ABG Interpretation, part 1: Introduction, ABG components
by Cathy Parkes June 01, 2021 Updated: January 08, 2022 8 min read
In this article, we'll give you an overview of what an ABG is, how it's measured, what needs to be done before it's drawn, and what the components of an ABG are (pH, PaO₂, PaCO₂, HCO₃, SaO₂) and the expected ranges of each component. Having this foundational knowledge will help you interpret ABGs in practice questions, on nursing school exams, and in the clinical setting!
What is arterial blood gas (ABG)?
An arterial blood gas (ABG) is a test run on an arterial blood sample which can provide information on a patient's acid-base balance and ventilation by measuring the amount of oxygen, carbon dioxide, and bicarbonate present in the blood. These tests are often ordered for critical care patients, but not always, and can also occur in a med-surg/telemetry unit.
How is an ABG measured (obtained)?
An ABG test uses 1mL blood from an artery, which can be obtained in one of two ways: from a functioning arterial line, or through an arterial puncture.
An arterial line is a small catheter inserted into an artery. A patient may already have an arterial line open, and this can be used to draw the ABG. Often, however, a patient will require a puncture for an ABG test.
If a patient does not already have an arterial line, an arterial blood sample can be obtained through an arterial puncture, typically to the radial artery.
The blood supply to the hand comes from two main arteries: the radial artery and the ulnar artery. Usually, if the blood supply from the radial artery is disrupted, the ulnar artery will supply adequate blood flow to the hand.
For some patients, the ulnar artery won't supply adequate blood flow, so alternative puncture sites may be needed. Before a puncture for ABG, a modified Allen is performed to assess the patency of the ulnar artery. The Allen test is modified meaning you are only checking patency of the ulnar artery. A true Allen test checks radial and ulnar arteries
After a 1mL sample is drawn from the artery, prolonged pressure must be applied. If prolonged pressure is not applied, then you might get squirted with blood — the arterial system is high pressure and requires prolonged pressure for at least 5 minutes to stop that bleeding.
If a patient is on anticoagulants, the pressure may need to be held longer than 5 minutes.
Who draws an ABG?
In many hospitals, a respiratory therapist is the one who does the blood draw for an ABG.
What is an Allen test?
An Allen test is a standard test used to assess whether a patient has adequate blood flow through the ulnar artery in the forearm, before the radial artery is used for a puncture.
How is an Allen test performed?
In an Allen test, both the radial artery and ulnar artery in the hand are compressed. The hand will get pale and cool from lack of blood flow. Let go of the ulnar artery, but keep compressing the radial artery, and ensure that blood flow comes back to the hand and it regains its color. This indicates that the ulnar artery by itself can give adequate blood supply to the hand.
After the arterial blood sample for an ABG is drawn, it gets sent to the lab, and the ABG results come back. ABG results contain the following components, and each of these components have a normal range. If you need to study these ranges for your exams, they are covered not only in our ABG flashcards, but in our Lab Values flashcards for nursing students!
pH in an ABG
The pH of an ABG is a measure of how acidic or basic the blood is, which indicates whether acidosis or alkalosis is present. The blood's pH is a measurement of hydrogen ion concentration.
On an ABG, the normal range is between 7.35 and 7.45.
PaO₂ in an ABG
PaO₂ on an ABG is partial pressure of oxygen in the arterial blood, which indicates how well oxygen can move from the lungs to the blood.
On an ABG, the normal range for PaO₂ is between 80 and 100 mmHg. A PaO₂ less than 80 mmHg indicates poor oxygenation in the arterial blood. Possible causes of this can be chronic obstructive pulmonary disorder (COPD), acute respiratory distress, and acute respiratory failure. If you'd like to know more about these conditions, check out our Med-Surg flashcards.
PaCO₂ in an ABG
PaCO₂ on an ABG is the partial pressure of carbon dioxide in the arterial blood.
On an ABG, the normal range for PaCO₂ is between 35 and 45 mmHg. PaCO₂ below 35 mmHg indicates presence of respiratory alkalosis due to hyperventilation OR compensation for metabolic acidosis. PaCO₂ above 45 mmHg indicates presence of respiratory acidosis due to hypoventilation, OR compensation for metabolic alkalosis.
HCO₃ in an ABG
HCO₃, also known as bicarbonate, is an important buffer in the blood, regulated by the kidneys.
On an ABG, the normal range for HCO₃ is between 22 and 26 mEq/L. HCO₃ under 22 mEq/L indicates metabolic acidosis, which can be caused by DKA, kidney failure, or diarrhea, OR compensation for respiratory alkalosis. HCO₃ greater than 26 mEq/L indicates metabolic alkalosis, which can be causes by excess antacids, vomiting, nasogastric suctioning, OR compensation for respiratory acidosis.
NOTE: Some textbooks and instructors use a slightly different range, and if your book or instructor uses a slightly different range, feel free to cross this off on your flashcards and put that specific range in. However, it shouldn't vary too much. And when you're given a bicarbonate level that is out of range on an exam, it will likely be very out of range, so the small differences should not be an issue.
SaO₂ in an ABG
SaO₂ on an ABG is oxygen saturation, which is a measure of the amount of oxygen that is bound to hemoglobin in the arterial blood.
On an ABG, the expected range for SaO₂ is between 95-100%. An SaO₂ level under 95% indicates hypoxemia which may be due to anemia, pneumonia, COPD, asthma, ARDS, pneumothorax, pulmonary embolism, or pulmonary edema.
It's important to note that patients with COPD are expected to have SaO₂ levels in the low 90s.
Which components are most important when interpreting an ABG result?
When we are interpreting an ABG result, the key components to pay attention to are the pH, PaCO₂, and HCO₃.
Hi. I'm Cathy, and in this video series, I will be covering arterial blood gas interpretation, or ABG interpretation.
I'll be covering a number of topics, including the ABG components and what the normal expected ranges of each component are.
We'll talk about buffers in the body that help maintain acid-base balance.
We'll go through the steps on how to interpret an ABG result, and we will also talk about the causes, the symptoms, and treatment for different acid-base imbalances.
And then finally, I'll go through a number of practice problems to really get you comfortable with interpreting ABG results.
So as I go through this video series, I will be following along with my cards that are available on leveluprn.com. It's kind of a mini-deck. You don't need to have these cards to find value in this video series. However, it is really helpful to have the information on these cards for reference, whether you're a nursing student or a nurse.
So what is an ABG? An ABG is an arterial blood sample that is used to assess the acid-base balance, the ventilation, and the oxygenation of a patient. These are often critical-care patients, but I also had patients when I worked on the Med-Surg/Tele floor who would get ABG tests done routinely.
So like I said, it's an arterial blood sample. So this is not a venous blood sample. We need to get it from an artery. So we're not pulling it out of a PICC line.
We either need to take that arterial blood sample out of a functioning arterial line - and patients who have arterial lines are typically in the ICU - or we can get it through a puncture.
So at my hospital and at many hospital systems, the respiratory therapist is the one who actually comes and does the blood draw for an ABG.
So they will typically get that blood sample from the radial artery. Before they do the puncture, however, they do what's called an Allen's test. So an Allen's test is done to make sure that we can get adequate blood flow through the ulnar artery while we're messing with that radial artery.
So with the Allen's test, we're compressing both of those arteries. And the hand will get kind of pale and cool. We'll let go of the ulnar artery but keep compressing the radial artery to make sure we get blood flow back to that hand and that that artery can supply that blood flow while we're getting the blood sample out of the radial artery. So that's an Allen's test.
So with ABGs, it's all As, right? We're getting our Arterial blood sample and we're using an Allen's test.
After we get the sample - and we want to get just about one milliliter of blood - we're going to need to hold prolonged pressure over that artery. If we don't, then probably the respiratory therapist or whoever is doing the blood draw is going to get squirted with blood, right, because this is your arterial system. It's your high-pressure system, and we're going to need to stop that bleeding, so we're going to have to hold prolonged pressure for at least five minutes.
If the patient is on anticoagulants, it may be a lot longer, but we want to make sure they're not going to bleed out.
Alright. So you have your blood sample, it gets sent to lab, and then you get your ABG results back, which will have the following components here, which we'll go into more detail now.
So the first thing you'll get is the pH, which is essentially telling you the acid-base balance of the blood, whether acidosis is present or alkalosis is present. It measures the hydrogen ion concentration in the blood, and we would expect a value somewhere between 7.35 and 7.45.
Then we have PaO2, which is the partial pressure of oxygen in the arterial blood that should be somewhere between 80 and 100 mmHg.
Then we have the partial pressure of carbon dioxide in the arterial blood, which is PaCO2. We would expect to have a value for PaCO2 somewhere within the range of 35 to 45 mmHg.
Then we have bicarb which is HCO3. So bicarbonate, it's an important buffer in the blood. It's regulated by the kidneys. And we would expect a value somewhere in the range between 22 and 26.
Some textbooks and instructors use a slightly different range, and if your book or instructor uses a slightly different range, feel free to cross this off on your card and put that specific range in. However, it shouldn't vary too much. And when you're given a bicarbonate level that is out of range, it will likely be very out of range, so it shouldn't trip you up too much.
Then we have SaO2 which is oxygen saturation. It basically measures the amount of oxygen that is bound to hemoglobin in the arterial blood. And we would expect that to be somewhere between 95 and 100%.
So when we are doing ABG interpretation, the key things we're going to be looking at are the pH, PaCO2, and HCO3. Those are the three key components that will help us to interpret our ABGs.
So in my next video, we will talk about the different buffers that are in our body that help to maintain acid-base balance, and then we'll get into the details of the different acid-base imbalances such as respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.
So stay tuned!
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