ABG values can be very intimidating! It’s hard to remember all the different normal values, what they mean, and which direction they’re supposed to be going. With so much information, it’s super easy to get mixed up and make a stupid mistake on an exam, even when you really DO know how to interpret ABGs. In this article, I’m focusing more on the “How to,” rather than understanding what’s going on with the A&P, which I’ve already done in previous articles. If you want to understand why these steps work (which you should do anyway to become a great nurse!), take some time to review my articles on Respiratory Imbalances and Metabolic Imbalances.
Here’s my 7-step method to interpreting ABGs. We have three puzzle pieces to put together:
A) acidosis or alkalosis
B) uncompensated, partially compensated, or compensated
C) respiratory or metabolic
Let’s get started:
1) Across the top of your page, write down the normal values for the three most important ABG lab results: pH (7.35-7.45), PaCO2 (35-45), and HCO3– (22-26).
2) Underneath pH, draw arrows to remind you which direction is acidic (down), and which direction is basic (down).
3) Underneath PaCO2, and HCO3–, draw arrows to remind you what abnormally high and low values would do to the body’s pH. When you’re done, your page should look something like this:
4) Now you can finally look at the patient’s ABG values. Check the pH and decide if the value is normal, high, or low.
4a) If the pH is normal, check PaCO2, and HCO3–. If they are both normal, then you patient is fine and you can stop here. But if one or both of these values is abnormal, then continue to step 5.
5) Identify if the patient has alkalosis or acidosis.
5a) If the pH is abnormal, then compare it to the arrows you wrote at the top of your paper and pick the imbalance that matches that direction (down is acidosis, up is alkalosis). Continue to step 6.
5b) If the pH is normal but at least one other value was abnormal, then choose acidosis/alkalosis based on which direction the pH is “heading.” For example, 7.35-7.39 is toward acidosis, while 7.41-7.45 is toward alkalosis. Continue to step 6.
6) Identify if the patient is uncompensated, partially compensated, or compensated.
6a) If pH is normal and PaCO2 and HCO3– are both abnormal, then the patient is compensated. Continue to step 7.
6b) If pH is abnormal and PaCO2 and HCO3– are both abnormal, then the patient is partially compensated. Continue to step 7.
6c) If pH is abnormal, and either PaCO2 or HCO3– are abnormal, then the patient is uncompensated. Continue to step 7.
7) Identify if imbalance is Respiratory or Metabolic. Finally time to look at the direction of the PaCO2 and HCO3– values!
7a) If only PaCO2 is abnormal, then it is respiratory.
7b) If only HCO3– is abnormal, then it is metabolic.
7c) If both PaCO2 and HCO3– are abnormal, then you need to refer to your arrows. Only one of these values will cause the pH to go in the direction that you identified (i.e. acidosis or alkalosis). That’s your culprit!
* This step-by-step process is a little bit simplified to reflect the types of ABG questions that nursing students and beginning nurses are most likely to see. There will be some exceptions to these general rules in real life practice, especially when you are dealing with patients facing complex and interacting medical conditions. For example, it is possible that a patient could have abnormal CO2, normal HCO3-, and normal pH if they are just beginning to show the signs of an imbalance. Or a patient could have two independent medical conditions that each cause an independent and “opposite” imbalance. However, these types of advanced situations are unlikely to be tested on because they are beyond the scope of a “safe, effective beginning nurse.” I simply include this note so that you’re aware it can occasionally get MUCH more complicated 🙂