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 the most accurate representation 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).
What is an 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. The test is used to check the function of the patient’s lungs and how well they are able to move oxygen 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.
How it differs from a 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
- Drug Overdose
- Metabolic Disease
- Chemical Poisoning
How Is An ABG Drawn?
An Arterial Blood Gas requires the nurse to collect a small sample of blood - generally a minimum of 0.5 ml, but a full 1 ml is preferred. Blood can be drawn via an arterial stick from the wrist, groin, or forearm.
The radial artery 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.
There are five key components to an ABG. They include:
- Partial pressure of oxygen (PaO2)
- Partial pressure of carbon dioxide (PaCO2)
- Bicarbonate (HCO3)
- Oxygen saturation (O2 Sat)
Blood gases can also measure hemoglobin and hematocrit as well as electrolyte values such as potassium, calcium, and sodium. If a clinician wishes to obtain these values, a SuperGas is run.
The aforementioned five 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 normal range, above, or below. If a patient’s pH > 7.45, the patient is alkalotic. If the pH < 7.35, then the patient is acidotic.
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 patient is suffering from a respiratory or a metabolic component. This will enable the medical team to treat the patient adequately.
|Respiratory acidosis with metabolic compensation||↓||↑||↑|
|Respiratory alkalosis with metabolic compensation||↑||↓||↓|
The acronym ROME is used to help nurses remember the relationship between pH and CO2.
The CO2 is the respiratory component of the blood gas: if the CO2 is low and the pH is high then the patient would have respiratory alkalosis. These two values move in opposite directions.
On the other hand, the HCO3 is the metabolic component of the blood gas. If the HCO3 is low and pH is low also the patient is in metabolic acidosis. These two values 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.