ABG Interpretation for Nursing Students

ABGs are also referred to as arterial blood gases or ABGs.  This is a blood test where the blood is not taken out of a vein as is normally the case but it is instead taken from an artery. It measures the pH or acidity of the blood as well as the amount of carbon dioxide and oxygen in the blood sample.  The test is used to see how well the patient is oxygenating and whether or not there is an acid-base disorder involved in the patient’s illness.  In this article, we are going to talk about ABG Interpretation.

While nurses may be called upon to draw an ABG on a patient, most of the time it is the doctor who does the interpreting.  ABGs can be tricky to interpret but if you understand the basics of physiology, you, too, can interpret ABGs in order to better help your patient be treated for whatever illness they have.

Normal Values of ABGs

Before getting into interpreting ABGs, it is important to understand what a normal ABG looks like.  In a normal ABG, the pH value should be somewhere between 7.35 and 7.45, the paCO2 should be between 35 and 45, and the HCO3 value should be between 22 and 26.  The pa CO2 is the respiratory component of the ABG, while the HCO3 value is the metabolic component of the AGB.

The paCO2 is controlled by the respiratory system and reflects the level of the CO2 gas in the arterial bloodstream.  The HCO3 value is controlled by the kidneys, which hold onto HCO3 or excrete HCO3, depending on the HCO3 level and the pH of the patient’s blood.

Determining if the Blood pH is Acidotic or Alkalotic

After reflecting on the paCO2 and the HCO3 values, the next thing the nurse must determine is whether the blood is acidotic (having a pH of less than 7.35) or alkalotic (having a pH of greater than 7.45.  Sometimes the patient has had abnormalities of the ABGs so long that the breathing and kidneys have compensated for the abnormalities in the arterial blood so that both the paCO2 is off and the HCO3 is off but the pH is normal.  As mentioned, a normal blood pH is between 7.35 and 7.45.

The patient uses the lungs to give off or hang onto CO2 and the kidneys are responsible for holding on to HCO3 or excreting HCO3 because the most important thing in the patient’s blood is that the pH of the blood is as normal as possible.  This is why sometimes the pH of the blood is normal because the patient’s system is actively trying to keep the pH normal.

Determining if the Acid Base Disorder is Metabolic or Respiratory

The next thing you do is to determine whether the patient’s ABGs are abnormal because of a respiratory condition or a metabolic condition.  If the patient has a respiratory problem causing the abnormalities in the ABGs, the paCO2 will be off and, if the patient’s ABGs are abnormal because of a metabolic problem, the HCO3 will be off.

There is a mnemonic you can use to determine whether or not the patient’s blood is from respiratory alkalosis, respiratory acidosis, metabolic acidosis, or metabolic alkalosis.  The mnemonic is called “RO-ME”.  What it boils down to is that the RO stands for “Respiratory Opposite”, while the ME stands for “Metabolic Equal”.

In respiratory causes of abnormal ABGs, the pH will be up when the paCO2 is down.  This is called respiratory alkalosis.  When the pH is down, the pa CO2 will be elevated.  This is called respiratory acidosis.

In metabolic causes of abnormal ABGs, when the pH is up, the HCO3 will be elevated.  This is called metabolic alkalosis.  When the pH is down, the HCO3 will be decreased.  This is called metabolic acidosis.

Making a Tic Tac Toe Chart

The best way to figure out a patient’s ABG status is to make a Tic Tac Toe chart with the top squares saying “Acidic”, “Normal”, and “Alkalotic”.  The rest of the Tic Tac Toe chart is filled in with the values of the HCO3 and the paCO2 levels in the patient’s arterial blood.

It should look like this:

It should then be marked in, filling in the squares of the Tic Tac Toe diagram.

For example, if the pH is 7.26, the paCO2 is 32, and the HCO3 is 18, you fill the Tic Tac Toe chart like this:

Fill in the values you have found on the ABG test into the ABG Tic Tac Toe Chart so that you can determine if the patient’s condition is metabolic acidosis, metabolic alkalosis, respiratory alkalosis, or respiratory acidosis.

Your chart should look like this:

Determining whether or not there is Compensation

The final step is to see if the patient has compensated for their disease by blowing off CO2, holding onto CO2, giving off HCO3 (in the kidneys) or excreting HCO3 (also in the kidneys).

If the pH is normal, then the paCO2 and HCO3 will both be abnormal, and the patient is said to have compensated their abnormality. If the pH is abnormal, the PaCO2 and the HCO3 will both be abnormal, and the patient is said to have partially compensated disease.  If the pH is abnormal and the paCO2 or the HCO3 is abnormal (but not both), the patient is said to have uncompensated disease.  This usually means that the problem is relatively acute and the patient hasn’t yet had time to compensate for their respiratory or metabolic disorder.

The example given above is that of partially compensated metabolic acidosis.  Next, try this one:

• pH is 7.44
• paCO2 is 30
• HCO3 is 21

The pH is slightly high so the patient has some type of alkalosis.  The paCO2 is low so place the paCO2 level under “alkalotic”.  The HCO2 level is low, so place this value under “acidic”. The paCO2 goes along with the pH so the patient is considered to have respiratory alkalosis.  The HCO3 is also not normal.  When the pH is a normal value, and the paCO2 and HCO3 levels are both abnormal, the patient is considered to have fully compensated for their condition.

In the above example, the ABG represents fully compensated respiratory alkalosis.

Now try this one:

• pH of 7.1
• paCO2 of 40
• HCO3 of 18

In this case the pH is low, which represents acidosis.  Place the pH level of 7.1 under the acidotic column in the Tic Tac Toe chart.  The PaCO2 is normal so place this value under the normal column in the Tic Tac Toe chart.  The HCO3 level is low, so place this under the acidic column in the Tic Tac Toe chart.

In this example, the HCO3 goes along with the pH and this is considered to be a metabolic disease, likely metabolic acidosis because the HCO3 level is abnormal.  The paCO2 level is normal in the face of an abnormal pH, so the patient is considered to have uncompensated metabolic acidosis.  Simple, right?

Try a few examples on your own.  Soon you will be figuring out ABGs with relative ease and you won’t look confused when the ABG test result returns from the laboratory.