Question about Pao2 in restrictive pattern lung disease?

[worldbeater]

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So I have a basic question about PaO2, what level is it suppose to be in a restrictive lung pattern? Normal or decreased?

If anybody could explain it in basic terms to me, I would appreciate it.

The question I working with was UWorld #1981, if you happen to have done it and want to follow the question along with me. I'll explain my thought process below, and you guys can point out where I am going wrong.

So it's a truck driver that has sudden dyspnea, with swelling over the calf . Which means it's a DVT that leads to a pulmonary embolism (dx). Which means it's a restrictive breathing pattern, which will be: you have a problem breathing in -> decreased 02 intake, causing a increase in respiratory rate, which means your blowing off CO2, and this leads to a respiratory alkalosis, since CO2 is an acid. This part looks good so far.

Now they are putting up columns, and asking the different values for pH, Pa02, pCO2, and plasma bicarb.

I'll do PCO2 first, since it's the easiest to work with. We said restrictive lung pattern, so pCO2 is decreased. Normal range is 33-45, so looking for a number below 33. This knocks out B,C and F.

Then we are looking at pH. We have blown off our acidic CO2, which means we are looking for a respiratory alkalosis. Normal pH is 7.35-7.45, so looking for a number above 7.45. This knocks out A. For those counting at home, we have D and E left.

Normal bicarb is 22-28, the kidney hasn't started compensating yet, so looking for a normal value here. D has 23, E is 17, so D is the winner.

To confirm the answer, I am looking at O2 and this is where I am sort of guessing. Looking at PaO2. A restrictive blood pattern means a problem breathing in, so decreased 02. Normal range is 75-105. Should be below 75, so that gives us D, which has 65. E is knocked out with a normal O2 range of 96.

Thanks.

 

[DrPicard]

Not on MCQs yet, but the way I understand it, PE is a problem with V/Q mismatch (less Q since flow has decreased). This should affect O2 and CO2 in a detrimental fashion, ie PaO2 should go down, and PaCO2 up, since gas exchange has been compromised. Because of the lower volume of CO2 that has to be moved as well as its much higher solubility, it should be affected less than Oxygen (at least in absolute terms). Acutely, you'd see PaO2 going down and PaCO2 going up. But the lowered PaO2 now becomes the drive for respiration - this is why you'll see people with PE generally presenting with tachypnea (among other signs and symptoms). The increased respiratory rate will attempt to normalize the PaO2, but a side effect will be decreased PaCO2. Hence such a patient should have low PaO2, low PaCO2, and resp alkalosis.

Once his resp muscles start fatiguing, his respiration won't be able to sustain itself at such a rapid phase. As this happens, his PaCO2 will begin to rise; first normalizing, then increasing above normal. Hence normal/increasing PaCO2 in such a patient (and ARDS, asthma, COPD exacerbations etc) is a sign the patient is fatiguing and needs to be intubated.

 

[seminoma]

So I have a basic question about PaO2, what level is it suppose to be in a restrictive lung pattern? Normal or decreased?

If anybody could explain it in basic terms to me, I would appreciate it.

The question I working with was UWorld #1981, if you happen to have done it and want to follow the question along with me. I'll explain my thought process below, and you guys can point out where I am going wrong.

So it's a truck driver that has sudden dyspnea, with swelling over the calf . Which means it's a DVT that leads to a pulmonary embolism (dx). Which means it's a restrictive breathing pattern, which will be: you have a problem breathing in -> decreased 02 intake, causing a increase in respiratory rate, which means your blowing off CO2, and this leads to a respiratory alkalosis, since CO2 is an acid. This part looks good so far.

Now they are putting up columns, and asking the different values for pH, Pa02, pCO2, and plasma bicarb.

I'll do PCO2 first, since it's the easiest to work with. We said restrictive lung pattern, so pCO2 is decreased. Normal range is 33-45, so looking for a number below 33. This knocks out B,C and F.

Then we are looking at pH. We have blown off our acidic CO2, which means we are looking for a respiratory alkalosis. Normal pH is 7.35-7.45, so looking for a number above 7.45. This knocks out A. For those counting at home, we have D and E left.

Normal bicarb is 22-28, the kidney hasn't started compensating yet, so looking for a normal value here. D has 23, E is 17, so D is the winner.

To confirm the answer, I am looking at O2 and this is where I am sort of guessing. Looking at PaO2. A restrictive blood pattern means a problem breathing in, so decreased 02. Normal range is 75-105. Should be below 75, so that gives us D, which has 65. E is knocked out with a normal O2 range of 96.

Thanks.

No idea why you're talking about restrictive and obstructive.

Most commonly you'll see low O2 and low CO2 on ABG in PE. The low O2 is due to the perfusion defect. The low CO2 is due to hyperpnea.

 

[worldbeater]

Hmmm..thanks for the above posts. I think what I read and understood about pulmonary embolism is the end result of the specific values will for PO2, C02 will have the same pattern as a restrictive lung disease, but the actual way the physiology gets there is the way you guys described above. You will still get a point on UWorld for the question either way though.