6 Easy Steps to ABG Interpretation

A proper arterial blood gas (ABG) analysis provides very important information regarding the acid/base balance of our bodies. It is important to understand that the pH in our bodies likes to stay within the very narrow range of 7.35 - 7.45. In the physiologically functioning body, this is achieved by the respiratory system maintaining a carbon dioxide (CO2) level between 35 - 45 mmHg and the metabolic system maintaining a bicarbonate level between 22 - 26 mEq/L.

When a pathophysiological process causes the CO2 or bicarbonate levels in our body to move outside of their normal ranges, the pH is affected and also moves outside of its normal range. The body is not happy with this and wants to return it's pH back to a normal range. The best way to do this is just to fix which component (CO2 or bicarbonate) of the system that has ventured outside the normal range. However, a pathophysiological process may not be able to be fixed instantaneously.

As a result, the next best method to get the pH back into the normal range is to have the component of the other system to have the opposite effect on the body in order to counteract the problem and return the body's pH back to normal. This is known as compensation. As bicarbonate is a base/alkalotic component, the more bicarbonate in the body leads to a more alkalotic pH. As carbon dioxide breaks down into hydrogen ions that is an acidotic component, the more carbon dioxide in the body leads to a more acidotic pH. The opposite is also true in that the less bicarbonate in the body leads to a more acidotic pH, while the less carbon dioxide in the body leads to a more alkalotic pH. Remember, the aim of the compensating system is to do the opposite thing to what the problematic system is doing.

Now for the fun part: practical application! Don't be overwhelmed by the copious amount of information available on the print out of an ABG; the only components that you need to focus on for ABG analysis are the pH, CO2 and bicarbonate...that's it!

Start your ABG Interpretation with these easy steps:

ㆍIs the pH normal?

ㆍIs CO2 normal?

ㆍIs the HCO3 normal?

ㆍMatch the CO2 or the HCO3 with the pH

ㆍDoes the CO2 or the HCO3 go the opposite direction of the pH?

ㆍAre the pO2 and the O2 saturation normal?

In order for our ABG interpretation to be effective, notes will have to be written next to the results on our lab slip.  Alternately, the ABG results can be transcribed onto another paper for analysis.

1. The first step in ABG interpretation is to look at the pH. Normal blood pH is 7.4, plus or minus 0.05, forming the range 7.35 to 7.45. If blood pH falls below 7.35 it is acidic. If blood pH rises above 7.45, it is alkalotic. If it falls into the normal range, label what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic. Label it.

2. The second step is to examine the pCO2. Normal pCO2 levels are 35-45mmHg. Below 35 is alkalotic, above 45 is acidic. Label it.

3. The third step in our ABG interpretation is to look at the HCO3 level. A normal HCO3 level is 22-26 mEq/L. If the HCO3 is below 22, the patient is acidotic. If the HCO3 is above 26, the patient is alkalotic. Label it.

4. Next match either the pCO2 or the HCO3 with the pH to determine the acid-base disorder.  For example, if the pH is acidotic, and the CO2 is acidotic, then the acid-base disturbance is being caused by the respiratory system.  Therefore, we call it respiratory acidosis. However, if the pH is alkalotic and the HCO3 is alkalotic, the acid-base disturbance is being caused by the metabolic (or renal) system. Therefore, it will be a metabolic alkalosis.

5. Fifth, does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system. For example, the pH is acidotic, the CO2 is acidotic, and the HCO3 is alkalotic. The CO2 matches the pH making the primary acid-base disorder respiratory acidosis.  The HCO3 is the opposite of the pH and would be evidence of compensation from the metabolic system.

6. Finally, evaluate the PaO2 and O2 sat. If they are below normal there is evidence of hypoxemia.

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