Persistent truncus arteriosus and right to left shunt

[loveoforganic]

Does the right to left shunt here develop following eisenmenger syndrome or is it a primary right to left shunt? If it's the latter, can someone please explain the physiology on that?

TYIA

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[spyderracing32]

Persistent Truncus is a little different than the other cardiac defects. Basically, you're missing the septum between the pulmonary arteries and the aorta, and thus you're simply dumping venous and arterial blood into a big mixing pot before it travels distally into the pulmonary and systemic circulations. So right from the get-go you have cyanosis because the contribution of blood from the right side of the heart introduces deoxygenated blood into the blood that will enter the systemic circulation.

 

[loveoforganic]

Thanks for the reply. I thought shunt was classified on whether more blood ended up going through the pulmonary or systemic circulation (rather than the physiological effect that you described, which is traditionally associated with right-left shunting). I thought in persistent truncus, you would get disproportionately more blood flowing through the pulmonary circulation than the systemic circulation due to the lower PVR, i.e. a left to right shunt (unless eisenmenger syndrome developed and reversed the PVR:SVR ratio). I see why you would have increased mixed blood and poor systemic SaO2, but what about the actual right to left shunt?

 

[Chaoticsheath]

It is a right to left shunt by definition because you have venous blood going directly through to systemic circulation w/o being oxygenated. There for you have cyanosis. All right to left shunts have cyanosis if substantial enough (Truncus arteriosus is substantial enough). Not a case of eisenmengers because as written above, cyanosis and right to left shunting starts immediately, as soon as the baby is born. it is not due to eventual RVHypertrophy.

that's the official stuff.

TECHNICALLY, you do have blood that is left to right shunting as well, but the dominant clinical features are due to the effects of right to left shunting. You are right in that more blood will probably go through the pulmonary vasculature at first, but if you think about it, this will lead to pulmonary edema REALLY fast because the pulmonary vasculature is seeing a LOT of fluid. I haven't looked this up anywhere, but I'm pretty sure that this leads to high Pulmonary VR in a pretty immediate sense as well. This might actually be the underlying reason why you right to left shunt (cyanosis) at birth now that i think about it.....

fascinated by cardiology

 

[spyderracing32]

Thanks for the reply. I thought shunt was classified on whether more blood ended up going through the pulmonary or systemic circulation (rather than the physiological effect that you described, which is traditionally associated with right-left shunting). I thought in persistent truncus, you would get disproportionately more blood flowing through the pulmonary circulation than the systemic circulation due to the lower PVR, i.e. a left to right shunt (unless eisenmenger syndrome developed and reversed the PVR:SVR ratio). I see why you would have increased mixed blood and poor systemic SaO2, but what about the actual right to left shunt?

I think it's probably a moot point. If the majority of the blood entering the truncus arteriosus enters the pulmonary circulation you could call this a L => R shunt (since CO from the left and right sides of the heart are supposed to be equal, and if we're mixing oxygenated and deoxygenated blood 50:50 with a larger portion entering the pulmonary circulation, we can say a larger portion is entering the pulmonary circulation from the arterial circulation than vice-versa), but it's still from pooled blood that has stepped down in oxygen due to the addition of deoxygenated blood from the right side (and while this is not a problem for the pulmonary circulation as the blood will be oxygenated just the same, it is a problem for the systemic circulation). Thus the physiological effects are going to be that of a R => L shunt, regardless of the volume of blood entering either circulation, and this is what we really care about. Any shunt causing cyanosis is (as far as I know) defined as R => L.

 

[loveoforganic]

Thank you both for the explanations, that makes sense