Interpreting ABG's

Nursing Students Student Assist

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Can someone please help me with this question.

ou are caring for a patient with a renal transplant who has come into the ED complaining of light-headedness and epigastric pain. The patient has a blood gas reading pH of 7.50, PaCO2 of 31mmHg, and a HCO3 of 20mmmol/L. The RN notifies the registrar because the patient is most likely experiencing:

a) uncompensated metabolic acidosis

b)uncompensated metabolic alkalosis

c)uncompensated respiratory acisosis

d)uncompensated respiratory alkalosis

For the answer I got compensated respiratory alkalosis.

This is how I figure it out:

pH 7.50- Alkalosis.

PaCO2 31mmHg- Alkalosis.

HCO3 20mmol/L- Acidosis.

pH and PaCO2 match (as in they are both alkalosis). This means it is respiratory relating.

And because the HC03 is acidic it is compensating.

BUT my lecturers answer to this is:

Just to clarify this question

pH 7.50 means it is uncompensated as patient is still in alkalosis

the cause is likely to be metabolic due to his kidney disease

and because the pH is above the reference range of 7.35-7.45 he is in alkalosis

The low pCO2 is due to the hydrogen ions being used up trying to compensate but the pH remains high so is not fully compensated as yet.

Hope that helps.

---- Now I am really confused. Am I working it out wrong?

Specializes in Pediatric Hematology/Oncology.
This is one of those thing where repetition is needed. Once you understand the pattern everything becomes simple.

For compensation, you should understand uncompensated, partial, and full compensation. For the primary problem the instructor most likely will just test your knowledge between respiratory and metabolic. Combined or mixed respiratory & metabolic is possible with critical injuries but unless the instructor mentioned that or is tough, you most likely do not need to go into that.

This site has a lot of practice with explanations: ABG Interpretation Quiz

I LOVE this quiz! LOVE LOVE LOVE LOVE!!!! It helped me get the hang of ABGs and I revisit it every now and then for a refresher. Once you get the hang of how things relate (and the quiz provides EXCELLENT rationale), this problem will make perfect sense. Practice practice practice!

So, let's work it out.

pH = 7.50 (alkalotic, no compensation as of yet)

PaCO2 = 31 mmHg

HCO3 = 20 mmol/L

Normal would be:

pH = 7.35-7.45

PaCO2 = 35-45 mmHg

HCO3 = 22-26 mmol/L

So, the pt is in alkalosis = 7.50

The CO2 is alkalotic = 31 (I also agree with GrnTea about the rebreathing CO2)

The HCO3 is acidotic = 20

Because we're talking about compensation v. uncompensation and you have the key piece of this being a renal transplant pt, the answer is likely metabolic (I know the values by ROME would lead to respiratory but we know this pt has major kidney problems and metabolic would be the culprit for this imbalance). The CO2 is low because it is in the process of compensating (RR on the pt would also be useful information - if they are breathing more slowly, it is because they are attempting to retain CO2 which is rapidly being used up by the H+ the kidneys are failing to excrete). Once the pt starts compensation, the CO2 will go back to normal, and you will see the bicarbonate value will be what indicates the imbalance (it's going to also be used up more quickly by the excess H+ and being turned into carbonic acid to help lower pH). (disclaimer: I am very likely explaining this very incorrectly and I would recommend a revisit on the acid-base blood buffer system!)

While I love the ABG interpretation quiz, you don't get background on real life conditions in which these situations actually occur and, for many of the random values, it is hard to see how this would actually happen. Though this problem kind of flies in the face of ROME (it is partially compensating so ROME would not necessarily apply!), I think the take home message is seeing the metabolic pH disturbances that can be caused by kidney disease (which is probably very confusing for you now and very typical of nursing school questions :sarcastic:).

Thank you for your help everyone. I have finally figured ABG's out although it took me awhile to wrap my head around this one!

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