HoleQ. I’m having a hard time wrapping my head around which way blood flows through congenital heart defects. In an atrial septal defect, blood moves from R-L in utero, but after birth, flow changes from L-R, and then to R-L. Does it keep playing this game of L to R and then when pressure gets too much it reverts again, so on and so forth?!

A. Good question! This is something that often trips up students when they start thinking about it.

To understand which way blood will flow through a defect (let’s just use ASD as an example), there are three important assumptions:
  1. In the developing fetus, pressure is higher on the right side than on the left (because the lungs are all collapsed and it’s hard to pump against them).
  2. In the adult, the pressure is normally higher on the left side than on the right (because after the lungs expand at birth, it’s way easier to pump blood through them).
  3. Lungs are highly sensitive to changes in blood flow. If lungs are exposed to more pressure than they are used to, the vessels can begin to constrict. This is probably a protective mechanism (you want to keep the capillaries from being exposed to massive blood pressure so they don’t rupture).

So let’s say you have a big ASD. In utero, blood flows through the hole from right to left (since pressure is higher on the right). When you’re born, the pressure on the right drops dramatically, and flow is now from left to right.

Over time, though, the right side of the heart gets exposed to a higher amount of pressure, since it’s got all this flow going through the hole from left to right. And if the right side of the heart is exposed to high pressure, then by extension, so are the lungs.

In response to this abnormally high pressure, the vessels in the lungs constrict. If there’s enough constriction, it can become very difficult to pump blood through those lungs – even harder than it is to pump blood through the body. In other words, the pressure on the right can become higher than the pressure on the left! And when that happens, the flow through the ASD will be from right to left. This is called Eisenmenger syndrome, by the way, and it’s a dangerous situation because it’s hard to get enough oxygen-rich blood to your organs and tissues.

That’s the end of the reversal process (flow doesn’t end up going from left to right again). This is because the longer you have the ASD, the higher the pressure on the right side becomes (due to lung constriction) There isn’t a corresponding change on the left side, in which it becomes hard to pump blood through the body. So flow just continues to go through the ASD from right to left.

Here’s a post I wrote on the direction of flow through the ductus arteriosus – maybe that will help a little too. Let me know if you still have questions!