PE vs. hyperventilation

[HereWeGo21]

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Got a question wrong in UWorld, got me thinking.

We have a patient with a PE, with:
Tracheal pO2 150 mm
Alveolar pO2 145 mm
Alveolar pCO2 5 mm

I had just thought this was primary hyperventilation. Say you had a panic attack, and increased rate of ventilation 8x. Wouldn't the picture look exactly like this? CO2 is low, because it's being breathed off. PAo2 is high, simply according to the alveolar gas equation. What am I missing?

Of course, the answer winds up being PE. The vessel is obstructed, so air isn't leaving the alveolus, and CO2 isn't entering the alveolus.
But I almost think that hypeventilation is a better answer. Because, we know Co2 diffuses much better than O2. So, you would expect 145 o2 in the alveolus. But you'd also think to expect more than 5 co2 in the alveolus as well, since Co2 dissolves better than O2. Does this make sense?

 

[StilgarMD]

I get what you are saying, and this Q tripped me up too. I've ended up settling on understanding it the following way:

There either is or isn't an Equilibrium between Air in the alveoli and the blood it sees.
If Alveolar O2 content matches the pulmonary veins, then diffusion is intact. - O2 is the hardest thing to move, so if it moves, it is fine (though as you noted "CO2 diffuses better" is something I just had to accept for this explanation, but idk why. O2 is a smaller, equally non-polar molecule)
If Alveolar O2 content matches the tracheal O2 content, it suggests there is a problem of alveolar perfusion or of diffusion.

If Alveolar CO2 is normal, than this suggests a diffusion problem disabling barrier which O2 can not overcome as easily.
If CO2 is low and O2 is high (air composition nearly identical to tracheal) this suggest the alveoli isn't seeing blood.

If the person were hyperventilating, given the typical ease with which CO2 moves, they'd have to have a blood CO2 of 5 to only have 5 in the alveoli and a pH of like 8, which seems like no-go. Come to think of it, I've heard about "Hypercapnic", but rarely "hypocapnia" (outside of a mild drop as a means of compensating for metabolic acidosis). Probably because the respiratory centers would slow down well before that low a point were reached.

 

[worldbeater]

Tracheal pO2 150 mm
Alveolar pO2 145 mm

I don't remember the specifics, but the normal difference between these two should be 10-12, not 5, thats why it's not primary hyperventilation.

Pulmonary embolism causes a restrictive pattern (increased respiratory rate due to lack of O2), same pattern as hyperventilation, so at least you were on the right track.