Everything ABG interpretation

On this page, you will begin to understand everything you need to know arterial blood gases (ABG) in order to accurately interpret them.

Table of Contents:

• Components of an ABG
• Normal ABG ranges
• Types of acid-base disorders
• Interpretation
• Practice Questions

Components

The components of an ABG that pertain to acid-base balance include the pH, the pCO2, and the HCO3

pH stands for potential of hydrogen and is a representation of the acidity of the blood. The lower the pH, the more acidic the substance. The higher the pH, the more basic the substance. In the human body variations of either extreme are life threatening and can cause significant physiologic disturbances

The PCO2 refers to the partial pressure of carbon dioxide dissolved in the blood. CO2 is regulated by the respiratory system, and can be impacted by pulmonary disease and respiratory rate. Changes in CO2 can occur within minutes depending on respiratory function.

HCO3 refers to bicarbonate, the main base of the body. Bicarbonate is regulated primarily by the kidneys, however, is regulated significantly slower than CO2. Bicarbonate can be excreted or retained by the kidneys depending on the body’s need, and is utilized in order to neutralize acids in the blood such as ketones and lactate. 

The following info is ONLY supplemental and NOT required to know: The balance of pH is accomplished by the following chemical reactions CO2 + H2O 🡨🡪  H2CO3 🡨🡪HCO3 + H. This sequence involves the conversion of carbonic acid, the most common acid found in the blood, into either bicarbonate or carbon dioxide. Conversion into carbon dioxide allows for it to be readily removed by the lungs during exhalation, and conversion into bicarbonate maintains serum levels. 

Normal Ranges:

Normal pH: 7.35-7.45

Normal pCO2: 35-45

Normal HCO3: 22-26

What abnormal ranges mean in ABG interpretation:

This portion often confuses students because the meaning of an abnormal value differs between the two variables.

For CO2, when interpreting the lab value, it is imperative to remember that this is effectively acid in the blood. Therefore, when the CO2 is elevated, > 45, there is too much acid in the blood. As discussed earlier, the lower the pH, the more acidic the sample. So elevated CO2 causes an acidotic state. In contrast, when you have a low pCO2, you have less acid in the body, which causes the pH to increase creating an alkalotic, or basic, state. 

For HCO3, it is important to remember that this is the body’s base or buffer. When the HCO3 is elevated, > 26, that means there are more bases in the blood. This will cause a rise in the pH creating an alkalotic state. When you have a decreased bicarbonate however, < 22, you have less base in your blood to buffer the acid. This causes your blood to have a lower pH as it becomes more acidic.

Types of acid-base disorders:

We categorize acid-base disorders based on the underlying cause of the disturbance. If CO₂ causes the disturbance, we label it respiratory. If HCO₃⁻ causes the disturbance, we label it metabolic. There are four main types of acid-base disorders: Metabolic Alkalosis, Metabolic acidosis, respiratory alkalosis, and respiratory acidosis. 

Metabolic alkalosis refers to an acid-base imbalance in which there is an excess amount of bicarbonate, causing the blood to become more basis and the pH to become elevated. 

Respiratory alkalosis refers to an acid-base imbalance in which there is a decreased amount of CO2, causing the blood to become more basic and the pH to become elevated

Metabolic acidosis refers to an acid-base imbalance in which there is a decreased amount of bicarbonate, causing the blood to become more acidic resulting in a decreased pH.

Respiratory acidosis refers to an acid-base imbalance in which there is an increased amount of CO2, causing the blood to become more acidic resulting in a decreased pH. 

Many people like to use tables to help them to remember these, if that assists you, I recommend using one however you should always understand the reasoning for why. See below for an example (INSERT BELOW)

ABG Interpretation

Before we start doing practice ABG’s, we need to talk about interpretation of the results. There are three main steps for interpretation of an ABG: 

Evaluate the pH

Determine which component is causing the shift in pH

Assess for any compensation.

Evaluation of the pH is simple: Is the pH less than or above the normal range. If the pH is < 7.35, then the condition is an acidosis. If the pH is above the normal range, > 7.45, then the condition is an alkalosis. 

Determining which component is causing the shift in pH requires you to remember what elevations and decreases in both numbers mean. If the pH is < 7.35 and your CO2 is > 45 then the acidosis is caused by CO2 and you have respiratory acidosis. When the pH is < 7.35 and your HCO3 is < 22 then the condition is caused by decreased bicarbonate and you have a metabolic acidosis. If your pH is > 7.45 and your CO2 is < 35, then the condition is caused by a decrease in CO2 and you have respiratory alkalosis. if your pH is above 7.45 and your bicarbonate is above 26, excess bicarbonate causes the condition—this means you have metabolic alkalosis.

Compensation refers to if the opposite component is moving towards the other spectrum of the pH. For example, when CO₂ levels rise in the blood (acidosis), the body compensates by retaining more bicarbonate (alkalosis). An easy way to remember this: if CO₂ and bicarbonate levels both increase or both decrease, the body is compensating. If there is no compensation, then the condition is termed uncompensated. When there is compensation, but the pH remains abnormal, then there is partial compensation. If compensation brings the pH back to normal, the condition is called fully compensated. In the case of fully compensated, we use 7.4 as a marker for which condition was causing the abnormality. If above 7.4, look at which component would cause an alkalosis and vice versa for acidosis.

Test your knowledge with our FREE ABG interpretation quiz: