Patent ductus arteriosus

Patent — not in the sense of a patent for an invention — refers to some opening. A patent ductus arteriosus, or PDA for short, refers to a blood vessel, the ductus arteriosus, which connects the pulmonary artery to the aorta during fetal development. The ductus arteriosus is right on the aortic arch where vessels branch off to the brain and upper extremities. All right, so to help visualize this, let’s look at a super simplified version of the heart to show the relationship between the aorta, branches, pulmonary artery, and ductus arteriosus. We’ll still keep the more anatomical heart for reference. So, the ductus arteriosus usually closes after birth because the walls collapse down; this vessel becomes a ligament, the ligamentum arteriosum. When it stays open after birth, we call it a patent ductus arteriosus because it’s still passing blood through it; in other words, it’s still patent.

Now, during development, the fetus doesn’t use the lungs yet; instead, it relies on oxygenated blood from the placenta, which comes into the right atrium. Most of that blood actually flows through the foramen ovale, an opening between the atria. Blood that doesn’t make it through the foramen ovale is pumped out of the right ventricle into the pulmonary artery, at which point most of it gets sent through the ductus arteriosus to the aorta instead of to the lungs. Remember, the fetus isn’t using the lungs yet.

During fetal development, the ductus arteriosus is kept open by high levels of a vasodilator prostaglandin E2, which is made 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. The lungs also start to release a small peptide called bradykinin, which constricts the smooth muscle wall of the ductus arteriosus and sort of helps the process along. Within the first day, the ductus arteriosus usually starts clamping shut; within three weeks, it’s completely closed off and turned into the ligamentum arteriosum. If the ductus arteriosus doesn’t close, then the baby is left with a patent ductus arteriosus. This condition accounts for about 10% of all congenital heart defects, of which the vast majority, about 90%, are isolated heart defects, meaning there aren’t any additional congenital defects. On the other hand, a PDA can be associated with other congenital problems, such as congenital rubella syndrome, which happens when the mother contracts rubella virus during her pregnancy.

All right, so now we’ve got this ductus arteriosus still open, and the patient is still using their lungs; what happens now? Well, as blood comes into the right atrium, and then goes to the right ventricle, it approaches the ductus arteriosus and has two options: keep going down the pulmonary artery, or reroute to the higher pressure system in the aorta. Since blood likes to move from high pressure to low pressure, and there’s higher pressure over in the aorta, it actually just keeps going on to the lungs. Next, that freshly oxygenated blood comes over into the left atrium and the left ventricle; now, again it has a choice, but this time it’s already in the higher pressure system, and now some of it is shunted back to the lower pressure in the pulmonary artery. At this point, the shunt is left-to-right, so oxygenated blood flows back and takes a second unnecessary trip to the lungs. What we don’t have is any deoxygenated blood escaping into the systemic circulation, which would cause a baby to appear cyanotic; therefore, we call this acyanotic, which means “not blue”.

Usually, this situation is asymptomatic, and patients can have what’s known as a holosystolic “machine-like” murmur from blood moving from the aorta to the pulmonary artery.

However, later in life, patients might develop pulmonary hypertension from years of increased pulmonary blood volume, or higher pressures on the pulmonary side.