The Med Book
Home of inspirational, thoughtful, interesting medicine.
Arterial Blood Gas (ABG) Interpretation
- 3 steps to diagnose acid-base balance
Reading time: 5 minutes
Introduction
Arterial blood gas (ABGs) interpretation is often made out to be very complex. You see a senior clincian working out a gas in their head … and voila – they say it’s ‘respiratory acidosis’! You are then left wondering how did that even happen? (see figure 1)
What they are really doing is breaking down the variables that you see on the print out. Broadly speaking there are two inferences that you can draw:
- Oxygenation problems
- Acid-base problems
We will be focusing on the acid-base problems in this blog. For oxygenation problems click here.
Figure 1
Figure 2 shows the components of an ABG printout and the variables that we are interested in are highlighted.
Figure 2
Now comes the secret: All we do is look at the 3 variables in sequence (see Figure 3).
Figure 3
Step 1: Look at the pH
If the pH is <7.35 it is acidosis and if it is > 7.45 it is alkalosis
Step 2: Look at the partial pressure of carbon dioxide (pCO2)
Assuming that the pH was acidic – bearing in mind that carbon dioxide is an acidic gas, if the pCO2 levels are high this will be in keeping with acidosis. Since carbon dioxide is a function of respiration, it is called respiratory acidosis. You can stop here if you wish and breathe! (If you would like to know more, read here).
On the other hand, if the pCO2 is normal or low, it means that this is not causing the acidosis. So we move on to step 3.
Step 3: Look at the bicarbonate (HCO3-)
Now HCO3– is a base as we know. Following on from the previous paragraph we are hoping that the HCO3– is accounting for the acidosis.
Hooray, the HCO3– is low! This will certainly explain the acidosis and we breathe a sigh of relief! We now proudly call it metabolic acidosis and move on to what could be causing this (Read here).
Summary
Interpretation of acid-base balance in an ABG follows 3 easy steps:
- pH → Is it acidosis or alkalosis?
- pCO2 (acid gas) → Is it in keeping with pH? If not, look at HCO3–
- HCO3– (base) → Is it in keeping with pH?
Assuming that we have metabolic alkalosis, what do you think the gas will look like? Take a guess!
What next?
- Learn more on ABG interpretation of Respiratory Failure
- Learn more on ABG interpretation of Respiratory Acidosis
- If you love to learn more on this topic, chapter 5 of “The Hands-on Guide to Clinical Reasoning in Medicine” is a good place to start.
Subscribe to Blog via Email
Join 56 other subscribers
Facebook
Twitter
LinkedIn
medbookprof
I'm a Consultant Pulmonologist having experienced medicine first-hand across two continents stretching from the South Indian peninsula to the British Isles, the emerald nation of Ireland, to the Mediterranean climes of Italy and to the happiest country on earth, Denmark. Along the way, I have served as an honorary Associate Clinical Teacher and Royal College Associate tutor at University Hospital of Wales, collected a Masters in Medical Education and a passion for teaching. My enthusiasm is matched only by my thirst for simplifying clinical knowledge for medical students and making it accessible on a global scale.
2 thoughts on “Arterial Blood Gas (ABG) interpretation- 3 steps to diagnose acid-base balance”
Dear Prof,
Excellent post. Great to hear from you after all this time. Wishing you all the best and thanks for your luminous teaching. 🙏🏽
Thank you! I’m glad you found it useful.