Arterial Blood Gas, or an ABG, is a blood gas analysis used as a diagnostic tool to assess the acid-base status of the patient and help provide further information in analysing the partial pressures of gas in the blood. Blood is obtained from the arterial circulating system in a specific syringe and is then ran through a machine where analysis occurs.

When caring for a patient who is deteriorating, it is vitally important to treat the signs and symptoms of the patient. Arterial blood gas interpretation is one of many additional diagnostic tools used to identify the problem.

Critical thinking skills when analysing ABG data are imperative. Linking the patient’s presentation with their results is crucial to patient recovery and improving patient outcomes.

When interpreting an ABG reading, it is essential to know what values are considered normal. From this baseline, you can then begin to recognise significant variations in a patient’s results.

pH (Power of Hydrogen)

The first value is the pH, which measures how many hydrogen ions (H+) are in the sample. This determines if the blood is acidotic or alkalotic.

Normal values for pH range from 7.35 - 7.45.

Carbon Dioxide

The next value is carbon dioxide level, and this will tell you if the problem is respiratory in origin. CO2 is the byproduct of cellular metabolism excreted via the lungs. Partial pressure of CO2 ,or PaCO2, in arterial blood is determined by how well the patient is ventilating. The normal range for partial pressure CO2 is 35 to 45 mmHg (Millimetres of mercury).

Bicarbonate

Bicarbonate ions, or HCO3-(negative), will tell you if the problem is related to metabolic changes in your patient and refers to the renal system. The kidneys secrete hydrogen ions and reabsorb bicarbonate when it needs to. Normal range is 22 to 26 mmol/L.

As the blood pH decreases, becoming more acidic the body retains bicarbonate (more base). As the blood pH rises, becoming more alkalotic, the body excretes bicarbonate in the urine. Be mindful that the compensatory mechanisms take hours and even days to occur.

Normal ABG Levels

Here is a table to clearly outline the values.

When the numbers in an ABG result fall outside of these ranges, you can then determine what type of problem the patient is experiencing. Remember to always treat the signs and symptoms of the patient first and escalate care.

Acidotic or Alkalotic

If the ABG results reveal pH numbers are not within the normal range, the patient’s pH level is either acidotic or alkalotic. The lower the number, the more acidotic the patient is. For instance, a pH of 7.30 signifies acidosis and requires further investigation.

Alkalosis is the opposite. The higher the pH, the more base is in the blood sample, which can disrupt the normal functioning of the body.

Once you’ve determined whether there is too much acid or too much base, you can move on to determine the cause.

So, is it a respiratory or metabolic problem?

After you’ve determined whether the sample is acidotic or alkaline, you need to work out if it’s due to a respiratory or metabolic condition.

If the cause is respiratory in nature, the partial pressure of CO2 will be out of the normal range, whereas for metabolic problems the HCO3- will be abnormal.

Low PaCO2 points to respiratory alkalosis, and high HCO3- can indicate metabolic alkalosis.

Arterial blood gas interpretations can be overwhelming to start with. So what does all this mean? How can we simplify it?

Try drawing up a grid that looks like this. Many clinicians find this method visually easy to assist in the analysing process. Write acidosis in the first column, normal in the second column and alkalosis in the third column.

Basic Interpretation

Using the grid, let’s work through an example. The pH level is 7.30, carbon dioxide is 50 and bicarbonate 30.0 pH is out of range, below 7.35, which represents acidosis. The CO2 is out of range, above 45, representing acidosis. Bicarbonate is out of range representing more of a base, placing HCO3- in the alkalosis column.

Always pair up the pH with the value that sits in the same column, in this case it is PaCO2. In this example, the patient is experiencing a respiratory acidosis. And the Bicarbonate has moved out of its normal range, trying to compensate for the acidosis present in the body. Remember, the higher the HCO3-, the more base is present.

Has this compensatory mechanism been successful? Bicarbonate has moved out of its normal range of 22-26 mmol/L, however, it hasn’t done a good job. This is known as a partially compensating respiratory acidosis.

If it was a fully compensating respiratory acidosis, the pH level would be back within the normal range of 7.35-7.45 The goal now is to rectify the pH abnormality so the body is able to function in optimal conditions.

Always remember to treat the signs and symptoms of the patient and escalate care.

ABGs should be thought of as a snapshot of how the body is interacting with its environment at a particular time. They should always be interpreted as part of a wider assessment of a patient’s respiratory function.