Wikipedia:Osmosis/Coarctation of the aorta



Author: Tanner Marshall, MS

Editor: Rishi Desai, MD, MPH

Coarctation’s a fancy way of saying “narrowing”, so a coarctation of the aorta means a narrowing of the aorta. If we look at the heart, we’ve got the right and left atria, the right and left ventricles, the pulmonary artery leaving the right ventricle to the lungs, and the aorta leaving the left ventricle and going to to the body.

There are two forms of aortic coarctation to be familiar with, an “infant” form and an “adult” form. With the infant form, which accounts for about 70% of cases, the coarctation comes after the aortic arch, which branches off to the upper extremities and to the head, and before the ductus arteriosus. Now you might be thinking, hey...what’s this ductus arteriosus thing doing here? Well, typically this guy only exists during fetal development, and closes after birth, but with infantile coarctation the ductus arteriosus is usually still open, or patent, so there’s a patent ductus arteriosus. In fact, sometimes this form is also called preductal coarctation.

So, if we draw out a little more simplified version of the heart, we’ve got deoxygenated blood coming into the right atrium, and then flowing into the right ventricle. Now, as it’s pumped out the pulmonary artery, it’s got two choices, right? One option is to go through the patent ductus arteriosus and continue down the aorta instead, and the other option is to continue on down the way it’s going. Hrm. Well, since it’s higher pressure over here on the left side, you might think that the blood would say “thanks, but no thanks”, and keep going down the lower pressure pulmonary artery. But this aortic coarctation adds a little twist. Since this spot, right before the ductus arteriosus, is narrower, blood flowing from the left side has a harder time going through, and so actually there’s a high pressure upstream of the coarctation, but low pressure downstream. So what happens is that blood decides to go this way through the Patent ductus arteriosus and into the lower pressure area over here in the systemic circulation and then continue down to the lower extremities, rather than the slightly higher pressure pulmonary artery. This gives you a real sense for exactly how much this coarctation reduces the pressure over here on the systemic side.

Alright since deoxygenated blood’s going to the lower extremities, infants typically present with lower extremity cyanosis, meaning a blue or purple-ish discoloration of the lower limbs, which is often present even at birth. This is a really important thing to catch because without intervention infants often don’t survive past the neonatal period. Infantile coarctation is something that happens during fetal development and can occur on it’s own or can be associated with other congenital changes. It’s worth mentioning that it’s highly associated with Turner syndrome, a genetic abnormality where females only have one X chromosome instead of two. Alright so this brings us to adult coarctation, which accounts for the other 30% of cases, and this typically develops...well...as an adult. Compared to infantile coarctation, in this type there usually isn’t a patent ductus arteriosus, and instead it’s been long since closed off and is now known as the ligamentum arteriosum. The coarctation usually happens in adults just distal to this ligament. So now, there’s no mixing of deoxygenated and oxygenated blood, but just like in infantile coarctation, the pressure is increased before the coarctation since blood has a harder time squeezing through this narrowed artery, and it’s going to be lower downstream from the coarctation. Alright so this results in a couple issues. First, upstream issues: blood flow increases into the aortic branches, and therefore blood pressure increases in the upper extremities and the head. Increased cerebral blood flow means an increased risk of berry aneurysms, where weak spots along blood vessels in the brain balloon out from the high pressures and become tiny sacs filled with blood. This increased pressure also tends to cause the aorta and aortic valve to dilate, or get larger, and the increased pressure means the aorta’s at risk of aortic dissection, or tearing of the inner layer of the aorta.

Now for downstream issues, since blood flow downstream from the constriction is decreased, there’ll be decreased blood pressure in the lower extremities and patients will have a weak pulse in those lower extremities. Since blood pressure’s lower, patients sometimes experience claudication in their legs, which is pain and cramping due to reduced perfusion. Also, when less blood is perfusing the kidneys, the kidneys respond by activating the renin-angiotensin aldosterone system, which results in water retention and ultimately increases blood pressure, and causes hypertension.

Coming off the aorta, you have these smaller arteries called intercostal arteries that run alongside the ribs and supply blood to the area between the ribs, the intercostal space. Keeping aortic coarctation in mind, some of these branch off above the constriction, and they’re called anterior intercostal arteries and they branch from the internal thoracic artery which is off the subclavian, and some of these branch off below the constriction, and these are the posterior intercostal arteries that come directly from the thoracic aorta and they supply ribs three and below. It turns out that the posterior intercostal arteries serving ribs 1 and 2 actually come off above the constriction as well. Ok so upstream, you have increased pressure in the aorta causing increased pressure in the subclavian, internal thoracic artery, and anterior intercostal arteries and the first two posterior intercostal arteries, and then you have decreased pressure in the posterior intercostal arteries for ribs three and below. Now what you need to know is that these arteries are normally linked up to form an anastomosis, which means that there is a direct connection between these anterior and posterior arteries. Under normal circumstances though pressure is equal in the anterior and posterior arteries so blood flows through away from the heart, as expected.

But now that we have high pressures in these anterior arteries and low pressures in the posterior arteries for three and below, you actually get reversed flow in the posterior intercostals.

These posterior intercostal arteries dilate to accommodate the high pressure, and when the heart beats, they pulsate and literally rub up against the ribs, slowly wearing away bone. This can be seen on x-ray as “rib notching”, typically only affecting ribs 3-9 and but particularly affecting ribs 3 and 4. Also, because there is high pressure in both the anterior and posterior intercostal arteries for ribs 1 and 2, there is no reversal of blood flow and you don’t typically see rib notching there.

Alright so there are a couple treatment options for adult aortic coarctation which are balloon dilation, where a tiny balloon is used to widen the aorta, as well as having the narrowed area of the aorta surgically removed, which can correct the hypertension.