V/Q Mismatch vs. Hypoventilation?

[bluesTank]

Just a quick question.

In terms of the A-a gradient:

Hypoventilation --> normal A-a
V/Q mismatch --> increased A-a

However, i am having trouble understanding why the A-a would be increased in a V/Q mismatch with decreased ventilation (ie: peanut lodged or something like that) when you wouldn't in hypoventilation.

Sorry if this is a dumb question.

Thanks!

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[tjaze]

because in hypoventilation, the alveolar oxygen is also low because they're not breathing. therefore, the A is about the same as the a, and the gradient is normal.

in V/Q mismatch, the A is normal because oxygen gets into the alveoli fine, but since the alveoli are not perfused at the proper ratio, the transfer of oxygen to the blood is impaired. the a is then lower than the A, and the gradient is increased.

 

[Captopril]

because in hypoventilation, the alveolar oxygen is also low because they're not breathing. therefore, the A is about the same as the a, and the gradient is normal.

in V/Q mismatch, the A is normal because oxygen gets into the alveoli fine, but since the alveoli are not perfused at the proper ratio, the transfer of oxygen to the blood is impaired. the a is then lower than the A, and the gradient is increased.

I think the OP was asking V/Q mismatch where the V drops, not the Q (hence his reference to a lodged peanut).

 

[Dirt]

With the aspirated peanut, you are still getting blood going to the non-functional lung parenchyma but it is not getting oxygenated at all. This non-oxygenated blood combines with the normally oxygenated blood from the rest of the lung yielding a decreased Pa02 since the Pa02 is the sum total of all the blood leaving the lungs. The functional portion of the lung is ventilated as normal (if not hyperventilated) which keeps the PA02 at a sufficient level.

Or think of it this way, PA02 is the average PA02 in every single alveolus. Likewise Pa02 is the average oxygenation of all the blood going through every part of the lung. In the case of V/Q mismatch, the average PA02 decreases less than the average Pa02.

 

[Magnetek Blue]

To build on what Dirt said, when you are hypoventilating, the air entering your lungs is normal and the lungs are perfused normally, so the blood leaving the alveoli is fully oxygenated. I think the oxygen level of the blood entering the lungs is unimportant, it is only the oxygen level of the blood leaving.

So with the peanut, you have no ventilation but still have perfusion in part of the lung, hence there is some blood that is not adequately oxygenated that is leaving. In hypoventilation, all blood leaving the lung is fully oxygenated.

At least that is the way I understand it... This thread was the first hit on my Google search when I had trouble figuring out a similar problem, so I hope this helps other people confused about A-a gradients!

 

[bakanoisha]

Yeah... I'm glad you brought this up. I hadn't thought about it before.

However, upon thinking about it I'm pretty convinced that V/Q mismatches are separate from hypo-ventilation b/c the V/Q mismatch is referring to any potential pathology or obstruction acting directly on the lung. After all, a hypoventilating patient would still be breathing, so when they inhale they should have pretty darn normal arteriolar gas partial pressures and nothing really inhibiting gas exchange.

I.e. if the pt was on a ventilator, lets say because of flaccid paralysis secondary to butulism, and didn't have a V/Q mismatch you can keep them well oxygenated with room air. A person with a V/Q mismatch breathing room air at the same respiratory rate would still be hypoxemic.

 

[Gray Mia]

Hello everyone, thank you so much for your explanations. This question had been confusing me for a long time. Thanks to the help from you and my friend Lingjia, I think I have understood it finally. And here is my summary. Please let me know if I make any mistake.

I think people who don't understand this question may have a similar thought as I did. When a peanut obstructs the airway, PAO2 in the distal alveoli decreases, so does PaO2. Why would it cause an increase in A-a gradient?

I think one way to think about it is that when there is an obstruction, there's no ventilation at all. We can simply regard the peanut as the dead end of the obstructed bronchus. In this case, if you wanna calculate the PAO2, it should be the pressure of oxygen proximal to the dead end, which is actually unchanged. However, when it comes to COPD and asthma, which are another 2 kinds of V/Q mismatch, this explanation does not fit these situations well.

A second way to put it is that during hypoventilation or at high altitude, the oxygen pressure anywhere inside of the whole lung decreases altogether. When it does not, for example, the alveolar PO2 decreases while the tracheal PO2 remains normal, consider it a V/Q mismatch.

Another way to think about it, similar to what Bakanoisha had mentioned, is that when there is an intrinsic problem of the lung, we choose V/Q mismatch.

I think the most confusing part of this whole thing, is the definition of PAO2, because, in both hypoventilation and some types of V/Q mismatch (eg. COPD), there is a decrease in the oxygen pressure in the alveoli, ie. PAO2. Then why would A-a gradient become different? Maybe when we do the calculation, we can regard PAO2 as the oxygen pressure in the trachea instead of the alveoli. Although it's not the correct definition, it helps us in this type of questions. For example, during hypoventilation, there's a weakness in breathing, so the PO2 in trachea would also decrease together with PaO2, resulting in an unchanged gradient. During peanut obstruction, COPD and asthma, the PO2 in the trachea is actually normal, since we still breathe in the air normally, if not more furiously. However, the PaO2 decreases. If we calculate the gradient between the trachea PO2 and PaO2, it will definitely increase.

Please let me know if I made any mistake so that I can stop misleading other people. Thank you again.