abg interpretation help

[bkell101]

Hey guys....ABA release some advanced sample questions....one deals with a man with 2 days of diarrhea from his UC flare presenting to the preop holding.....wants to know what the most likely abg is.

one of the wrong answers given is pH 7.22 PaCO2 38 HCO3 22

if you had this abg, how would you interpret it? what type of mixed disorder is it? how do you work out the math?

or is this not a possible abg and a dummy answer? thanks!

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[Wiscoblue]

Well going by the pH the primary disorder is an acidosis. Not at all compensated. Hardly any base deficit. Doesn't add up.

 

[pgg]

You really can't say anything 100% definitive about a gas without a set of electrolytes too.

Unknown anion gap. Unknown delta gaps. Acidosis.

But this looks like bad data. A quick way to check a gas for internal consistency is to calculate a notional CO2 = 24 x PaCO2 / HCO3 and see what pH you'd expect to see from that CO2 level, all else being equal.

In this case you'd get 24 x 38 / 22 = 41.5 ... from that you'd expect a pH near 7.4, which it isn't. So, dubious data.

Chronic diarrhea you'd expect a hyperchloremic hypokalemic non gap metabolic acidosis from bicarb loss.

 

[bkell101]

You really can't say anything 100% definitive about a gas without a set of electrolytes too.

Unknown anion gap. Unknown delta gaps. Acidosis.

But this looks like bad data. A quick way to check a gas for internal consistency is to calculate a notional CO2 = 24 x PaCO2 / HCO3 and see what pH you'd expect to see from that CO2 level, all else being equal.

In this case you'd get 24 x 38 / 22 = 41.5 ... from that you'd expect a pH near 7.4, which it isn't. So, dubious data.

Chronic diarrhea you'd expect a hyperchloremic hypokalemic non gap metabolic acidosis from bicarb loss.

The right answer gives an abg with a mixed metabolic and respiratory alkalosis (I'm assuming the guy is anxious in preop or something of that nature)

 

[bkell101]

Before calling it nonsense data I just wanted to make sure this isn't some crazy three metabolic disorder crap mixed together....is the following logic crazy?...I tried working backwards too...similar to what was posted above....what bicarbonate drop would give u a decrease in pH of 1.8 (I think I recall .1 for every six of hco) .....let's say it takes a bicarbonate drop of 10 to create a decrease in ph to 7.2 from 7.4....if we add 10 to the current bicarbonate (22) we would get 32....so maybe his baseline bicarbonate is 32 because he is a copd dude and his kidney makes his bicarbonate high at baseline....so let's say this baseline guy has a co2 of 54 and a bicarbonate of 32 at baseline ....then he has aspirin overdose ....his co2 drops to 38 and his bicarbonate drops to 22 and now he has mixed metabolic acidosis and respiratory alkalosis ....I mean that would be some ridiculous logic right? A mixed metabolic and respiratory alkalosis superimposed on a partially compensated chronic respiratory acidosis? .....I doubt the math even adds up...even if that was the case....there would be two right answers!

 

[bkell101]

"But this looks like bad data. A quick way to check a gas for internal consistency is to calculate a notional CO2 = 24 x PaCO2 / HCO3 and see what pH you'd expect to see from that CO2 level, all else being equal.

In this case you'd get 24 x 38 / 22 = 41.5 ... from that you'd expect a pH near 7.4, which it isn't. So, dubious data."

I think this is the key....how do you go from 41.5 to 7.4? ...I guess just explain the notional co2 portion

 

[pgg]

"But this looks like bad data. A quick way to check a gas for internal consistency is to calculate a notional CO2 = 24 x PaCO2 / HCO3 and see what pH you'd expect to see from that CO2 level, all else being equal.

In this case you'd get 24 x 38 / 22 = 41.5 ... from that you'd expect a pH near 7.4, which it isn't. So, dubious data."

I think this is the key....how do you go from 41.5 to 7.4? ...I guess just explain the notional co2 portion

It's a rule of thumb loosely based on the Hendersen Hasselbalch equation

Ka = [H+][A-]/[HA]

[H+] = Ka * [HA]/[A-]

which via some handwaving is roughly approximated to = 24 * CO2/HCO3

And where the rule of thumb comes in is that you treat the resulting number as a notional CO2 and you mentally consult a table to predict the pH

60 --> 7.2
50 --> 7.3
40 --> 7.4
30 --> 7.5
20 --> 7.6

If the measured pH is not close to the predicted pH then the data itself is suspect. Tends to be less accurate the further from 40 and 7.4 you get.

 

[Ignatius J]

Those are inaccurate numbers.

 

[lunaire]

Interesting & tricky ABG. Not necessarily bad data.

The only way to explain acidosis with normal Pco2 and HCO3 is anion gap.

I'd interpret this as acidemia; lack of respiratory compensation; anion gap metabolic acidosis with mixed metabolic alkalosis.

The delta ratio for this guy would be >>2. The anion gap would be pretty profound.

Not consistent with diarrhea (primarily non-gap acidosis), and thus, wrong answer.

 

[Temple2020]

this helped me with ABG analysis: Respiratory Guide | Free training and quizzes for spirometry, blood gases, hypertension and auscultation.

also merlot.org has a great program you can download that has practice and lessons. Can't find the link right now but I'm sure if you google it it will pop right up.

 

[deleted171991]

Typical irrelevant board question.

I always use an app for crap like this. The calculation is way too academic for real life - all the compensatory numbers are so easy to forget. (I can calculate the square root of a number on paper, too, but why wouldn't I use a calculator?)

All I care about is what the underlying acid-base disorders are, and that's when I start to think about the possible etiology.

 

[nimbus]

Typical irrelevant board question.

I always use an app for crap like this. The calculation is way too academic for real life - all the compensatory numbers are so easy to forget. (I can calculate the square root of a number on paper, too, but why wouldn't I use a calculator?)

All I care about is what the underlying acid-base disorders are, and that's when I start to think about the possible etiology.

But isn't getting off on nerdy s*** like this what separates us from the nurses?

 

[epidural man]

follow the chloride - this tells "all".

 

[deleted171991]

follow the chloride - this tells "all".

Actually is the Na-Cl difference (normally 35), poor man's SID.