Diagnose ANY Arterial Blood Gas in 5 Easy Steps: Get it Right, Every Single Time!

It is important to understand that the pH in our bodies likes to stay within the very narrow range of 7.35 – 7.45. In the physiologically functioning body, this is achieved by the respiratory system maintaining a carbon dioxide (CO2) level between 35 – 45 mmHg and the metabolic system maintaining a bicarbonate level between 22 – 26 mEq/L.

When a pathophysiological process causes the CO2 or bicarbonate levels in our body to move outside of their normal ranges, the pH is affected and also moves outside of it’s normal range. The body is not happy with this and wants to return it’s pH back to a normal range. The best way to do this is just to fix which component (CO2 or bicarbonate) of the system that has ventured outside the normal range. However, a pathophysiological process may not be able to be fixed instantaneously.

As a result, the next best method to get the pH back into the normal range is to have the component of the other system to have the opposite effect on the body in order to counteract the problem and return the body’s pH back to normal. This is known as compensation. As bicarbonate is a base/alkalotic component, the more bicarbonate in the body leads to a more alkalotic pH. As carbon dioxide breaks down into hydrogen ions that is an acidotic component as explained in this post, the more carbon dioxide in the body leads to a more acidotic pH. The opposite is also true in that the less bicarbonate in the body leads to a more acidotic pH, while the less carbon dioxide in the body leads to a more alkalotic pH. Remember, the aim of the compensating system is to do the opposite thing to what the problematic system is doing.

Now for the fun part: practical application! Don’t be overwhelmed by the copious amount of information available on the print out of an arterial blood gas; the only components that you need to focus on to diagnose the arterial blood gas is the pH, CO2 and bicarbonate…that’s it!

ABG InforgraphicIf you follow the 5 steps listed above, you will diagnose your arterial blood gas correctly every single time! Don’t believe that it can be that simple? Why not put to practice what you have just learnt with these various arterial blood gas questions?

pH 7.25, CO2 58, Bicarbonate 25
Click for the answer…
 Uncompensated Respiratory Acidosis
1) pH 7.35 – 7.45, CO2 35 – 45, Bicarbonate 22 – 26
2) pH acidotic, CO2 acidotic, Bicarbonate alkalotic
3) pH and CO2 are matching components therefore respiratory acidosis
4) Bicarbonate within normal range therefore uncompensated
5) N/A as uncompensated
pH 7.45, CO2 50, Bicarbonate 30
Click for the answer…
 Fully Compensated Metabolic Alkalosis
1) pH 7.35 – 7.45, CO2 35 – 45, Bicarbonate 22 – 26
2) pH alkalotic, CO2 acidotic, Bicarbonate alkalotic
3) pH and Bicarbonate are matching components therefore metabolic alkalosis
4) CO2 outside normal range therefore compensated
5) pH is within the normal range therefore fully compensated
pH 7.32, CO2 65, Bicarbonate 35
Click for the answer…
 Partially Compensated Respiratory Acidosis
1) pH 7.35 – 7.45, CO2 35 – 45, Bicarbonate 22 – 26
2) pH acidotic, CO2 acidotic, Bicarbonate alkalotic
3) pH and CO2 are matching components therefore respiratory acidosis
4) Bicarbonate outside normal range therefore compensated
5) pH still not within the normal range therefore partially compensated
pH 7.50, CO2 43, Bicarbonate 32
Click for the answer…
Uncompensated Metabolic Alkalosis
1) pH 7.35 – 7.45, CO2 35 – 45, Bicarbonate 22 – 26
2) pH alkalotic, CO2 acidotic, Bicarbonate alkalotic
3) pH and Bicarbonate are matching components therefore metabolic alkalosis
4) CO2 within normal range therefore uncompensated
5) N/A as uncompensated
pH 7.37, CO2 43, Bicarbonate 25
Click for the answer…
 Normal Arterial Blood Gas
1) pH 7.35 – 7.45, CO2 35 – 45, Bicarbonate 22 – 26
2) All following steps are not required as all values within normal range 

If you want to try your hand at a few more practice arterial blood gases, knock yourself out on this website. Honestly, it is one of the best sites out there to practice arterial blood gas analysis!

References

  • Pilbeam, S. P. (2006). Oxygenation and acid-base evaluation. In S. P. Pilbeam & J. M. Cairo (Eds.), Mechanical ventilation: Physiological and clinical application (4th ed., pp 1-13). Missouri: Mosby Elsevier.
  • Theodore, A. C. (2015). Arterial blood gases. Retrieved from: http://www.uptodate.com/contents/arterial-blood-gases

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10 thoughts on “Diagnose ANY Arterial Blood Gas in 5 Easy Steps: Get it Right, Every Single Time!

  1. Good day! I could have sworn I’ve been to this blog before but
    after reading through some of the post I realized it’s
    new to me. Anyways, I’m definitely delighted I found it and I’ll
    be book-marking and checking back often!

    Like

  2. Thank you so much. I’m a student nurse in Australia and this helps me a lot. Would you be able to discuss about types of mechanical ventilator used in ICU and their indications etc.? Thank you 😊

    Like

  3. Excellent training section- have incorporated in our regular blood gas training. We are always searching for things that add value to our training. We always look forward to your new posts-

    Liked by 1 person

    • Thank you Carol! I’m so glad that you find these posts helpful and it’s a lovely thing to hear that I have people looking forward to reading my posts 🙂 Are there any specific topics that you would like discussed; I’m always thinking of what people want to learn about and suggestions are always welcome!

      Like

  4. QUESTION ASKED ON MY FACEBOOK PAGE: What if you have a respiratory alkalosis that develops on top of a metabolic alkalosis?

    ANSWER: In certain patients, especially the critically ill, you can sometimes have both a respiratory and metabolic system malfunction. For example, a DKA patient that is actively vomiting. Or a lactic acidosis superimposed on severe diarrhoea. These patients do not have the compensatory mechanisms that you would see in most cases, and this is known as a mixed acid base disorder.

    The general rule of thumb is this: whenever the CO2 and bicarbonate are abnormal in opposite directions, a mixed respiratory and metabolic acid-base disorder exists. So if you go back to step 2 in the above article, both the CO2 and bicarbonate will match the pH…whether it is acidotic or alkalotic. This is true even if one of the components are within the normal range but “trending” towards a match with the pH.

    At the end of the day, we want to diagnose an arterial blood gas in order to see what we can fix and how quickly we are going to fix it. If you identify that you have both systems causing the problem, you identify that your patient is going to become acidotic or alkalotic very quickly due to there being no compensatory buffer. The end game is still the same, we aim to fix the problem.

    I will put up a blog post on advanced diagnosis of mixed acid base disorders to explain this in more detail. The method outlined in this post above will cover the majority of your patients and having a good knowledge of compensatory mechanisms and extent of compensation will aid in the diagnosis of more complicated acid base disorders in the future.

    Like

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