Step I question

Started by cosine
Get help with your application

Use all the free resources available to you from SDN: articles, guides, expert advising, forums discussions, and school research.

Browse articles Compare schools My application plan Read Interview Feedback
Practice for interviews Ask an Advisor
C

cosine

Full Member
10+ Year Member
15+ Year Member
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?
 
Sort by date Sort by votes
Knicks said:
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.
Click to expand...

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.
 
Upvote 0 Downvote
lord_jeebus said:
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.
Click to expand...

....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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
lord_jeebus said:
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.
Click to expand...

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
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
cosine said:
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?
Click to expand...

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.
 
Upvote 0 Downvote
lord_jeebus said:
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.
Click to expand...

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.
 
Upvote 0 Downvote
psu07 said:
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.
Click to expand...

Excellent, I stand corrected.
 
Upvote 0 Downvote
psu07 said:
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.
Click to expand...

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
 
Upvote 0 Downvote
You must log in or register to reply here.