Step II ASA Toxicity ABG

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Ronin786

Ronin786

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In Uworld QID: 2803, it asks which would be the ABGs for a patient with ASA toxicity. Their answer is:

pH of 7.36, PaCO2 of 22 and HCO3 of 12.​

In MTB, they ask the same exact question, but their answer is:

pH of 7.46, PaCO2 of 22 and HCO3 of 16​



However, one of their answer choices has a pH of 7.35, PaCo2 of 32 and HCO3 of 20, which to me looks like an earlier stage of Uworlds answer.

To make matters worse, Uworld has another Q, ID 2820, where they say a patient with ASA toxicity would have a pH of 7.43, PaCO2 of 25 and an HCO3 of 16. And that is the same as MTB's answer.

Basically how do I know what the ABG for ASA toxicity is going to look like, especially if I have no idea how long ago this happened?

Is it going to be Primary Resp Alkalosis (elevated pH) with mixed Metabolic Acidosis (low bicarb) or is it going to be a Primary Acidosis (low pH) with mixed Respiratory Alkalosis (low PaCo2)?

I used to consider myself solid with ABGs, but in doing Uworld questions they seem to pay attention to the bicarb before the PaCo2 which is the opposite of what I'd been taught and its really messing my head.

Thanks!
 
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ASA toxicity is basically Primary Respiratory Alkalosis with Primary metabolic Acidosis. In order to confirm that you have to use the Winter's formula to find the expected CO2 (which is roughly the given HCO3+15). So, with a given CO2 of less than expected CO2 makes it primary Respiratory Alkalosis and a low HCO3 makes it a mixed Respiratory Alkalosis with metabolic Acidosis.
In the earlier stages the Primary Respiratory Alkalosis commponent is predominant (hyperventilation) and later on Primary metabolic Acidosis due to accumulation of lactic acid. So, the pH will differ.
Hope this helps.
 
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MY BAD!! DIDN'T REALIZE THIS WAS IN THE STEP 2 THREAD. DONT LISTEN TO ME LOL.



Ronin786 said:
In Uworld QID: 2803, it asks which would be the ABGs for a patient with ASA toxicity. Their answer is:

pH of 7.36, PaCO2 of 22 and HCO3 of 12.​

In MTB, they ask the same exact question, but their answer is:

pH of 7.46, PaCO2 of 22 and HCO3 of 16​



However, one of their answer choices has a pH of 7.35, PaCo2 of 32 and HCO3 of 20, which to me looks like an earlier stage of Uworlds answer.
Click to expand...

Thats not the same thing as being an earlier stage of U-worlds answer.... that doesn't show a metabolic acidosis like you would get in ASA toxicity, as the bicarb is elevated (not "elevated" but too high for metabolic acidosis). The U-world example you gave shows a metabolic acidosis as would be seen in the later stages of ASA toxicity. The MTB answer shows a respiratory alkalosis which would be seen in the earlier stages of ASA toxicity. My guess would be that if they didn't give you a timeline as to how early/late the toxicity was occuring that you should be able to get the correct answer just by determining which answer choice is either 1) a metabolic acidosis or 2) a respiratory alkalosis. If they didn't give you a time line and then gave you these as answer choices:

A. pH of 7.36, PaCO2 of 22 and HCO3 of 12.
B. pH of 7.46, PaCO2 of 22 and HCO3 of 16​

Then that is seriously messed up because i don't know how you could determine the answer except to say that A is a mixed disturbance (metabolic acidosis + resp alkalosis) as the PaCO2 is below the compensated level expected by using Winter's Formula [(1.5 x HCO3-) + 8 +/- 2].... which I would think was beyond the scope of Step 1.

MY BAD!! DIDN'T REALIZE THIS WAS IN THE STEP 2 THREAD. DONT LISTEN TO ME LOL.
 
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^^ Still making the right points.

I thought I read somewhere that first ASA causes a respiratory alkalosis, followed by a secondary metabolic acidosis. This is worse than them both happening together, because apparently the body somewhat compensates for the respiratory alkalosis by dumping bicarb, then the metabolic acidosis hits and just kills it even more.

I don't know how clinically relevant anything I just typed is, but yeah. Let's take a look at every single ABG you posted.

pH of 7.36, PaCO2 of 22 and HCO3 of 12.

I usually start by looking at pH then decide what supports that pH. In the above example, the bicarb supports the pH first, while the CO2 is either compensating or adding to the disorder.

Acidosis --> Metabolic Acidosis --> Winter's says 1.5*12 + 8 = 26 > 22 --> Additional Respiratory Alkalosis (not Respiratory Compensation, took me a while to differentiate these 2 concepts)

pH of 7.46, PaCO2 of 22 and HCO3 of 16

Alkalosis --> Respiratory Alkalosis (since Bicarb 16 = acidosis) --> For every 10 point drop in CO2, HCO3 should decrease by ~ 2 in respiratory alkalosis (for the record, I had to look this up just now) --> HCO3 should be around 20 (24 = normal). Since it's lower, there's a combined metabolic acidosis.

pH of 7.35, PaCo2 of 32 and HCO3 of 20

Acidosis --> Metabolic Acidosis --> 1.5(20) + 8 = 38 > 32 --> Respiratory alkalosis.... wait. Now I've confused myself. Clearly I'm not understanding this either, or MTB/UWorld are some serious dicks. If they're definitely not telling you how long it's been since ingestion, I'll lean towards them being dicks.

pH of 7.43, PaCO2 of 25 and an HCO3 of 16. Just to check.

Alkalosis --> Resp. Alkalosis --> HCO3 should be 21 > 16 --> Metabolic Acidosis. Well, crap.

Clearly we actually have the same understanding of this, and if they don't give you timings, you really can't specify a difference. I'm going to look up that question real quick on Step 2 UWorld and see what it says.

OK - just took a look at both the questions. 2820 gives you a set of lab values and asks you what condition it would be. They give you some form of respiratory alkalosis. Out of the 5 answer choices, ASA tox is the only one that includes any sort of respiratory alkalosis as the primary process.

in 2803, you saw that her respirations were at 24/min, with an unremarkable lung exam. This would lead you to expect a PCO2 < 40. I picked the right answer (7.36, 22, 12), but I'm not sure why I thought it was that and not the 7.45, CO2 = 30, HCO3 = 20 if that's all I was basing it on. While her sx (tinnitus and vertigo) are apparently sx of ASA tox, I know I wasn't thinking about that when I answered the question.
 
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Looks like we're basically in the same boat :laugh:

I just cant seem to find out how to tell whether the primary is going to be Acidosis or Alkalosis without being given the time. I guess since its a mixed disturbance, its most likely going to be the one with both values extremely depressed?

And for the record the MTB questions is almost exactly like the 2803 Q from UWorld except there actually is no stem. They just ask, which of these pH values would be seen in a patient with ASA toxicity. And whats worse is for the explanation of the answer choice, MTB describes the (7.35/32/20) as Primary Acidosis with Respiratory Compensation, even though Winter's formula would expect the PaCo2 to be 38.

My guess is that the MTB question was too lazy to calculate the compensation and just put that in there. However, that doesn't solve the original query of the primary disturbance in the mixed ASA disorder.
 
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Ronin786 said:
However, that doesn't solve the original query of the primary disturbance in the mixed ASA disorder.
Click to expand...
The pH will tell you which of the event is primary in general. e.g. if the pH is acidotic 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, ASA toxicity is unique since both the events are primary (as they are happening independent of each other (ASA causing resp alkalosis due to hyperventilation and ASA causing metabolic acidosis due to lactic acidosis)--these two events are NOT related to each other (as in compensation).

Ronin786 said:
MTB describes the (7.35/32/20) as Primary Acidosis with Respiratory Compensation, even though Winter's formula would expect the PaCo2 to be 38.
Click to expand...
That is due to the fact that pH approaches normal but will never be normal in any form of compensation although I agree that the CO2 should be higher than 32 (it should be at least 35 by the thumb rule).
 
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in 2803 said:
Hi,

In regards to when you said, " I am not sure why I thought it was that and not the 7.45, Co2 30, 20 HCO3, for ASA tox. As you know, asprin toxicity causes metabolic acidosis as well as resp alkalosis. In the ABG that you pointed out, it shows resp alkalosis, but does not show metabolic acidosis, as 20 bicarb would not be considered metabolic acidosis , and therefore would rule that ABG out for asprin toxicity. In the other ABG you pointed out, 7.36, Co2 22, and 12 bicarb, you see there is both resp alkaois and metabolic acidosis, pointing toward ASA toxicity.
Click to expand...
 
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