Persistent truncus arteriosus

Definitions & Key takeaways

Persistent truncus arteriosus (PTA) is a rare congenital heart defect in which the embryonic structure that normally divides into the pulmonary artery and aorta fails to form properly, resulting in a single large vessel that supplies blood to both the systemic and pulmonary circulations.

Symptoms of PTA may include cyanosis, rapid breathing, poor feeding, and difficulty gaining weight. If left untreated, PTA can lead to heart failure and other complications. Treatment typically involves surgical repair of the defect, which involves separating the pulmonary artery from the aorta and creating a connection between the right ventricle and the pulmonary artery.

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The truncus arteriosus is this big structure that’s present during fetal development, and later in development it divides to form two separate arteries—the aorta and pulmonary artery.
Once it’s fully developed, the aorta comes from the left ventricle and the pulmonary artery comes from the right ventricle.
A persistent truncus arteriosus is when when this developmental structure doesn’t divide off into the two separate arteries, and so the baby’s left with this one giant artery that branches off from both the right and the left ventricles, and then splits off into the aorta and pulmonary artery.
Sometimes this condition is simply referred to as truncus arteriosus, or TA. The cause of this TA is unknown, although a lot of cases seem to be associated with 22q11.2 deletion syndrome, also known as DiGeorge Syndrome.
So let’s switch to a little more simplified view of the heart, alright usually deoxygenated blood comes in from the body to the right atrium, where it goes to the right ventricle and is pumped through the pulmonary artery to the lungs to get re-oxygenated.
Then fresh oxygenated blood comes from the lungs to the left atrium, goes to the left ventricle and is pumped through the aorta to the body.
And then the circuit repeat, right? If these two great arteries, the aorta and pulmonary artery, don’t divide, you essentially have this massive artery coming from both ventricles.
Notice though, that this one big artery does split into the aorta and pulmonary artery. Even though they eventually split off, before that it’s just one single vessel, so the oxygenated and deoxygenated blood mix.
When deoxygenated blood mixes into the systemic circulation, it’ll often present as cyanosis, a bluish-purple discoloration of the skin, which can be seen in a baby within the first days after birth.
Before birth though, things actually work a little differently. So what happens is that deoxygenated blood gets sent to the mother and then oxygenated blood comes from the mother, and since they’re not using their lungs yet, the fetal heart uses a few tricks like sending blood through the foramen ovale.
What’s important though is that since the fetus doesn’t use their lungs yet, there’s relatively high vascular resistance and therefore high pressure in the pulmonary circulation.
So even though there’s this huge main blood vessel and mixing of oxygenated and deoxygenated blood, the fetus actually does pretty well since the pressures in the two circuits are relatively similar, meaning that both circulations get about the same amount of blood.
After birth, the mother stops supplying blood, obviously, the foramen ovale typically closes, and the baby starts to rely on his or her own lungs, all of which leads to a decrease in vascular resistance and pressure in the pulmonary circuit, which is actually normal.
That being said though, now there’s this pressure differential between the left and right side, where the left’s higher than the right, which is also normal.
What’s not normal, though, is that with TA the blood’s still able to mix. So what ends up happening is that since the pressure’s lower in the pulmonary circuit to the lungs, blood tends to shunt toward the lungs, so more blood volume goes to the lungs and less goes to the body.
This extra blood volume on the right side leads to fluid overload, and with all this extra volume, the heart begins to fail.
Babies with TA therefore quickly develop heart failure, potentially within weeks after birth. Due to the severity of complications with TA, newborn babies requires surgical repair, with the main goal being to restore normal blood flow through the heart.