100% O2 improve PaO2 in V/Q mismatch due to physiologic dead space

[hsk013]

Can someone tell me why this is the case?

The way I am thinking is that in physiologic dead space, 100% oxygen goes to alveoli, but it is not perfusing. Like there is a barrier that prevents oxygen to cross alveoli and into the capillary.

So no matter how much oxygen we have in alveoli, it won't be able to cross to capillary? That was my logic. If someone can correct me, I would appreciate it.

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[SMIGuy]

You are describing shunt when you say there is a barrier preventing diffusion. Shunt is classically due to inadequate ventilation of the alveoli however. ARDS can cause a problem similar to what you are describing but even then there is some diffusion and o2 can help. A situation with zero diffusion of O2 into the capillaries in the presence of a ventilated alveolus would be difficult to produce. It could perhaps occur in IPF I suppose or from lung scarring from a fire. In that case you have something similar to shunt but I suppose it is technically v/q mismatch. Shunt is a subset of v/q mismatch but not the whole picture. Shunt is poor ventilation of alveoli while dead space determined mismatch is inadequate perfusion or contact with air due to decreased surface area (emphysema). As long as the alveoli are ventilated even at very low perfusion rates increased o2 can improve paO2 since room air is only 21% O2. You could get into perfusion versus ventilation limitations here but the above is probably enough. This article is an excellent explanation. Ventilation Perfusion Mismatch

 

[DoctorG2020]

Can someone tell me why this is the case?

The way I am thinking is that in physiologic dead space, 100% oxygen goes to alveoli, but it is not perfusing. Like there is a barrier that prevents oxygen to cross alveoli and into the capillary.

So no matter how much oxygen we have in alveoli, it won't be able to cross to capillary? That was my logic. If someone can correct me, I would appreciate it.
View attachment 225403

So, you have three different scenarios.

1. Shunt - this is a block of some of the alveoli that prevents gas diffusion into the capillary. If you give 100% O2, there will be no change in the PaO2 as some alveoli are not "working properly" to allow diffusion. The alveoli that are working properly are already working at their maximum capacity to make up for the others, so there is no additional gain in PaO2.

2. Physiologic dead space - this is where the alveoli are working properly, but there is a blood flow obstruction preventing perfusion of some of these alveoli (thus these vessels are not contributing to PaO2 at all). However, because the alveoli are working properly at their maximum capacity in other areas that are perfused, you would not get hypoxemia with dead space alone. The main problem is increased PCO2 since the alveoli near the obstructed vessels cannot extract the CO2 from the obstructed vessels.

3. V/Q Mismatch due to physiologic dead space - again, dead space in and of itself cannot cause hypoxemia. However, after some time passes, the obstruction of blood flow will be so great that it will actually divert the excess blood flow to the capillaries that are not obstructed. This leads to a V/Q of infinity in the obstructed areas (dead space) but a decrease in V/Q in other areas due to excessive increase in flow (thus V/Q mismatch).

Your question relates to scenario 3. When you give 100% O2 to someone with V/Q mismatch due to physiologic dead space, the PaO2 improves. This is because the areas with the low V/Q due to excessive blood flow are now benefiting. Normally, blood is moving too fast to properly extract oxygen from the alveoli (normally inspired air has 21% O2). If you give 100% O2, you essentially have better odds of extracting more O2 (thus increasing PaO2). Again, the obstructed side is not perfusing and thus not even contributing to the PaO2.

Hope this long explanation helps!