Know Your ABGs - Arterial Blood Gases Explained
Arterial Blood Gases (ABG) can be difficult and confusing to understand at first. However, it's a crucial skill for nurses, physicians, respiratory therapists, and nursing students to learn.
An ABG is one of the most commonly used tests to measure oxygenation and blood acid levels, two important measures¹ of a patient’s clinical status and correct interpretation can lead to quicker and more accurate changes in the plan of care.
Before learning how to analyze an ABG, it’s important to understand what exactly it is, the different components and their corresponding values, and how it differs from a Venous Blood Gas (VBG).
This article has been reviewed by our panel of experienced registered nurses:
- Tyler Faust, MSN, RN
- Chaunie Brusie, BSN, RN
- Kathleen Coduvell Gaines, BSN, RN, BA, CBC
What is an Arterial Blood Gas (ABG)?
An ABG is a blood test that measures the acidity, or pH, and the levels of oxygen (O2) and carbon dioxide (CO2) from an artery.² The test is used to check the function of the patient’s lungs and how well they are able to move oxygen into the blood and remove carbon dioxide. This test is commonly performed in the ICU and ER setting; however, ABGs can be drawn on any patient on any floor depending on their diagnosis.
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.
ABGs are drawn for a variety of reasons. These may include concern for:
- Lung Failure
- Kidney Failure
- Uncontrolled diabetes
- Chronic Obstructive Pulmonary Disease (COPD)
- Drug Overdose
- Metabolic Disease
- Chemical Poisoning
- To check if lung condition treatments are working
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 sample⁴.
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.
Key Components to an ABG
There are six key components to an ABG. They include:
- Partial pressure of oxygen (PaO2)
- Partial pressure of carbon dioxide (PaCO2)
- Bicarbonate (HCO3)
- Oxygen saturation (O2 Sat)
- Oxygen content (O2CT)
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 an ABG
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.⁵ 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.⁶
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 determining 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.
|Compensated respiratory acidosis||↓||↑||↑|
|Compensated respiratory alkalosis||↑||↓||↓|
* Source: American Thoracic Society
Respiratory Opposite -- In respiratory disorders, the pH and CO2 arrows move in opposite directions.
Metabolic Equal -- In metabolic disorders, the PH and CO2 arrows will move in the same direction.
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.
There’s a wealth of information available for nurses seeking to deepen their understanding of this important test. Here are some great resources you might use for further study.