Reading the ABG

Subscribe to Resus

Get the latest updates on our Conferences PLUS our Webcasts and Education Newsletters. And it's FREE!

  • We respect your privacy
  • This field is for validation purposes and should be left unchanged.

Register Now:

Sorry, we couldn't find any posts. Please try a different search.

The ABG is one of the most basic tests we run in the Emergency Department.
It may be an ABG or a VBG. Regardless, it is an opportunity to gain information quickly.
​This module will assist you in learning to read the ABG.
Look for my overall approach to the ABG at the end of the exercise

This is a question from a previous Fellowship Examination.
Without much stress, the diagnosis can be reached.
What is the diagnosis?

(a)  Diabetic Ketoacidosis
(b)  Lactic Acidosis
(c)  Organophosphate Toxicity
(d)  Ethylene Glycol Toxicity
(e)  Renal Tubular Acidosis

Scroll Down…….
The key here is to be able to read the gas and make some assumptions that common sense will support.


​We will work through the above example in a moment.
The answer of course is (d)

Work through the steps to interpreting a gas


If the pH < 7.35 Consider Acidosis
If the pH is > 7.45 Consider Alkalosis

The major disturbance is reflected by the way the HCO3 or CO2 moves. It must move in the same direction as the pH.

In the above example the pH =7.15. So it is an ACIDOSIS
What is the primary disturbance? pCO2 = 20, HCO3 =8. A drop in HCO3 causes acidosis. A drop in pCO2 decreases acid(Co2 is acidic). Therefore the major disturbance is METABOLIC


If metabolic- correct CO2, If Respiratory correct HCO3

Given that this is a METABOLIC ACIDOSIS, we need to use the HCO3 to correct the pCO2.
​The expected pCO2 is 1.5xHCO3+8 = (1.5 x 8) + 8 = 24
​The actual CO2 is 20. That is vey close. We can say that there may be a respiratory alkalosis, however I would say compensation has occurred.

​If this was a RESPIRATORY Driven issue, where the acidosis or alkalosis, was due to the CO2, then we would need to correct the HCO3 using the CO2.

Example of a respiratory cause:

A 65 yo male presents to the emergency department with SOB. His ABG is:
pH 7.16
PCO2 68
HCO3 28
How do we interpret this?

1. It is an ACIDOSIS
The primary disturbance is: Respiratory as the pCO2 is high. It is a RESPIRATORY ACIDOSIS

2 BECAUSE IT IS A RESPIRATORY ACIDOSIS, we need to correct the HCO3.
The COs is 68 ie., 48mmHg more than 40(baseline)
In ACUTE respiratory acidosis the HCO3 would rise by 1 mol/L for every 10mmHg rise above 40. i.e. 4.8 x 1 = 4.8
The baseline of Bicarb we use is 24. You would expect the bicarb to be 28.8. It is 28…close enough


THE ANION GAP is Na-Cl-HCO3 and should be equal to 12. A HIGH Anion Gap by definition must mean that a Metabolic Acidosis is present.

The DELTA RATIO allows us to look for mixed disturbances as well as to pick up Metabolic Alkalosis.
Delta Ratio = [Change in AG/ change in HCO3]     =        AG- 12
What is the Ratio?
​<0.4 = Normal Anion Gap Metabolic Acidosis (NAGMA)
0.4-0.8 = NAGMA + RAGMA(Raised Anion Gap Metabolic Acidosis)
2 = There is a concurrent metabolic alkalosis

The ANION GAP for the case above is: 135-95-8 = 32- There is a raised anion gap.
The causes of a raised anion gap metabolic acidosis are several. We often use the mnemonic CATMUDPILES:
C: Cyanide, carbon monoxide
A: Alcoholic Ketoacidosis
T: Toluene
M: methanol, metformin
U: Uraemia
P: Paraldehyde, Paracetamol
I: Iron, isoniazid
L: Lactic acid
E; Ethylene glycole
S: Salicylates
The DELTA Ratio is 32-12 / 24-8 = 20/16 = 1.25


A-a gradient:       A-a = [(760-vapour pressure) x FiO2 – 1.25 x PaCO2] – PaO2

At FiO2 of 21% ie room air (760-47) x .21 = 150
You can estimate the others
–At 40% ~ 300mmHg
-At 60% ~ 450mmHg
IF ELEVATED A-a GRADIENT, What might be the causes? Think of the Following:
–Other resp disease

A-a = 150-(1.25×20) -115 = 150-25-115=10

Expected is age/4 i.e. 26/4=6. Slightly raised.


Na –correct for gluc Na + (Gluc-5)/3
K – For each 0.1 pH fall below 7.4 the K should rise by 0.5mmol/l above 5
U:C ratio
Osmolality 2Na + U + Glc
Large osmolar gap = methanol, ethylene glycol


Na: is 135
K: the pH is 7.15 i.e. .25 below 7.4. Therefore expect K to be 2.5 x 0.5 above 5mmol/L ie., 6.25. The actual K is 6.5.
U:C ratio- make sure both are in the same units. In this case creatinine is not given


The osmolar gap is useful for trying to identify osmotically active toxins (alcohols and others)
Does not detect small amounts of toxic alcohols
Does not detect late presentations – dangerous active metabolites present but not osmotic.

Calculated Osmolarity =2xNa + urea + glucose [ + EtOh ] Usually 270-290mmol/L normal range.
Osmolar gap = osmolality – calc osm [ n<10] Large gap implies presence of an osmotically active unmeasured compound. Multiply by (molecular weight/10) for the mg/dL Osmolality = 2×135 + 8.1+6.2 = 284.3. The measured osmolality is 320mosm/kg, so an osmolar gap of 36. This is large- one thing causes this: a toxic alcohol.

Interpretation of the Case
This is a raised anion gap metabolic acidosis, with respiratory compensation and a large osmolar gap. The cause is a toxic alcohol

The 5 Possibilities:

The ABG or VBG will have certain abnormalities. At most 3 abnormalities. We must know the potential causes of these.

We have already covered this. The causes are CATMUDPILES

This occurs as the loss of HCO3 is replaced by Chloride, thus the anion gap stays constant, however there is a hyperchloraemic acidosis. I remember the causes as anything that decreases bicarb (HCO3) or increases Chloride:

  • Bicarb loss:
    • GIT Losses as in diarrhoea
    • Renal Tubular Acidosis
    • Drugs that stop the reabsorption of HCO3
      • 85% of HCO3 is reabsorbed in the pro tubule and 15% in the rest of the tubule. Acetazolamide stops proximal tubular reabsorption of HCO3
  • Addition of Chloride
    • As fluid
    • As acid: CaCl, NH4Cl
  • Addison’s Disease: Adrenal insufficiency due to adrenal failure; look for low Na and high K(not always there)

When the bicarbonate rises above 24mmol/l, the kidney rapidly excretes it. If you give IV HCO3, there is a transient rise in HCO3 in the plasma, then the kidney excretes it.
This has been called the ‘waterfall effect’. You fill a bucket with water and no water is lost, however after the bucket is full, all the excess water spills over the side. This is the way the kidneys behave with HCO3.
So metabolic acidosis is due to:

  • increased addition of HCO3
  • Loss of acid- from the kidney or the gut

The maintenance of the disorder is due to one of two major factors:

  1. Chloride depletion- they need chloride to correct
    1. Loss of Gastric acid secondary to vomiting
    2. Diuretics: frusemide and thiazides interfere with reabsorption of Cl and Na
  2. Potassium depletion is due to mineralcorticoid excess. Potassium depletion results in increased bicarbonate reabsorption.

Most cases are due to decreased ventilation.
This may be due to lung damage, nerve or muscle disorder, or depressed conscious state.

This is due to hyperventilation. This can be due to pulmonary causes such as airways disease(asthma) or PE, or central causes, such as stroke, head injury, pregnancy or related to drugs such as salicylate.


Posted in

Dr Peter Kas

Emergency Physician, Educator. Key Interests: Resuscitation, Airway, Emergency Cardiology, Clinical Examination. Creator


  1. wiwztzx on 29/08/2021 at 10:32 pm

    Reading the ABG – Resus

  2. Baby T-Shirt on 02/09/2021 at 9:15 am
  3. Bluecut on 03/09/2021 at 9:58 am
  4. PS on 11/09/2021 at 2:00 pm
  5. Extreme Keto EFX Ingredients on 19/09/2021 at 11:13 am

    Incredible story there. What happened after? Good luck!

    Here is my webpage Extreme Keto EFX Ingredients

  6. on 19/09/2021 at 11:33 am

    What i do not understood is actually how you’re not actually a lot more neatly-liked than you may be right now.
    You are so intelligent. You recognize therefore significantly relating to this subject, produced me
    personally imagine it from so many numerous angles. Its like women and men aren’t interested unless it’s one thing to accomplish with Woman gaga!
    Your own stuffs great. Always deal with it up!

  7. Otto on 19/09/2021 at 11:33 am

    Attractive component to content. I simply stumbled upon your
    blog and in accession capital to assert that I
    acquire in fact enjoyed account your weblog posts.

    Any way I will be subscribing to your augment or even I fulfillment you get entry to persistently quickly.

    my website Erectesto XL Reviews (Otto)

  8. UltraXTend Wifi Extender on 19/09/2021 at 11:44 am

    Good day! Do you use Twitter? I’d like to follow you if that would be ok.
    I’m absolutely enjoying your blog and look forward to new posts.

    Also visit my blog post: UltraXTend Wifi Extender

  9. Terra Xtract CBD Tincture on 19/09/2021 at 12:01 pm

    That is a great tip particularly to those fresh to the blogosphere.
    Brief but very accurate info… Thank you for
    sharing this one. A must read article!

    Take a look at my blog post – Terra Xtract CBD Tincture

  10. Extreme Keto EFX Reviews on 19/09/2021 at 12:05 pm

    I savor, cause I found exactly what I was looking
    for. You have ended my four day lengthy hunt!
    God Bless you man. Have a nice day. Bye

    Feel free to surf to my blog post – Extreme Keto EFX Reviews

  11. Keto-X3 on 19/09/2021 at 12:13 pm

    Spot on with this write-up, I absolutely believe this site needs far more attention.
    I?ll probably be back again to read more, thanks for the information!

    Also visit my homepage; Keto-X3

  12. selengkapnya disini on 19/09/2021 at 12:43 pm

    Ꮐreat goods from you, man. I’ve consider your ѕtuff prior to and you’re just extremely mаgnificent.

    Ι really like what yоu’ve got here, really like what
    yoս’re statіng and the ᴡay during which you aare saying it.
    You make it enjoyable and you continue to care for to stay it sensible.
    I cant wait to learn far morе from you. Thhat is really a tremendous web site.

    Takе a look at my website … selengkapnya disini

  13. TestoXmen Reviews on 19/09/2021 at 12:49 pm

    Hello, Neat post. There is a problem along with your
    site in web explorer, could test this? IE still is the
    marketplace chief and a good part of people will miss your
    wonderful writing because of this problem.

    My blog: TestoXmen Reviews

  14. Pure CBD Softgels Reviews on 19/09/2021 at 12:57 pm

    Thanks a lot for sharing this with all people you really recognise what
    you’re speaking approximately! Bookmarked. Kindly additionally talk over with my web site =).
    We can have a hyperlink change contract between us

    Feel free to surf to my web blog Pure CBD Softgels Reviews

  15. Murray on 19/09/2021 at 1:19 pm

    hey there and thank you for your info ? I have definitely picked up something new from right here.
    I did however expertise several technical issues using this site, as I
    experienced to reload the site lots of times previous
    to I could get it to load correctly. I had been wondering if your web host is OK?
    Not that I’m complaining, but sluggish loading instances times will often affect your placement
    in google and can damage your high quality score if ads and marketing with Adwords.
    Well I am adding this RSS to my e-mail and could look out for a lot more of your respective fascinating content.

    Make sure you update this again very soon..

    Check out my web-site Ultra Quick Keto (Murray)

  16. on 19/09/2021 at 2:10 pm

    Hello There. I found your blog using msn. This is a really
    well written article. I’ll be sure to bookmark it and come back to
    read more of your useful info. Thanks for the post. I’ll certainly

    My webpage Erectesto XL Reviews (

  17. A1 Keto BHB Ingredients on 19/09/2021 at 3:03 pm

    wonderful post, very informative. I wonder why the opposite experts of this sector don’t realize this.

    You must proceed your writing. I am confident, you have a
    huge readers’ base already!

    my homepage: A1 Keto BHB Ingredients

  18. You ought to take part in a contest for one of the best sites on the internet.
    I most certainly will highly recommend this website!

    Feel free to visit my website: Pure CBD Softgels Review (

  19. GrownMD CBD Reviews on 19/09/2021 at 3:39 pm

    I believe this site contains very fantastic composed
    subject material articles.

    Also visit my site; GrownMD CBD Reviews

  20. Bioneo Farms CBD on 19/09/2021 at 5:27 pm

    You should take part in a contest for one of the finest blogs online.
    I most certainly will recommend this web site!

    my web page – Bioneo Farms CBD

  21. As I site possessor I believe the content matter here is rattling
    wonderful , appreciate it for your efforts. You should keep it up forever!
    Good Luck.

    my homepage; Mega Male Enhancement Review (

  22. Devon on 20/09/2021 at 7:52 am

    Hi! I just wanted to ask if you ever have any issues with
    hackers? My last blog (wordpress) was hacked and I ended up losing months of hard work due to no back up.
    Do you have any methods to stop hackers?

  23. Franklyn on 20/09/2021 at 9:50 am

    This design is steller! You certainly know how to keep a reader entertained.
    Between your wit and your videos, I was almost moved to start my own blog (well, almost…HaHa!) Fantastic job.
    I really enjoyed what you had to say, and more than that, how you presented it.
    Too cool!

Leave a Comment