Transposition of the Great Arteries? I understand that ultimately any shunt (ASD, VSD, or PDA) is just a temporary solution until the the patient can get corrective surgery. It seems like a PDA will only cause significant mixing downstream of the origin of the Left Subclavian artery and basically your head (and more importantly, brain) and UEs would be screwed. Am I missing something here?
So I'll get more at the anatomy and logic than others have stabbed at.
Blood goes from the Right Heart to the Lungs, Back to the left heart, than up and around the aorta to the rest of the body. You got that.
In the case of a PDA, oxygenated blood going into the aorta goes BACK into pulmonary circulation through the little connection. It is a "left to right shunt." Afterall, the aorta is a high pressure system, the pulmonary artery is a low pressure system. So, if blood is going to flow anywhere, its from a high pressure system to a low pressure system.
In transposition, the RIGHT ventricle-aorta-RIGHT atria is just one continuous circuit. Its a
low pressure system (because the right heart is weak), and the blood never sees the lungs to get oxygenated. Meanwhile, The LEFT ventricle - pulmonary artery - LEFT atrium is also just one continuous circuit. It is a
high pressure system (because of the left heart is strong). It ONLY oxygenates, and never perfuses the body.
Now put transposition and PDA together. Pay very close attention to right vs left (low and high pressure systems respectively). The HIGH pressure, left ventricle pushes most of its blood through the lungs to get oxygenated (yay!). Then, because of a persistent PDA, there is a connection between the high pressure oxygenated blood and the low pressure unoxygenated blood. The oxygenated content flows down its pressure gradient and into the systemic system.
See what happened? In PDA alone it flows left to right (from the aorta to the pulmonary arter), because the aorta is a high pressure system because the left ventricle is high pressure. But in a TGA, the aorta is connected to the right ventricle. So its a low pressure sytem because the right ventricle is low pressure, and now the pulmonary artery is a high pressure system because its connected to the left ventricle. So, when you put PDA and TGA together you have again have a "left to right" shunt. "Left" is oxygenated, "Right" is unoxygenated. But "
left" in this new case
means "f
rom the pulmonary artery" and "
from the fully oxygenated" while "
right"
means "
to the aorta" and "
to the unoxygenated periphery"
This mixing of the circulations gives some oxygenation to the RV-Aorta-RA system. This blood gets circulated again and again, which means that there is at least oxygen coming out of the right ventricle in to the aorta even BEFORE the PDA.
of course, if you don't buy that, then you could try:
1. Left ventricle is a higher pressure system so can fill BACKWARDS from the PDA because there is no pressure resisting it
2. There are collaterals
and if you don't buy that:
1. Who cares why. Understanding physics isnt that important. babies live when you keep the PDA open. Maybe you can't see why it works, but the fact is that it does. So we do it.