USMLE V/Q Mismatch question and 100% O2 treatment

[tpsreport]

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Edit: figured it out. I guess the increased perfusion to other parts of the lung does play a huge role after all.

Hi there,

I seem to be missing out on a pretty basic point here so please help me if you get it:

Two conditions (First aid 2015, page 606):

1. V/Q = ZERO (basically, no ventilation due to foreign body aspiration). Blood flow is still going through the capillaries supplying the dead space, but there's no oxygen exchange.
100% O2 will NOT change PaO2 because the increased O2 in working alveoli can't get to the blood flowing through the unventilated alveoli

2. V/Q = INFINITY (no perfusion due to a PE). Blood flow is not going through the blocked capillaries.

Here's the conceptual part that's confusing me: oxygenation is perfusion limited, so if there's increased blood flow through other working capillaries because of the PE, does that mean that normal ventilation can't get O2 fast enough to the suddenly increased blood flow? (I thought that perfusion doesn't matter until cardiac output is >4X normal rate anyway?) How will 100% O2 help in the second condition?

Thanks 🙂

 

[Transposony]

You are on the right track.
In PE the alveoli doesn't have enough O2 to meet the demand of increased blood flow. But if you increase the PAO2 then the blood in pulmonary capillaries will be able to extract more O2 as they have less time in contact with the alveoli due to increased flow.

 

[seminoma]

It might help to think about it from a different perspective. In PE ventilation is normal. Thus, if you augment ventilation, you can help the patient. In obstruction, ventilation is blocked. Thus, if you augment ventilation (without fixing the obstruction) you aren't doing anything for the patient.

Also, it's best to think of "perfusion limited" as "reaches equilibrium" rather than "won't increase unless I increase rate or volume of perfusion". When you think of it as "reaches equilibrium" then it's easy to see why increasing PAO2 results in increased PaO2. This isn't just a way to simplify the concept, it is actually the whole idea of the concept. Perfusion limited gases equilibrate almost instantaneously and the "perfusion" thing means "the capillary will get maximal gas out of the alveolus as long as there isn't a diffusion defect". Thus, the properly perfused alveoli will pass O2 into the capillaries until equilibrium is reached.