ABG Interpretation

Hello all.....I am usually pretty good with ABG interpretation. However, I have one here I am completely stuck on.

pH 7.48 alkalotic

PaCO2 50 acidic

HCO3 18 acidic

a. metabolic alkalosis - uncompensated

b. metabolic alkalosis - partially compensated

c. metabolic alkalosis - fully compensated

d. metabolic acidosis - fully compensated

I have never run into one where both the carbon dioxide and the bicarbonate were acidic, yet the pH was alkalotic. PLEASE HELP!! Thanks for your time!

[Esme12, ASN, BSN, RN]

Think about it.....

respiratory opposite

• ph elevated pco2 diminished = respiratory alkalosis

  
  

  
  
  

• ph diminished pco2 elevated = respiratory acidosis

  
  
  

metabolic equal

• ph elevated hco3 elevated = metabolic alkalosis

  
  
  

• ph diminished hco3 diminished = metabolic acidosis

  
  
  

[Esme12, ASN, BSN, RN]

now.....you can rule out they are compensated

Interpretation:

Uncompensated: pH and one other value (either PaCO2 or HCO3) are abnormal. Abnormal pH determines whether acidosis or alkalosis. Abnormal PaCO2 or HCO3 determines whether respiratory or metabolic.

PaCO2 abnormal = respiratory; HCO3 abnormal = metabolic.

Partial compensation: all values are abnormal. Two values “agree” (pH and either PaCO2 or HCO3 are BOTH acid or BOTH base), and the remaining value (either PaCO2 or HCO3) is the OPPOSITE of the pH. The value (PaCO2 or HCO3) that “agrees” with the pH is correct (determines whether respiratory or metabolic).

Compensation: pH is normal, both PaCO2 and HCO3 are abnormal. One will be acidotic, the other base. The value that most closely matches the pH identifies the true (underlying) state. If the fully compensated pH is between 7.35-7.40, then the normal pH is closer to the acid end and the acidotic value (either PaCO2 or HCO3) is correct. If the pH is between 7.40-7.45, then the normal pH is closer to the alkalotic end and the base value (either PaCO2 or HCO3) is correct.

https://allnurses.com/nursing-student-assistance/abg-help-821378.html

[nurseprnRN, BSN, RN]

You're not crazy. There's a typo in there somewhere. It's not possible to be alkalotic with an elevated CO2 AND a decreased bicarb.

[May 2014]

First it's uncompensated. Secondly, a mixed disorder.

[swissmiss4720]

Thanks for the feedback! I am not going to choose A, B, C or D. But instead write in next to this problem what it is that I do know. Hopefully the teacher will see that at least I'm really thinkin about what I'm doing.

[nurseprnRN, BSN, RN]

First it's uncompensated. Secondly, a mixed disorder.

It's physiologically impossible to have alkalosis AND high CO2 (acidotic) AND low bicarbonate (acidotic). Saying "mixed and uncompensated" nonsensically doesn't recognize the basic disorder, which is an alkalosis. pH always comes first in acid/base analysis.

[Esme12, ASN, BSN, RN]

I have looked at this....I have seen some crazy gasses but not like this....LOL

Good for you OP let us know what your instructor says.

[chare]

First it's uncompensated. Secondly, a mixed disorder.

Please explain how you've come to this conclusion.

pH 7.48

PaCO2 50

HCO3 18

a. metabolic alkalosis - uncompensated

b. metabolic alkalosis - partially compensated

c. metabolic alkalosis - fully compensated

d. metabolic acidosis - fully compensated

First it's uncompensated. Secondly, a mixed disorder.

It's physiologically impossible to have alkalosis AND high CO2 (acidotic) AND low bicarbonate (acidotic). Saying "mixed and uncompensated" nonsensically doesn't recognize the basic disorder, which is an alkalosis. pH always comes first in acid/base analysis.

Let's unpack this down to its fundamentals....

First, the equation that we're talking about:

CO₂ + H₂O -> H₂CO₃ -> H⁺ + HCO₃^-

What does this mean? Simply that carbon dioxide spontaneously combines with water to form carbonic acid.

What are acids? This is actually a very complicated question with diverse answers but in this case we'll stick with the simple one: Acids are proton (H⁺) donors... that is, they spontaneously dissociate to liberate a proton and an anion... in this case, bicarbonate ion.

What is so important about this equation - and all equations in chemistry - is that they are EQUILIBRIUM equations and that the acquisition and liberation of the protons is happening all the time and that the balance of the equation can shift back and forth between CO₂ + H₂O and H⁺ + HCO₃^-

1. Look at the pH... in this case it's 7.48 which is, of course, alkalotic (though very slightly)... before we go any further, a very slight alkalosis means one of two things... either the disturbance has just started and you just happened to catch your gas sample in the earliest stages (which is not very likely), or... compensation is occurring.
   

1. So, we have an alkalotic condition... what does that mean, though? It simply means that there are fewer H⁺ ions floating freely in the solution than there are under 'normal' conditions.
   

How could this be?

Well, only three ways... either there is less CO₂ around which 'pulls' the equation to the left or there is an abundance of HCO₃^- around which 'drives' the equation to the left... or, H⁺ is being lost directly somehow.

Let's look at the other components to see what they tell us...

1. Look at the CO₂... in this case, it's elevated which would drive the equation to the right...
   

OK, so it's going to the right... which means more H⁺ floating around... that is, an acidic environment... but we don't have that here so something else is going on... but those ions are being produced... they have to be... the equilibrium demands it... so why don't we see them?

The only reason that we don't see them is because they are offset by a large number of HCO₃^- ions floating about... which bind with H⁺ to drive the equation back the other way... all good... so we should see an elevated HCO₃^- level...

Which we don't... we see a low HCO₃^- level... which means that a bunch of H⁺ that would normally be bound up are instead floating about... Lots of H⁺ means a low pH or an 'acidic' environment.

Which contradicts what we see...

Which is why Ms. GrnTea says, "It's physiologically impossible to have alkalosis AND high CO2 (acidotic) AND low bicarbonate (acidotic)"

I dislike the memorization tricks and ROME and all that... just reason it out...

Though it does work algorithmically (if you take the time to study the Henderson-Hasselbalch equation and consider the other biological buffers (which is why base excess is always reported on ABG reports).