V/Q defect

[lDanny]

So I have two sources and they seem to be saying two opposite things. When V/Q is high, is the PaO2 highest or lowest? thanks

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[WarriorMD]

My understanding is that it would be a high PaO2 and low PaCo2.

My kaplan books and my gunnertraining seem to have a bit of a discrepancy on this topic as well so I am glad you brought it up, when I come across the gunner questions again I will make up a post to get some feedback on it.

 

[chet]

I think the discrepancy may be due to looking at the V/Q defect as a physiologic process in different areas of the lung compared to a pathologic process.

V/Q is highest at the apex of the lung, so in pulmonary venules leaving that region you would see higher oxygen content compared to the base of the lung.

In the case of a massive PE V/Q approaches infinity, so you have a shunt and you'd expect that Pa02 would decrease.

Maybe I'm wrong or don't understand the question, hope that helps though.

 

[loveoforganic]

So I have two sources and they seem to be saying two opposite things. When V/Q is high, is the PaO2 highest or lowest? thanks

Edit: Much more correct and thorough answer below

 

[Phloston]

So I have two sources and they seem to be saying two opposite things. When V/Q is high, is the PaO2 highest or lowest? thanks

The question you're asking is very general. Are you inquiring as to where in the lung Pa is greatest, or are you asking about whether a perfusion vs ventilation defect results in a greater ebb of arterial oxygen content?

In terms of lung location (physiological):

V/Q is ~3.0 at the lung apex and ~0.6 at the lung base when upright. It's not that ventilation (PA) is greatest at the apex; perfusion (Pa) is merely lesser at that location due to gravity. Therefore, arterial pressure is lesser than alveolar pressure, and PA > Pa > Pv. As you move inferiorly in the lung (mid-lung zone), when upright, arterial pressure surpasses alveolar pressure and Pa > PA > Pv. At the lung base, perfusion is greatest such that even venous pressure can surprass alveolar pressure (Pa > Pv > PA). Furthermore, physiologically, when considering the higher V/Q at the lung apex vs the lower V/Q at the lung base, Pa is greatest at the lung base where V/Q is low.

In terms of ventilation vs perfusion defect (pathological):

If V/Q is elevated, a perfusion defect is present. If V/Q is low, a ventilation defect is present. PaO2 may decrease with both perfusion and ventilation defects, but it classically decreases more with a ventilation defect (PaO2 is lower with low V/Q). This is because under-ventilated regions have a substantially decreased PAO2 such that properly ventilated regions, even if hyper-ventilated, cannot fully compensate (i.e. if net PAO2 should be ~100mm Hg, if one area of under-ventilated region can only yield PAO2 = 40mm Hg, it doesn't matter whether other hyper-ventilated regions yield PAO2 = 120mm Hg, because shunting of blood from high O2 to low occurs, and the net result is PAO2 < 100mm Hg). Therefore, since the hyper-ventilated regions cannot compensate in a ventilation defect, putting the patient on 100% O2 won't change his or her PaO2 because any additional O2 merely enters a decompensated shunt. In a perfusion defect, blood not reaching one area of lung is transferred to other areas of lung. Since healthy lung is perfusion-limited, augmented perfusion in areas not affected by the perfusion defect results in sufficient compensation. Therefore, giving O2 to a patient with a perfusion defect does increase his or her PaO2.

The take away message: Pathologically, PaO2 decreases with both high and low V/Q, however it classically decreases more with low V/Q. Low V/Q is not subject to PaO2 augmentation with 100% O2 administration, whereas high V/Q is. Physiologically, Pa is greater at the lung bases than at the lung apices due to greater longitudinal distance below the level of the left atrium, when upright, resulting in higher proximal vascular pressures and lesser vasoconstrictive effects. Healthy lung is perfusion-limited.

 

[lDanny]

Thank you for taking the time out to answer my question.

I guess the confusion was that in one text it had the ratio for the apex having high V/Q and having high arterial pO2. The text described the high PaO2 and low PaCO2 at high V/Q (not yet dead space), the reasoning was that since there was still perfusion the CO2 is low.

But in another text, stated since there was less perfusion more CO2 remains in the blood, therefore the high V/Q will have a high PaCO2.

I will try and get the text later today. I am probably just misinterpreting the information.

Thanks again