Which direction does blood flow through the ductus arteriosus?

Q. I just had a quick question for you. Our notes say that a ductus arteriosus allows flow from the pulmonary artery to the aorta, which I knew.
However, they also say that it’s a left to right shunt, and that it can become right to left. This confuses me, since from what I know, flow would be going from right (pulmonary artery) to left (aorta).

A. When we talk about the ductus allowing flow from the pulmonary artery (right) to aorta (left), we’re talking about intrauterine flow through the ductus. Before birth, the pressure on the right side of the heart is greater than the pressure on the left – so blood flows from pulmonary artery to aorta (through the ductus).

After birth, though, the pressure on the left becomes greater than the pressure on the right. In most babies, the ductus closes (probably in response to the new levels of oxygen in the blood). In some babies it remains patent, in which case flow would now be from the aorta (left; higher pressure) to the pulmonary artery (right; lower pressure).

If the ductus is widely patent, then after a while, that left to right shunt can put enough pressure on the lungs that they react by closing down vessels, effectively making it more difficult to push blood through. Now the right heart has to work really hard to push blood through the lungs – and it can get to the point where the right heart is actually bigger and stronger than the left, making the shunt reverse and go from pulmonary artery (right; higher pressure) to aorta (left; lower pressure).

Why would you wash red cells for patients with PNH?

Q. I’m currently doing my rotations at Children’s Memorial Hospital’s blood bank and I was reading the standard operating procedure for washing red cells. One of the conditions in which they need to be washed is paroxysmal nocturnal hemoglobinuria. Do you know why this is?

A. The reason for washing red cells for people with PNH is to get rid of any ABO incompatible plasma.

In a person without PNH, ABO incompatible plasma doesn’t cause any perceptible hemolysis. The antibodies in the donor unit probably just get diluted out enough that they don’t have much of an effect. Or perhaps they get sopped up by other ABO antigens on other cells (did you know that you have A and B antigens on cells besides red cells?! Weird.).

But patients with PNH are super susceptible to complement-induced red cell destruction. They lack the ability to anchor certain proteins (including proteins that protect the red cell against complement) to the red cell membrane. Patients with PNH have a hard time down-regulating even a small amount of complement activation – so theoretically, a transfusion of even a small amount of non-ABO compatible plasma could lead to hemolysis. There have been a few cases of hemolytic transfusion reactions in patients with PNH that have been attributed to this phenomenon…so in 1948, blood banks began washing red cells before giving them to patients with PNH.

However, this practice has been called into question. The Mayo Clinic reviewed 38 years of experience with transfusing patients with PNH, and only found one documented episode of post-transfusion hemolysis). Their conclusion was that the important thing is to use group-specific blood products for patients with PNH; washing seems to be an unnecessary precaution.