R --> L Intrapulmonary Shunt

[Redpancreas]

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In a Step 2 CK UWorld question, it asks what causes someone with lobar pneumonia to have a low pulse ox when they lay on the side with the consolation and the answer is a R --> L Intrapulmonary Shunt. I am so confused about this concept having learnt in vaguely in various classes in various ways and I just want the get the definition straight.

Is an R->L Intrapulmonary shunt:

A) When one side is low on O2 due to a consolidation, that side constricts causing a shunt of blood within the lungs (hence intrapulmonary shunt).

B.) When blood can't be oxygenated because ventilation is being blocked so blood is theoretically traveling from the right to the left side of the heart without really getting oxygenated.

C.) Is it some gravity mediated affect where blood flows from R ->L lung

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If you chose B, then is it possible to have a L->R INTRAPULMONARY Shunt?

 

[recycledpaper]

I understand how it can be confusing because when we think shunt, we're historically learned it in the context of an intracardiac shunt. But the name works because physiologically it sort of works the same way, just not in the same location.

When there is pneumonia/consolidation, ventilation is effectively zero because there is gunk on the alveoli (V=0). As a result, the blood that flows next to the alveoli does not get oxygenated. Essentially, it's as if the blood had never entered the lungs in the first place, which is why its physiologically similar to a R-L intracardiac shunt.

When you lay on a certain side, blood will want to go there because gravity affects flow (think of a river flowing downstream). If you lay on the side with the pneumonia, you are essentially increasing flow to an already oxygen-deprived, poorly ventilated region. So of course you are going to get worsening hypoxia as a result.

 

[recycledpaper]

And no, there is no such thing as a L to R intrapulmonary shunt. You are confusing the terminology used to describe intracardiac shunts.

Intrapulmonary shunts are not called LR or RL because of direction of flow (like cardiac shunts). Its called a RL shunt because it behaves like a RL cardiac shunt. If you have a VSD, deoxygenated blood will flow through that instead of being oxygenated in the lungs. Same thing if you have pneumonia, except the blood is still flowing to the lungs, its just not being oxygenated.

 

[iBS1972]

Recycledpaper gave very good explanations. I agree with most. But:
VSDs actually result in L->R shunting of blood, because the pressure in LV (110-130) is so much greater than systolic pressure of RV (25), resulting in blood being pumped from LV to RV during systole. This is until very late VSD (decades), when the increased in volume in RV increases the pressure in pulmonary circuit so much (after years) that the RV pressure actually equals that of LV, and eventually greater than the LV pressure, resulting in a R->L shunt, with blood from RV shunted into LV. This is as you might recall Eisenmenger syndrome, and occurs very late in disease.