Wikipedia:Osmosis/Patent ductus arteriosus



Author: Tanner Marshall, MS

Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS

“Patent” (not “patent” like an invention) refers to some opening, and a patent ductus arteriosus, which I’m going to call 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 are branching off to the brain and upper extremities. Alright, so to help visualize this I’m going to switch to this super simplified version of the heart since it’s better at showing the relationship between the aorta, branches, pulmonary artery, and ductus arteriosus. But we’ll keep the more anatomical heart for reference. So this vessel, the ductus arteriosus, usually closes after birth because the walls collapse down and it 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, and relies on oxygenated blood from the placenta that 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 right ventricle to the pulmonary artery, at which point, most of it gets sent to the ductus arteriosus to the aorta instead of to the lungs. Because 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 by the ductus arteriosus itself. At birth a bunch of things change, though—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, and within 3 weeks, it’s completely closed off and turned into the ligamentum arteriosum. If that ductus arteriosus doesn’t close off, then the baby is left with a patent ductus arteriosus, and 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, for example, congenital rubella syndrome, where the mother contracts rubella virus during her pregnancy.

Alright, so now we’ve got this still-open ductus arteriosus, and we’re using our lungs, now what happens? 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. Well, since blood likes to move from high pressure to low pressure, and it’s higher pressure over in the aorta, it actually just keeps going on to the lungs. Then, that freshly oxygenated blood comes over into the left atrium, the left ventricle, and 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 is flowing back and taking a second unnecessary trip to the lungs. What we don’t have is any deoxygenated blood escaping into the systemic circulation and causing a baby to appear cyanotic, so 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.

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

If that happens, pressure might eventually increase to a point where right-sided pressures are greater than left sided pressures, and so the flow of blood through the PDA can be reversed. When this changes from a left-to-right shunt to a right-to left shunt, it’s referred to as an Eisenmenger’s syndrome. Now, you’ve got deoxygenated blood flowing into the aorta and generally heading to the lower extremities since the upper extremities and head typically have arterial branches upstream from the PDA. This results in cyanosis in the lower extremities, a blue or purple discoloration of the skin that’s the result of deoxygenated blood mixing with oxygenated blood. This situation is actually sometimes called differential cyanosis, since only the lower body is cyanotic.

Patent ductus arteriosus can actually be treated early on. Remember how prostaglandin E2 keeps the ductus arteriosus open during fetal development? Well, the drug indomethacin can be used to close a PDA because it’s a type of non-steroidal anti-inflammatory drug, or NSAID, and it inhibits prostaglandin E2. In some cases, the PDA may be closed by surgical ligation as well.