The first test that should come to mind in a deteriorating or critical patient, is an ABG (Arterial Blood Gas). This test will give you an instant view of the patient’s physiology. Despite their popular use, a lot of nurses still struggle to interpret an ABG. So this article will break it down in a few simple explanations.
1. What is an ABG?
An ABG is a type of blood test that is taken from an artery, most commonly with a small syringe and an orange needle. If the patient is in a high dependency unit, the blood sample can be collected directly from an arterial line.
After collection, the blood is taken to an ABG machine. This machine will aspirate a small sample, examine it, and produce a small piece of paper with the results. In most cases, an ABG is taken to check the levels of pH, oxygen and carbon dioxide in the blood.
2. Why should I take an ABG?
Depending on the setting, you can take an ABG can to monitor the patient’s physiology, or to identify the cause of deterioration.
An ABG can tell you if there is:
Lung failure,
Kidney failure
Shock
Trauma
Diabetes
Asthma or COPD
Hemorrhage
Metabolic disease
Chemical poisoning
3. How is an ABG collected?
Taking an ABG is pretty simple, the only challenge is finding the artery. Most commonly, unless the patient has an arterial line, the ABG is taken from the wrist, groin or forearm. Depending on the hospital’s policy, ABGs can be taken either by doctors or nurse.
4. What results does an ABG give?
The ABG machine has different settings, you can either select a “Blood Gas Panel” to get the values of the ph, oxygen saturation and partial pressure , carbon dioxide partial pressure and bicarbonate. Or you can select a “full panel” test which will give you the values mentioned above and the hematocrit, heamoglobin, potassium, calcium, sodium, magnesium, lactate, and glucose.
The arterial blood gas machine
5. What is the normal range?
pH: 7.35-7.45
Partial pressure of oxygen (paO2) 75 to 100mmHg
Partial pressure of carbon dioxide (PaCO2) 35-45mmHg
Bicarbonate (HC03) 22-26mEq/L
Oxygen sat (o2 sat) 94-100%
6. How do I read an ABG result?
So the first thing you should look at is the PaO2, and determine if the patient is hypoxic (aka. Low on oxygen).
Next, look at the pH. If it is high, your patient is alkalotic. If it is low, your patient is acidotic.
Once you determine the pH, you now need to ask yourself what is causing this change? The pH can vary because of respiratory issues or metabolic issues.
If the cause is respiratory, you will notice an abnormal level of CO2If the cause is metabolic, you will notice an abnormal HCO3-
Good. So by now you should have a clear picture if the patient is acidic or alkaline, and if its coming from a respiratory problem or a metabolic problem. The next step is to check if the body is trying to solve the problem by creating another one.
What does this mean? Well sometimes the body will try to regulate the pH disturbance caused by a respiratory problem, by increasing the metabolic work and vice versa.
Now without going into too much detail behind the biochemistry, it’s best if you memorise the following tables so that you can quickly identify what is wrong with your patient.
Results | pH | CO2 | HCO3- |
Respiratory Acidosis | Low | High | Normal |
Respiratory Alkalosis | High | Low | Normal |
Resp. Acidosis with Metabolic Compensation | Low | High | High |
Resp. Alkalosis with Metabolic Compensation | High | Low | Low |
Results | pH | CO2 | HCO3- |
Metabolic Acidosis | Low | Normal | Low |
Metabolic Alkalosis | High | Normal | High |
Metabolic Acidosis with Resp. compensation | Low | Low | Low |
Metabolic Alkalosis with Resp. Compensation | High | High | High |
In Summary:
An ABG is taken to monitor a high dependency patient or identify the cause of deterioration.
It is an arterial blood sample.
It is worked in an ABG machine, and it takes just a few minutes to get the results.
To read an ABG:
Check the oxygen level
Check the pH, low = acidic, high = alkaline
Check the CO2, if abnormal = respiratory cause
Check HCO3-, if abnormal = metabolic cause
Compare the CO2 and HCO3- to look for compensation
And that’s pretty much it. You simply need practice now to start to identify the problems quicker!
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References
Know Your ABGs – Arterial Blood Gases Explained. Nurse.org. https://nurse.org/articles/arterial-blood-gas-test/ Published 2017.
ABG Interpretation | A guide to understanding ABGs | Geeky Medics. Geeky Medics. https://geekymedics.com/abg-interpretation/. Published June 12, 2016. Accessed January 3, 2020.
Puri S, Paul G, Sood P. Interpretation of arterial blood gas. Indian Journal of Critical Care Medicine. 2010;14(2):57-64. doi:10.4103/0972-5229.68215
Singh V, Gupta P, Khatana S. Blood gas analysis for bedside diagnosis. National Journal of Maxillofacial Surgery. 2013;4(2):136. doi:10.4103/0975-5950.127641