ABG Interpretation, part 9: Example problems 5-8

June 29, 2021 Updated: August 11, 2021 8 min read

In this video, Cathy goes through four example ABG Interpretation problems and answers. You can download the questions and answers below to print out and follow along. We've also explained the steps for interpretation below.

Need help interpreting ABGs? Check out our Arterial Blood Gas Interpretation Flashcards for Nursing Students.

ABG practice question 5

A patient's arterial blood gas measurements read pH = 7.48, PaCO₂ = 31 mmHg, and HCO₃ = 20 mEq/L. How would you interpret this?

Steps for interpretation

  1. Check the pH to determine if we have acidosis or alkalosis. The normal range for pH is 7.35 - 7.45.
    1. The pH here is 7.48, which is out of range on the alkalotic side. Therefore, we have ALKALOSIS.
  2. Determine which system, metabolic or respiratory, is causing the alkalosis. Check the PaCO₂, which represents the respiratory system, first. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. The PaCO₂ here is 31 mmHg, which is out of range on the alkalotic side, which is in the same direction as pH.
    2. Since the PaCO₂ is in the same direction as pH we know it is the respiratory system causing it.
    3. Therefore, the respiratory system is causing the alkalosis and we have RESPIRATORY ALKALOSIS.
  3. Check if there is compensation. Because it's respiratory alkalosis, it's the metabolic system that would be compensating. The metabolic system is represented by HCO₃. So we'll check HCO₃ to check for compensation. The normal range for HCO₃ is 22 - 26 mEq/L.
    1. The HCO₃ here is 20 mEq/L, which is out of range on the acidic side. This means there is metabolic compensation.
    2. To determine if the compensation is partial or full, we check the pH again. The pH was 7.48 which is out of range on the alkalotic side, so the metabolic system has not succeeded in fully compensating.
    3. Therefore, we have PARTIALLY COMPENSATED RESPIRATORY ALKALOSIS.

ABG practice question 6

A patient's arterial blood gas measurements read pH = 7.45, PaCO₂ = 30 mmHg, and HCO₃ = 19 mEq/L. How would you interpret this?

Steps for interpretation

  1. Check the pH to determine if we have acidosis or alkalosis. The normal range for pH is 7.35 - 7.45.
    1. The pH here is 7.45, which is normal but on the alkalotic side. Therefore, we have ALKALOSIS.
  2. Determine which system, metabolic or respiratory, is causing the alkalosis. Check the PaCO₂, which represents the respiratory system, first. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. The PaCO₂ here is 30 mmHg, which is out of range on the alkalotic side, which is in the same direction as pH.
    2. Since the PaCO₂ is in the same direction as pH we know it is the respiratory system causing it.
    3. Therefore, the respiratory system is causing the alkalosis and we have RESPIRATORY ALKALOSIS.
  3. Check if there is compensation. Because it's respiratory alkalosis, it's the metabolic system that would be compensating. The metabolic system is represented by HCO₃. So we'll check HCO₃ to check for compensation. The normal range for HCO₃ is 22 - 26 mEq/L.
    1. The HCO₃ here is 19 mEq/L, which is out of range on the acidic side. This means there is metabolic compensation.
    2. To determine if the compensation is partial or full, we check the pH again. The pH was 7.45 which is normal but on the alkalotic side, so the metabolic system has succeeded in fully compensating.
    3. Therefore, we have FULLY COMPENSATED RESPIRATORY ALKALOSIS.

ABG practice question 7

A patient's arterial blood gas measurements read pH = 7.32, PaCO₂ = 40 mmHg, and HCO₃ = 16 mEq/L. How would you interpret this?

Steps for interpretation

  1. Check the pH to determine if we have acidosis or alkalosis. The normal range for pH is 7.35 - 7.45.
    1. The pH here is 7.32, which is out of range on the acidic side. Therefore, we have ACIDOSIS.
  2. Determine which system, metabolic or respiratory, is causing the acidosis. Check the PaCO₂, which represents the respiratory system, first. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. The PaCO₂ here is 40 mmHg, which is normal.
    2. Since the PaCO₂ is normal, we know it is not the respiratory system causing it.
    3. Therefore, the metabolic system is causing the acidosis and we have METABOLIC ACIDOSIS.
  3. Check if there is compensation. Because it's metabolic acidosis, it's the respiratory system that would be compensating. The respiratory system is represented by PaCO₂. So we'll check PaCO₂ again to check for compensation. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. The PaCO₂ here is 40 mmHg, which, again, is within the normal range. This means there is no compensation.
    2. Therefore, we have UNCOMPENSATED METABOLIC ACIDOSIS.

ABG practice question 8

A patient's arterial blood gas measurements read pH = 7.31, PaCO₂ = 30 mmHg, and HCO₃ = 18 mEq/L. How would you interpret this?

Steps for interpretation

  1. Check the pH to determine if we have acidosis or alkalosis. The normal range for pH is 7.35 - 7.45.
    1. The pH here is 7.31, which is out of range on the acidic side. Therefore, we have ACIDOSIS.
  2. Determine which system, metabolic or respiratory, is causing the acidosis. Check the PaCO₂, which represents the respiratory system, first. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. The PaCO₂ here is 30 mmHg, which is out of range on the alkalotic side , which is in the opposite direction as pH.
    2. Since the PaCO₂ is opposite direction as pH , we know it is not the respiratory system causing it.
    3. Therefore, the metabolic system is causing the acidosis and we have METABOLIC ACIDOSIS.
  3. Check if there is compensation. Because it's metabolic acidosis, it's the respiratory system that would be compensating. The respiratory system is represented by PaCO₂. So we'll check PaCO₂ again to check for compensation. The normal range for PaCO₂ is 35 - 45 mmHg.
    1. Remember the PaCO₂ here is 30 mmHg, which is out of range on the alkalotic side. This means there is respiratory compensation.
    2. To determine if the compensation is partial or full, we check the pH again. The pH was 7.31 which is out of range on the acidic side, so the respiratory system has not succeeded in fully compensating.
    3. Therefore, we have PARTIALLY COMPENSATED METABOLIC ACIDOSIS.

Full Transcript

We're going to work through some more ABG problems. I have a new pen, so hopefully that'll work better.

Number five, we have a pH of 7.48, PaCO₂ of 31, HCO₃ of 20. First step, do we have acidosis or alkalosis, based on the pH? pH should be between 7.35 and 7.45. Because it is out of range on the high side, we know we have alkalosis. So that is step one. Step two, who's to blame? The respiratory system or the metabolic system? So let's look at PaCO₂ to see if it's the respiratory system acting up. So PaCO₂ should be between 35 and 45. 31 is out of range, and it's out of range on the basic side. So we know right away that we have respiratory alkalosis. So I'm going to write that down here. Now we need to see if the metabolic system is doing anything to compensate for this situation. So HCO₃ should be between 22 and 26. We have an HCO₃ of 20, which is out of range on the low side, so this is acidic so we have respiratory alkalosis. We have the metabolic system trying to compensate, but it is not fully compensated because our pH is out of that normal range. So in this case, we have partially compensated respiratory alkalosis. So write that up here, partially compensated respiratory alkalosis. So again, the metabolic system is trying to compensate. It's acidic, trying to compensate for the alkalosis, but it's not quite getting the job done, okay?

Number 6. pH is 7.45, which we know is actually within range - right? - because our range is 7.35 to 7.45. But it's kind of on the alkalosis side. So it's normal, but it's on the alkalosis side of things, okay? So see what's going on with respiratory system and the metabolic system. So PaCO₂ should be between 35 and 45. 30 is out of range, and it's out of range on the basic side. So we do have respiratory alkalosis going on and let's see what the metabolic system is doing. HCO₃ should be between 22 and 26. It's 19, which is on the low side on the acidic side. So in this case, we have respiratory alkalosis that the metabolic system has compensated for, and they were able to bring the pH up to a normal level. So we have fully compensated respiratory alkalosis in this case. So the respiratory system was acting out, causing this alkalosis, and the metabolic system came in, saved the day, became more acid to help compensate for that and was able to bring that pH into normal range. So again, we have fully compensated respiratory alkalosis. And that's problem six.

Alright. Number seven. We have a pH of 7.32, which is out of range, right? Our range is 7.35 to 7.45. It's out of range on the low side, so we know we have acidosis, so acidosis. Now we need to see who to blame, the respiratory system or the metabolic system. Let's first look at the respiratory system. PaCO₂ is 40 in the normal range because our normal range is 35 to 45, so we have normal respiratory. So we know this is not causing the acidosis. So let's go down here to HCO₃, or the metabolic system should be between 22 and 26. It is out of range on the low side, so it is causing the acidosis. So we have acidosis, and it is caused by the metabolic system. So we have metabolic acidosis. Is it compensated? Is the respiratory system trying to compensate for the issue the metabolic system has caused with metabolic acidosis? No. It's just chilling out in its normal range, right? It's 40. It's not trying to do anything. So in this case, we have uncompensated metabolic acidosis, uncompensated metabolic acidosis.

Alright. Let's do problem eight together. PH is 7.31. So, again, it's out of range, on the low side. So, again, we have acidosis. Let's see who to blame. Alright. So step two. Look at the respiratory system. PaCO₂ is 30 which is out of range, right? The range should be between 35 and 45. It's out of range on the low side, so it is basic-- or you could say alkalosis. Those are kind of interchangeable.

Now let's see what the metabolic system is doing. We should have somewhere in the range between 22 and 26. It is low. It is out of range on the low side, so we have acidosis here. So we know that the metabolic system is responsible for the acidosis. And we can also see that the respiratory system is trying to compensate, right? The respiratory system is becoming more basic, more alkaline, to try to compensate for this metabolic acidosis, but it has not been able to fully do the job because our pH is still out of range. So in this case, we have partially compensated metabolic acidosis-- so partially compensated metabolic acidosis. So the metabolic system is responsible for the acidosis. The respiratory system is trying to compensate by becoming more basic, but it has not fully done the job because our pH is still out of range. Okay. We'll pick it up with more problems soon!


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