Examples of Real-World ABG Interpretation

Let’s look at some common gases and what they would look like in real-world practice. We will start easy and slowly increase the difficulty. If you’re still having trouble understanding ABGs at this point, try working through the first example or two and see if things “click.” For the purposes of these examples, we will comment on the acid-base status only and fill in the first three “blanks” of an ABG interpretation. After you have a good understanding of this concept, you will learn how to comment on the oxygenation status.

As you work through this page, you will need to refer back to the normal ranges of ABG values to figure out your answers. Here are those values again now:

If you can, take a moment to jot these numbers down on a sticky note, or save it in a handy file on your computer so you can easily refer to these ranges when needed.

Level 1: Uncompensated gases

The easiest ABGs to interpret are uncompensated ABGs. In this case, an illness or problem is causing a swing in the pH out of normal range. The body has not had time to try to even out the imbalance yet. Therefore, only one of either the CO₂ or HCO₃ is outside of the normal range. These uncompensated gases are easy to identify because the abnormal pH is caused by one abnormal value (CO₂ OR HCO₃).

Let’s see this ABG type in practice! Remember, ABGs are often written with just the values pH/pCO₂/pO₂/HCO₃. We will be filling out only the acid-base portion of the ABG interpretation so that we can focus on acid-base interpretation, so there are three blanks to fill:

Since we are leaving out the oxygen level for now, the placeholder text “oxygen level” will appear in each of the following examples. Oxygen level will be covered on the next page.

Patient A | 7.31/57/Oxygen level/24

Steps to complete:

1. Start with the pH and determine if it is abnormal or normal (or fully compensated).

In this example, the pH is less than 7.35. This is considered an acid. This ABG is an acidosis:

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

2. Look at the pCO₂ and the HCO₃. How many are abnormal?

In this example, only the pCO₂ is outside of normal ranges. The HCO₃ is normal. There is too much CO₂. It is higher than normal. Only one is abnormal. There is no compensation with the other value starting to change too. This gas is uncompensated.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

3. Determine which change (in CO₂ and HCO₃) is causing the acidosis/alkalosis.

In this case, we know the CO₂ is too high. We have already learned that CO₂ = acid and the more we have, the more acidic the pH will be. This makes sense. We know CO₂ relates to the respiratory component and HCO₃ relates to the metabolic component. This abnormality is of the CO₂, therefore the cause of the acidosis is respiratory.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

If you use the Tug O’ War Analogy: In this case, the rope has been pulled past 7.35 so Team Acid is winning! Why are they winning though? Is it because the CO₂ side added players? Or because the HCO₃ side lost players? In this case, the CO₂ side added players. They are the reason for the change, so it is a respiratory acidosis.

ANSWER: 7.31/57/Oxygen level/24 is an Uncompensated Respiratory Acidosis

Level 2: Partially compensated gases

After an illness or physiologic process causes an acid-base imbalance, the body will start to try to “fix” the imbalance by altering the levels of the opposite value—increasing or decreasing the amounts of the opposite variable to compensate for the change in the other value. The pH will partially correct but has not returned back to normal, so the body’s compensation is only partially done.

The trickiest part of this interpretation is deciding what the problem is—is it respiratory or metabolic? To decide this, identify whether the pH is high (alkalosis) or low (acidosis) and whatever altered value—CO₂ or HCO₃—would drive the pH that way. The other value would be the one to try to pull the pH back to normal.

Let’s see this ABG type in practice! Remember, ABGs are often written with just the values pH/pCO₂/pO₂/HCO₃. We will be filling out only the acid-base portion of the ABG interpretation so that we can focus on acid-base interpretation, so there are three blanks to fill:

Patient B | 7.32/30/oxygen level/18

Steps to complete:

1. Start with the pH and determine if it is abnormal or normal (or fully compensated).

In this example, the pH is less than 7.35, so this is an acidotic pH.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

2. Look at the pCO₂ and the HCO₃. How many are abnormal?

In this example, both pCO₂ and HCO₃ are outside of the normal limits. We still have an abnormal pH, but both values have moved from “normal”—one causing the acidosis, and one trying to fix the acidosis. It is definitely a compensated-type gas. But is it fully or partially compensated? Since the pH is still abnormal, the imbalance is only partially fixed.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

3. Determine which change (in CO₂ and HCO₃) is causing the acidosis/alkalosis.

In this example, we have low CO₂ and low bicarbonate. Which one of these would cause an acidic situation? Remember CO₂ = acid and HCO₃ = base. Low CO₂ would not cause an acidosis but low HCO₃ (not enough base) would! HCO₃ is the metabolic component, so this is a metabolic acidosis.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

If you use the Tug O’ War analogy, in this ABG, one side is winning Tug O’ War. The pH/rope has been pulled lower than 7.35, so Team Acid is winning at this time. Why are they winning—did team CO2 gain players? No, it looks like the CO₂ is lower. This should mean they would be losing, but both teams have changed numbers! The HCO₃ has lost players as well. That would definitely cause Team Acid to be winning. Losing CO₂ is the attempt to try to even out the game of Tug O’ War, but it is not enough to fully bring it back to an even game. Is it respiratory or metabolic? Remember if the pH rope is pulled to the acidosis side, whatever player number changes that would cause Team Acid to win is the primary mechanism and any other player number changes are the body’s response, and if you think about it, that change would cause the opposite reaction.

Level 3: Fully compensated

Abnormalities that cause pH imbalances will, over time, be normalized with normal compensation of the body. We have described early in this chapter how the brain can regulate the CO₂ levels by triggering breathing differently, and it can conserve or eliminate HCO₃ to help even out pH. When an illness causes an issue with the pCO₂ or HCO₃ levels, the body will drive a change for the other value to cause the opposite effect. Is the pCO₂ too high, causing an acidotic gas? The body will conserve HCO₃ to add more base to the body. This will drive the HCO₃ up to help bring the pH back to normal range. You can recognize a fully compensated blood gas by a normal pH, but both the pCO₂ and HCO₃ levels are abnormal.

The trickiest part of a fully compensated gas is determining what the problem originally was that caused the imbalance. Textbook theory states that the clinician would look at what side of the normal range the pH is on. Meaning, if a normal range is 7.35-7.45, the middle is 7.4. anything 7.35-7.39 is acid “ish” and anything 7.41-7.45 is alkaline “ish.” Then, you would look at which abnormal value would cause that imbalance between the pCO₂ level and the HCO₃ level.

Let’s see this ABG type in practice! Remember, ABGs are often written with just the values pH/pCO₂/pO₂/HCO₃. We will be filling out only the acid-base portion of the ABG interpretation so that we can focus on acid-base interpretation, so there are three blanks to fill:

Patient C | 7.37/27/oxygen level/16

Steps to complete:

1. Start with the pH and determine if it is abnormal or normal (or fully compensated).

In this example, the pH is between 7.35-7.45. This is a normal pH. But is it a normal gas? Move on the next step to see if the rest of the values are within normal ranges.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

Nothing selected because we are not sure yet. Remember, the blood gas could be totally normal, unless you see something off in the other values.

2. Look at the pCO₂ and the HCO₃. How many are abnormal?

In this example, both pCO₂ and HCO₃ are outside of the normal limits, but we have a normal pH.  Both CO₂ and HCO₃ values have moved from “normal”. Low HCO₃ would cause acidosis, and low CO₂ would cause alkalosis. It is definitely a compensated-type gas. But is it fully or partially? Since the pH is still normal, it is fully fixed.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)

3. Determine which change (in CO₂ and HCO₃) caused the imbalance in the first place.

In this example, we have low CO₂ and low bicarbonate. First, decide if the pH is more acidic or alkaline. Since the pH is less than 7.4, it is more acidic. This must be a compensated acidosis. Between the CO₂ and HCO₃ levels, an elevation in which one would cause an acidic situation? Remember CO₂ = acid and HCO₃ = base. Low CO₂ would not cause an acidosis, but low HCO₃ (not enough base) would! HCO₃ is the metabolic component so this is a metabolic acidosis.

(un/partially/fully compensated)  (respiratory/metabolic)  (acidosis/alkalosis)