uworld question 4535

[ctizzle13498]

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It give you a patient with a pH of 7.15 and a PCO2 of 60 and a HCO3- of 18. When I see this, i immediately think BOTH respiratory AND metabolic acidosis. However, the answer was metabolic acidosis first, and LACK of respiratory compensation. Why can't it be respiratory acidosis first, with LACK of metabolic compensation?

 

[foofighter7]

good question...anyone know the answer?

 

[lira126]

was there any supporting information in the vignette aside from the lab values?

 

[Transposony]

It give you a patient with a pH of 7.15 and a PCO2 of 60 and a HCO3- of 18. When I see this, i immediately think BOTH respiratory AND metabolic acidosis. However, the answer was metabolic acidosis first, and LACK of respiratory compensation. Why can't it be respiratory acidosis first, with LACK of metabolic compensation?

It depends on the given clinical scenario.
The pH will tell you which of the event is primary in general. e.g. if the pH is low and the CO2 is high it is primary respiratory acidosis and if the pH is low with low HCO3 it is primary metabolic acidosis.
However, since metabolic compensation takes time while respiratory compensation is rapid it is more likely the above mentioned scenario is Primary metabolic acidosis with lack of respiratory compensation an example being alcohol intoxication.
The expected CO2 should be about (33 +/- 3) which means that he is retaining CO2.

 

[evilbooyaa]

OP, I took a look at the question itself, and the question states that the patient is in ARF. Sure, she has some hx of respiratory issues, but with her BUN/Cr, her kidneys are likely giving her a bigger problem than her lungs. She is oliguric. She is not diuresing massively, which is when you are more likely to get non-potassium electrolyte abnormalities. It's not the best question/answer, but even if this patient had poor bicarb reabsorption and appropriate hyperventilation, she probably wouldn't be lethargic.

 

[foofighter7]

OP, I took a look at the question itself, and the question states that the patient is in ARF. Sure, she has some hx of respiratory issues, but with her BUN/Cr, her kidneys are likely giving her a bigger problem than her lungs. She is oliguric. She is not diuresing massively, which is when you are more likely to get non-potassium electrolyte abnormalities. It's not the best question/answer, but even if this patient had poor bicarb reabsorption and appropriate hyperventilation, she probably wouldn't be lethargic.

Evilbooyaa....Thanks so much. You're right, his BUN was like 40 and creatinine was 4. Thanks again!

 

[thefritz]

Look at the history.

To get a pH that low from a primary respiratory disorder, the respiratory problems would have to very acute, and this would be obvious from the history. Chronic respiratory acidosis has near-normal pH values.

Additionally, look at the electrolytes if given. Respiratory acidosis usually doesn't have much of an effect on electrolyte levels.

The answer to your question is that this is most likely a primary CHRONIC metabolic acidosis with a superimposed ACUTE respiratory acidosis. This is evidenced by the mildly reduced bicarb and markedly increased CO2. It would be FAR more unlikely to have a chronic respiratory acidosis with a superimposed acute metabolic respiratory acidosis (although I suppose it's possible to create a scenario like that -- guy has COPD exacerbation and doesn't want to live anymore and OD's on aspirin or something).

 

[evilbooyaa]

Look at the history.

To get a pH that low from a primary respiratory disorder, the respiratory problems would have to very acute, and this would be obvious from the history. Chronic respiratory acidosis has near-normal pH values.

Additionally, look at the electrolytes if given. Respiratory acidosis usually doesn't have much of an effect on electrolyte levels.

The answer to your question is that this is most likely a primary CHRONIC metabolic acidosis with a superimposed ACUTE respiratory acidosis. This is evidenced by the mildly reduced bicarb and markedly increased CO2. It would be FAR more unlikely to have a chronic respiratory acidosis with a superimposed acute metabolic respiratory acidosis (although I suppose it's possible to create a scenario like that -- guy has COPD exacerbation and doesn't want to live anymore and OD's on aspirin or something).

Aha! But salicylate OD induces both metabolic acidosis and respiratory Alkalosis. Let's go with he has a COPD exacerbation then eats a dozen donuts without taking his insulin to send him into DKA.