Arterial Blood Gases (ABGs) Explained

An ABG is one of the most commonly used tests to measure oxygenation and blood acid levels - two important measures of a patient’s clinical status - and correct interpretation can lead to quicker and more accurate changes in the plan of care. It's commonly performed in the ICU and ER setting; however, ABGs can be drawn on any patient on any floor, depending on their diagnosis.

Before learning how to analyze an ABG, it’s important to understand what exactly it is, the different components and their corresponding ABG values, and how it differs from a Venous Blood Gas (VBG).

Arterial blood gases can be confusing to understand at first. However, it's a crucial skill for nurses, physicians, respiratory therapists, and nursing students to learn.

How to Draw an ABG

An Arterial Blood Gas requires the nurse to collect a small sample of blood - generally, a full 1 ml³ is preferred. Blood can be drawn via an arterial stick from the wrist, groin, or above the elbow.

The radial artery on the wrist is most commonly used to obtain the sample. However, the femoral artery and brachial artery can be used if necessary. If the patient already has a pre-existing arterial line, this can be used to obtain the sample. 

Once the blood is obtained, it is either sent to the hospital’s central lab for analysis or tested by the respiratory therapist on the unit’s blood gas analyzer. Most ICUs have one on the unit for a quick turnaround.  

While arterial samples are the best for diagnostic reasons, they do provide some challenges for nurses and providers. The main issue is if the patient does not have a functioning arterial line, a frontline clinician has to draw the arterial sample. Some hospitals allow specially trained nurses or phlebotomists to perform this skill but only after an intense training program. If a provider is not available to perform the arterial stick, treatment could be delayed. 

What is Measured in an ABG?

There are six key components to an ABG. They include,

1. pH - This measures the balance of acids and bases in your blood.
2. Partial pressure of oxygen (PaO2) - This measures the pressure of oxygen dissolved in your blood.
3. Partial pressure of carbon dioxide (PaCO2) - This measures the amount of carbon dioxide in your blood and how well carbon dioxide can move out of your body.
4. Bicarbonate (HCO3) - This is calculated using the measured values of pH and PaCO2 to determine the amount of the basic compound made from carbon dioxide (CO2.)
5. Oxygen saturation (O2 Sat) - This measures how much hemoglobin in your blood is carrying oxygen.
6. Oxygen content (O2CT) - This measures the amount of oxygen in your blood.
7. Hemoglobin - This measures the amount of hemoglobin in your blood. 

ABG Normal Values

The aforementioned components all have different normal values and represent different aspects of the blood gas. According to the National Institute of Health, typical normal values are:

• pH: 7.35-7.45
• Partial pressure of oxygen (PaO2): 75 to 100 mmHg
• Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg
• Bicarbonate (HCO3): 22-26 mEq/L
• Oxygen saturation (O2 Sat): 94-100% 

How to Interpret ABG Results

The first value a nurse should look at is the pH to determine if the patient is in the normal range, above, or below. If a patient’s pH > 7.45, the patient is in alkalosis. If the pH < 7.35, then the patient is acidosis. Remember, the lower the pH number, the higher the acid level in the body. And even with a normal pH level, acidosis or alkalosis may still be present, as the body can compensate to balance the pH. 

Next, examine the PaCO2. This will determine if the changes in the blood gas are due to the respiratory system or metabolically driven. In combination with the HCO3, the nurse will be able to fully comprehend the blood gas.

Below is a chart that contains the different values and determines if the cause is respiratory or metabolic-driven, and if, to what degree, the patient is compensating for the pH. This will enable the medical team to treat the patient adequately. 

* Source: American Thoracic Society

There are many different ways to remember how to analyze arterial blood gas. It’s a nurse’s responsibility to be able to identify key components in order to be prepared for the next step. Asking the help of more senior clinical nurses and respiratory therapists will allow novice nurses to master this skill. 

ABG vs. VBG

A VBG on the other hand, tests the venous blood and can accurately determine pH and CO2 but is unable to provide reliable O2 data. For this reason, arterial testing has become the gold standard in sick patients who are at risk for sudden decompensation or those with a respiratory component. 

Why is an ABG Done?

ABGs are drawn for a variety of reasons. These may include concerns for:

What Happens During an ABG?

If an ABG is ordered by the healthcare team, then blood drawn will be performed by either,

• Medical Physician
• Physician Assistant
• Nurse Practitioner
• Registered Nurse
• Respiratory Therapist

During the blood draw, the healthcare provider will locate the radial artery in either the right or left arm. The radial artery is the preferred artery of choice because it is easier to locate and is superficial.  Then a small needle is inserted into the artery and blood is withdrawn. Approximately 1 ml is required. Once the blood is taken, it is sent to a specialized laboratory to determine the values. 

Alternative sites for drawing an ABG include the brachial or femoral artery; however, these do have disadvantages because they are, 

• Harder to locate
• Poor collateral circulation

What Risks are Associated with an ABG?

The risks associated with drawing an ABG, if done correctly, are minimal. Possible risks include, 

• Arteriospasm
• Hematoma
• Nerve damage
• Fainting
• Vasovagal response
• Sweating
• Pallor
• Drop in blood pressure
• Bleeding at the site

ABG Resources

A wealth of information is available for nurses seeking to deepen their understanding of this important test.  Here are some great resources you might use for further study.

• MediStudents ABG Tutorial
• American Thoracic Society: Interpretation of ABGs

FAQs

• Who performs an ABG?
  • An ABG can be performed by a doctor, nurse practitioner, physician assistant, registered nurse, and/or respiratory therapist. It will depend on the hospital and the specific training of the healthcare provider.  
• What happens before an ABG test?
  • Before an ABG, the patient will be informed about the test and why it is being drawn. All supplies are gathered and the blood is drawn. 
• What happens after an ABG test?
  • After the ABG is sent to the lab for analysis, the results are analyzed by the healthcare team. Changes to respiratory support or fluids are determined as a result of the laboratory findings 
• When do patients get the results?
  • Depending on the healthcare facility and the urgency of the results, they can be delivered in as little as 30 minutes. However, if it is a routine lab draw, the results come back later in the day.
• What does an ABG reveal?
  • An ABG will reveal several things including, pH, partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2), bicarbonate (HCO3), oxygen saturation (O2 Sat), oxygen content (O2CT), and hemoglobin. 
• What are normal ABG levels?
  • pH: 7.35-7.45. Partial pressure of oxygen (PaO2): 75 to 100 mmHg. Partial pressure of carbon dioxide (PaCO2): 35-45 mmHg. Bicarbonate (HCO3): 22-26 mEq/L. Oxygen saturation (O2 Sat): 94-100%
• What is a normal ABG for a COPD patient?
  • Ideally, a patient that has well-controlled COPD should in fact have a normal ABG finding. However, there are many factors involved with this. Age is a very large contributing factor. It is said that a PaO2 of 10.6-11.3 kPa (80-85 mmHg) is most likely normal for a patient with COPD. 

*This website is provided for educational and informational purposes only and does not constitute providing medical advice or professional services. The information provided should not be used for diagnosing or treating a health problem or disease.

Sources

1. National Center for Biotechnology Information

2. University of Michigan Health Library 

3. Journal of Clinical and Diagnostic Research

4. Sydney Local Health District

5. Physiology, Respiratory Alkalosis - Joshua E. Brinkman; Sandeep Sharma

6. Interpretation of Arterial Blood Gases (ABGs) - David A. Kaufman, MD