases, PDAs occur as isolated defects, but in the remaining 10%, they are associated with other congenital heart defects.
In fact, in certain cyanotic congenital heart defects, where the right ventricular outflow is obstructed like in Tetralogy of Fallot, or where the aorta and pulmonary artery swap locations, like in transposition of great vessels, PDA is necessary for survival.
That’s because it forms a connection between the aorta and pulmonary artery that allows mixing of oxygenated and deoxygenated blood during fetal development.
Now, here’s where anatomy knowledge kicks in. The ductus arteriosus originates in the arch of the aorta, below where the subclavian artery branches, and remember the subclavian artery supplies the upper limbs.
So, over time if a PDA is not corrected, babies eventually present with normal, pink upper limbs, but pale or even cyanotic lower limbs, which is called differential cyanosis.
Okay, now comparing it to the septal defects, oxygen saturation will be increased only in the pulmonary artery, and will be normal in the right ventricle and atrium.
On auscultation, a PDA produces a continuous “machine-like” murmur that’s best heard over the left clavicle. PDAs usually consist of treatment with indomethacin a non-steroidal anti-inflammatory medication which blocks the synthesis of prostaglandin E.
If this fails, surgery is often necessary to close it. Alright, now if any left-to-right shunt is not corrected, then a long-term complication called Eisenmenger syndrome can occur.

Eisenmenger Syndrome9:05–10:24

Over time, constant shunting of blood to the right heart increases pulmonary blood flow. Because the pulmonary vessels are now receiving more blood than they are used to, they remodel their anatomical structure, known as vascular remodeling.
Although this seems normal, vascular remodeling causes an increase in the blood pressure in the pulmonary vessels, in other words, pulmonary hypertension.
Now the right heart has to pump against much higher pressures than it’s used to, so it too undergoes remodeling in the form of right ventricular hypertrophy, This causes the pressure in the right heart to go up until it reaches a point where it’s higher than the left heart, which switches the direction of the shunt.
Now that it’s a right-to-left shunt, individuals begin to develop cyanosis and hypoxia. In response to the decrease in oxygen saturation, or hypoxia, the interstitial cells of the kidney make more erythropoietin, which signals to the bone marrow to produce more RBCs to compensate for the hypoxia.
This results in polycythemia or elevation of the hematocrit. Another sign of chronic hypoxia is the clubbing of the fingers and toes.
So again, although these are termed acyanotic diseases, without proper treatment individuals will eventually develop cyanosis due to Eisenmenger syndrome.
Okay, now the final acyanotic congenital heart disease is coarctation of the aorta. Unlike the disorders we talked about, there is no shunt here.

Coarctation of the Aorta10:24–13:35

Coarctation of the aorta is narrowing of the aorta and is classified into two types: pre-ductal also called the infantile type, and post-ductal, also called the adult type.
This is based on the location of the aortic narrowing in relation to the ductus arteriosus. In the infantile form, the coarctation develops early in life and the ductus arteriosus is still open.
So we have a PDA. Now in a PDA we mentioned that blood is shunted from the higher pressure left side, or the aorta, to the lower pressure right side, or the pulmonary artery.
But when there is a coarctation before the ductus arteriosus, the pressure is now lower in the aorta compared to the pulmonary artery.
So, now deoxygenated blood moves from the pulmonary artery to the aorta, and because this happens after the aortic arch branches, the infant develops differential cyanosis, or cyanosis in the lower extremities.
On the other hand, the adult type of coarctation happens after what is now the ligamentum arteriosum, so they do not develop differential cyanosis.
This is the main difference between the two types. Alright, in both types, because the narrowing occurs after the aortic arch branches, blood has a hard time getting through.
So, the blood pressure is higher proximal to the narrowing, but is lower distal to the narrowing. This is reflected as high blood pressure in the upper extremities, but low blood pressure in the lower extremities.
But the thing is, we don’t usually check lower extremity blood pressure routinely. So, exams can bring coarctation as a case of hypertension that was not resolved with medications.
Now, if you put one finger on the brachial artery and another on the femoral artery, you should normally feel the pulses right around the same time, and they should be of equal strength.
But in coarctation, the femoral pulse is quite delayed and it’s relatively weaker than the brachial pulse, and this feature is called brachio-femoral delay.
In addition, the narrowing increases blood flow to the brain, which increases the pressure in the cerebral circulation putting individuals at risk of cerebral hemorrhage.
The narrowing also causes the afterload to increase, which is the pressure the left ventricle has to pump against. So a logical complication of this is left ventricular hypertrophy and potentially heart failure.
Other more rare complications include aortic rupture and endocarditis of the aortic valve. Another important clue is rib notching.
See normally the intercostal arteries supply the ribs. The anterior intercostal arteries come from the internal thoracic artery, a branch of the right subclavian, and the posterior intercostals come from the descending aorta.
This anatomy actually provides a collateral circulation to bypass the narrowing, which is good. But the problem is these intercostal arteries are now under such high pressure that they eventually dilate.
These dilated arteries rub up against the ribs and over time, the ribs can develop notches that can be seen on a chest x-ray.
For your test, there are some high yield disorders associated with coarctation of the aorta such as Turner syndrome and bicuspid aortic valve.Alright, as a quick recap!

Review13:35–15:04

Congenital heart diseases are defects in the embryological development of the heart, and can be broadly classified into cyanotic and acyanotic defects.
Acyanotic defects are characterized by a left-to-right shunt, with the exception of coarctation of the aorta. The most common acyanotic defect is ventricular septal defect, which usually occurs in the membranous portion of the septum.
On auscultation, a holosystolic murmur is heard over the left sternal border. Atrial septal defect usually happens in the septum secundum, and auscultation may reveal a fixed, wide splitting of the S2 heart sound.
A patent ductus arteriosus usually happens in infants born prematurely and is also associated with congenital rubella. Auscultation may reveal a continuous machine-like murmur over the left clavicle.
Over time, any acyanotic defect with a left-to-right shunt can be complicated by Eisenmenger syndrome, which is reversal of the shunt direction to become a right-to-left shunt, causing the individual to develop cyanosis.
Finally, coarctation of the aorta is narrowing of the aortic arch and is classified into the infantile and adult forms. In infants, the narrowing happens proximal to a patent ductus arteriosus, and infants present with differential cyanosis, which is cyanosis in the lower extremities.
In the adult form, individuals often present with hypertension in the upper extremities, and are at risk of complications like heart failure and cerebral hemorrhage.
Back to our case, if we put Tara’s clinical picture together, she has shortness of breath and fatigue on exertion which can be caused by the various congenital heart defects we’ve discussed.

Summary15:04–15:26

The key clue is that on examination, there’s a holosystolic murmur over the left sternal border. This is consistent with a ventricular septal defect.
Tara underwent surgical correction of the defect and is now