ABG Interpretation, part 3: Steps for Interpretation
by Cathy Parkes June 08, 2021 Updated: July 29, 2021 5 min read
In this article, we'll give you an overview of the expected ranges of the most important components in an ABG (pH, PaCO₂, and HCO₃) and we'll explain the three-step process for interpreting ABG results.
Expected ranges for ABG components
When you are interpreting an ABG result on an exam, the NCLEX, or in your nursing practice, it will be important to know the three key components and the significance of higher or lower values. The three key components in ABG results are pH, PaCO₂, and bicarbonate (HCO₃).
pH on an ABG is a measure of how acidic or basic the blood is, which indicates whether acidosis or alkalosis is present. pH measures hydrogen ion concentration in the blood.
On an ABG, the normal range is between 7.35 and 7.45. pH values under 7.35 indicate acidosis, while pH values over 7.45 indicate alkalosis.
PaCO₂ on an ABG is the partial pressure of carbon dioxide in the arterial blood, and can be used to represent the respiratory system and its function.
On an ABG, the normal range for PaCO₂ is between 35 and 45 mmHg.
PaCO₂ below 35 mmHg indicates presence of respiratory alkalosis due to hyperventilation OR the respiratory system is compensating for metabolic acidosis.
PaCO₂ above 45 mmHg indicates presence of respiratory acidosis due to hypoventilation, OR the respiratory system is compensating for metabolic alkalosis.
HCO₃, also known as bicarbonate, is an important buffer in the blood, regulated by the kidneys. HCO₃ can be used to represent the metabolic system and its function.
On an ABG, the normal range for HCO₃ is between 22 and 26 mEq/L.
HCO₃ under 22 mEq/L indicates metabolic acidosis, which can be caused by DKA, kidney failure, or diarrhea, OR compensation for respiratory alkalosis.
HCO₃ greater than 26 mEq/L indicates metabolic alkalosis, which can be caused by excess antacids, vomiting, nasogastric suctioning, OR compensation for respiratory acidosis.
What if pH, PaCO₂, and HCO₃ are within the expected range?
If, on an exam, you are given ABG results to interpret where the pH, PaCO₂, and HCO₃ are all within the expected ranges, that means the patient has homeostasis.
The chance of encountering an ABG interpretation problem with all three of these values within the normal range is pretty small. But if you do, that would be good news for that patient!
Three-step process for ABG interpretation
When you are interpreting ABG results, whether in the classroom, on an exam, at clinicals, or in your nursing practice, you can use this three-step process.
Step 1 - Is acidosis or alkalosis present?
Look at the pH. pH values under 7.35 indicate acidosis, while pH values over 7.45 indicate alkalosis. This helps us get narrowed down quickly. You can already do this step right away with what you've learned so far in this series.
Step 2 - Is the respiratory system or metabolic system to blame?
If acidosis or alkalosis is present, you now need to determine which system is to blame. Is it the respiratory system or the metabolic system causing this alkalosis or acidosis?
For example, in the case of acidosis, if PaCO₂ is acidic, then the respiratory system is to blame. But if HCO₃ is acidic, the metabolic system is to blame.
You can learn how to determine this in Videos 4-7 of this series.
Step 3 - Is the system compensating?
Once you have determined whether it's the respiratory system or metabolic system behind the acidosis or alkalosis, you then need to determine if the opposite system is compensating.
If the respiratory system is causing acidosis or alkalosis, is the metabolic system trying to compensate?
If the metabolic system is causing acidosis or alkalosis, is the respiratory system trying to compensate?
And then, if compensation is occurring, you must determine if it's partial compensation or full compensation.
You can learn how to detect compensation in Videos 4-7 of this series.
- ABG Interpretation, part 4: Respiratory Acidosis
- ABG Interpretation, part 5: Respiratory Alkalosis
- ABG Interpretation, part 6: Metabolic Acidosis
- ABG Interpretation, part 7: Metabolic Alkalosis
The ABG Interpretation video tutorial series is intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI and NCLEX
As we get into interpreting ABG results, it's going to be really important for you to know the three key components that you need to look at when doing your interpretation and what the significance of higher or lower values means.
So we talked in my previous video about the normal ranges of these different components, but let's now go a step further and talk about what it means when they're high or low.
So with the pH, pH should be between 7.35 and 7.45.
If it is under 7.35, that means we have acidosis.
If it is over 7.45, that means we have alkalosis present.
Then when we look at PaCO2, which really represents the respiratory system, if we find that PaCO2 is below 35, so remember PaCO2 should be between 35 and 45.
If it is below 35, that means we have respiratory alkalosis present, or it could mean that the respiratory system is compensating for metabolic acidosis.
If PaCO2 is over 45, that means we have respiratory acidosis, but it could also mean that the respiratory system is compensating for metabolic alkalosis.
And then finally when we look at bicarb, which is HCO3, it kind of represents the metabolic system, it should be between 22 and 26.
However, if HCO3 is under 22, that means we have metabolic acidosis, or it means the metabolic system is compensating for respiratory alkalosis.
If HCO3 is over 26, this means we have metabolic alkalosis or it also could mean that the metabolic system is compensating for respiratory acidosis. And we're going to go through a lot more examples and break this down even more.
If for some reason you're given a pH that is within normal range as well as the PaCO2 and the HCO3, if they're all within the expected ranges, that means we have homeostasis. The chance of you getting an ABG interpretation problem with all three of these values within the normal ranges is pretty small. But if you do, that's definitely cause for celebration.
So as we are interpreting ABGs, we're going to use a three-step process.
The first step is we're going to look at the pH and see whether acidosis or alkalosis is present.
The second step is if we have acidosis or alkalosis present, who is to blame? Is the respiratory system to blame or is the metabolic system to blame?
And then the third step is to see whether compensation is happening. If the respiratory system is acting out and causing either respiratory acidosis or respiratory alkalosis, is the metabolic system trying to come in and save the day? Are they trying to compensate for that respiratory system acting up?
Or vice-versa. If we have metabolic acidosis or metabolic alkalosis, is the respiratory system trying to compensate and save the day? So that is the third step. Is compensation occurring, and do we have no compensation, partial compensation, or full compensation?
So in the coming videos, we will go through each acid/base imbalance and look at the different values to be able to very easily and accurately tell whether we have acidosis or alkalosis, respiratory or metabolic acidosis or alkalosis, and whether we have full, partial, or no compensation going on.
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