question

[cosine]

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I was doing a question on world and needed some help

A 45 y/o male presents to the ER with Po2 of 54 and a Pco2 of 26. they wanted to know what was process was most likely occuring and the answer was alveolar hyperventilation.

I thought that hyperventilation would mean there is an increase of 02 above normal value and a decrease in Co2 below normal value. In this case the guy has low o2 and co2. what gives?

 

[Nematode]

something blocking the air way?

 

[lord_jeebus]

Hyperventilation will not typically increase pO2 as O2 uptake is usually perfusion-limited.

 

[Knicks]

Hyperventilation will not typically increase pO2 as O2 uptake is usually perfusion-limited.

Still, shouldn't the Alveolar AND arterial PO2 be increased? Judging from the OP, it seems as if there is nothing in the question that implied any perfusion problems.

 

[lord_jeebus]

Still, shouldn't the Alveolar AND arterial PO2 be increased? Judging from the OP, it seems as if there is nothing in the question that implied any perfusion problems.

I'm not saying that there's a perfusion problem or not (there really isn't enough info here to explain the etiology of the blood gas); I'm just saying that you can hyperventilate all you want and it won't boost your pO2.

 

[Knicks]

I'm not saying that there's a perfusion problem or not (there really isn't enough info here to explain the etiology of the blood gas); I'm just saying that you can hyperventilate all you want and it won't boost your pO2.

....if there's a diffusion/perfusion problem, then yeah I can see that. But as I said (and you corroborated), there's no indication that there is a perfusion problem.

 

[lord_jeebus]

Basically, the OP asked if hyperventilation would increase pO2, and barring unusual hyperdynamic scenarios which don't apply here, the answer is no.

There is no information given that tells us why pO2 is low. It is useless to speculate, and unnecessary to answer the question.

 

[Knicks]

Basically, the OP asked if hyperventilation would increase pO2, and barring unusual hyperdynamic scenarios which don't apply here, the answer is no.

There is no information given that tells us why pO2 is low. It is useless to speculate, and unnecessary to answer the question.

LoL, OMG, am I in some twilight zone here?

You're saying hyperventilation would NOT increase po2?? Maybe I just didn't get enopugh sleep last night 😳 lol

 

[lord_jeebus]

Arterial pO2 is primarily determined by the alveolar pO2, and they more or less reach equilibrium at the alveolus.

Hyperventilation does not change the alveolar pO2, and hence can not increase arterial pO2.

Even if there is an a-A gradient, hyperventilation will not correct this.

Remember this when you're managing vent settings in the ICU someday - increasing FiO2 (oxygen concentration) or PEEP will improve oxygenation, but increasing minute ventilation will not.

 

[Knicks]

Thanks.

Then can you clarify this: I thought that when you hyperventilate, your pCO2 goes down because you're increasing the pO2 in the alveoli.

 

[BoneNibbler]

Thanks.

Then can you clarify this: I thought that when you hyperventilate, your pCO2 goes down because you're increasing the pO2 in the alveoli.

Silly girl, you have been listening to to much goljan.

 

[Knicks]

Silly girl, you have been listening to to much goljan.

I'm a man, and I don't listen to Goljan. 😎

 

[lord_jeebus]

Thanks.

Then can you clarify this: I thought that when you hyperventilate, your pCO2 goes down because you're increasing the pO2 in the alveoli.

I would recommend reading the recent thread and/or your textbooks regarding the subject of perfusion- vs. diffusion-limited gas absorption.

 

[Knicks]

^^ I just did last week. I thought the way that I understood it was correct, and not the way you explained it.

 

[lord_jeebus]

The OP just posted the full text of the question but I had to delete it as a copyright violation.

Nonetheless, here is the key.

Of the answer choices, only hyperventilation can cause a decrease in pCO2. The others would all cause decreased ventilation, which increases pCO2. The pO2 is irrelevant.

 

[RisingSun]

I was doing a question on world and needed some help

A 45 y/o male presents to the ER with Po2 of 54 and a Pco2 of 26. they wanted to know what was process was most likely occuring and the answer was alveolar hyperventilation.

I thought that hyperventilation would mean there is an increase of 02 above normal value and a decrease in Co2 below normal value. In this case the guy has low o2 and co2. what gives?

People have given some pretty good advice here, so I'll just add that I think that it is probably more important to think about why the pO2 is low rather than why the pCO2 is low in order to get to the bottom of what is going on.

With decreased pO2, you have got to think about hypoventilation (not the problem here), R-L shunt, V/Q mismatch, or diffusion abnormality. My guess is that this guy had a PE, there is a V/Q mismatch (so low paO2 but NORMAL pAO2) and he is hyperventilating because he is short of breath.

 

[psu07]

Arterial pO2 is primarily determined by the alveolar pO2, and they more or less reach equilibrium at the alveolus.

Hyperventilation does not change the alveolar pO2, and hence can not increase arterial pO2.

I agree with everything you are saying here up to the point when you say hyperventilation does not change alveolar pO2 and here is the reason why. alveolar pO2 = FiO2 (atm pressure - 47) - PACO2 / R ( R being the respiratory coefficient) according to this equation, if you were to have a decreased PACO2, you would end up having a higher alveolar PO2. Somebody please care to explain if my logic is wrong.

 

[lord_jeebus]

I agree with everything you are saying here up to the point when you say hyperventilation does not change alveolar pO2 and here is the reason why. alveolar pO2 = FiO2 (atm pressure - 47) - PACO2 / R ( R being the respiratory coefficient) according to this equation, if you were to have a decreased PACO2, you would end up having a higher alveolar PO2. Somebody please care to explain if my logic is wrong.

Excellent, I stand corrected.

 

[Knicks]

Excellent, I stand corrected.

LoL. What psu07 said was what I was trying to get at.

 

[pseudoknot]

I agree with everything you are saying here up to the point when you say hyperventilation does not change alveolar pO2 and here is the reason why. alveolar pO2 = FiO2 (atm pressure - 47) - PACO2 / R ( R being the respiratory coefficient) according to this equation, if you were to have a decreased PACO2, you would end up having a higher alveolar PO2. Somebody please care to explain if my logic is wrong.

The value of R changes with hyperventilation. This is why PaO2 does not increase with hyperventilation.

http://www.chestjournal.org/content/81/3/391a.full.pdf