ABG interpretation made easy: acid base balance

So you made it this far.  Now you must interpret the results.  

Looking for some tips to ease your anxiety over an upcoming test that covers arterial blood gas (ABG) interpretation?  Well, look no further. The goal of this blog is to make your life easy.

ABG interpretation is as easy as remembering four basic questions, and then answering them in sequence.  Of course then you'll have to practice, practice, practice. By the time your test comes up you should be an ABG interpretation expert.

To make things simple, I will only refer to the three basic ABG values in this post

1. Acid Base Balance (pH)
2. Carbon Dioxide (CO2)
3. Bicarbonate (HcO3)

Normal ABG values are as follows:

1. pH  = 7.35 to 7.45  
2. CO2 = 35 to 45 
3. HcO3  = 22 to 26

You also must note the following:

1. CO2 greater than 45 is acidotic
2. HcO3 less than 22 is acidotic
3. Co2 less than 35 is alkalotic
4. HcO3 greater than 26 is alkalotic

To interpret these results, all you have to do is memorize these four basic questions, and then answer them in order.

A. Is the ABG normal?

1. If all the values fall within the normal parameters, then you have a normal ABG and you can stop here: The ABG is normal.  
2. If any one of the values is out of the normal range, then you must move on to the next question.

B. Is the pH Acidotic or Alkalotic? To determine this you look only at the pH.

1. Alkalotic: If the pH is greater than 7.45 the patient is Alkalotic.
2. Acidotic: If the pH is below 7.35 the patient is acidotic.

C. Is the cause respiratory or metabolic? To determine this you look at pH and compare it with HcO3 and CO2. If the pH is acidotic, you look for whichever value (HcO3 or CO2) is also acidotic. If the pH is alkalotic, you look for whichever value (HcO3 or CO2) is also alkalotic.

In this sense, you match the pH with HcO3 and CO2. If the pH matches with the CO2, you have respiratory. If the pH matches with the HcO3, you have metabolic.

Or, put more simply:

1. Metabolic Alkalosis: If the pH is alkatotic and the HcO3 alkalotic.
2. Respiratory Alkalosis: If the pH is alkalotic and the CO2 is alkalotic
3. Metabolic Acidosis: If the pH is acidotic and the HcO3 acidotic.
4. Respiratory Acidisis: If the pH is acidotic and the CO2 is acidotic.

A special case is when the pH, CO2 and HCO3 are all alkalosis or all acidotic.  In this case you have a case of combined alkalosis or combined acidosis.  

1. Combined Alkalosis:  If the pH is alkalotic, CO2 is alkalotic, and HCO3 is alkalotic
2. Combined Acidosis:  If the pH is acidotic, CO2 is acidotic, and HCO3 is acidotic

D. Is the cause compensated or uncompensated?

1. Compensated: pH is anywhere inside the normal ranges (Anything between 7.35 to 7.45)
2. Uncompensated: pH is anywhere outside the normal ranges (greater than 7.45 or less than 7.35).  Also, the value (CO2 or HCO3) that does not match the pH will still be in the normal range.  
3. Partially compensated: pH is anywhere outside the normal range, and the value that does not match the pH (CO2 or HCO3) will be outside its normal range.  This indicates the body is attempting to get the pH back to normal.  Example: A patient is in respiratory failure and his CO2 is 50 (acidotic) and pH is 7.24 (acidotic).  An HCO3 of 27 (alkalotic) means the body is attempting to get the pH back to normal, and this is considered compensation.   

Put A, B, C, and D together and you have your basic ABG interpretation.  That's it.  It's easy.

So, here are some examples:

1. Ph 7.40, CO2 37, HcO3 23

What do you have here? All the number are within normal range, so you have a normal ABG.

That was easy enough. You need to go no further in analyzing this ABG.

2. ph 7.23, CO2 50, HcO3 22

What do you have here?

A. Is the ABG normal? You can see right away that the pH and CO2 are out of the normal range, so you must move on to the next question.

B. Is the pH acidotic or alkalotic? Since the pH is less than 7.40 it is acidotic.

C. Is is metabolic or respiratory? Since the pH is acidotic and the CO2 also acidotic, then you have respiratory acidosis.

D. Is it compensated or uncompensated? Well, the pH is outside the normal range of 7.35 to 7.45, and the HCO3 is still in the normal range, the ABG is uncompensated. You don't have to look at any other values. You are done.

The ABG is uncompensated respiratory acidosis

2. pH 7.36, CO2 50, HcO3 29

A. Is the ABG normal? You can see right away that both CO2 and HcO3 are out of the normal range, so you move on to the next question.

B. Is is acidotic or alkalotic: The pH is less than 7.40, so it is acidotic

C. Is the cause respiratory or metabolic? The pH is acidotic and the CO2 is also acidotic, so you have respiratory acidosis.

D. Is it compensated or uncompensated? Since the pH is within normal limits, it is compensated.

In this example you have compensated respiratory acidosis.

3. pH 7.50, CO2 42, HcO3 33

A. Is the ABG normal? No. Some of the values are outside the normal ranges.

B. Is it acidotic or alkalotic? The pH is greater than 7.40, so it is alkalotic.

C. Is the cause respiratory or metabolic? You know the pH is alkalotic, so you look for the matching value. The HcO3 is alkalotic, so it matches the pH. So, what you have is a metabolic problem.

D. Is it compensated or uncompensated? Since the pH is outside the normal range of 7.35 to 7.45, it is uncompensated.

Thus, you have uncompensated metabolic alkalosis.

4. pH 7.50, CO2 18, HcO3 24

A. Is the ABG normal? No, pH and CO2 are both out of the normal range.

B. Is it acidosis or alkalosis? Since the pH is greater than 7.40 it is alkalosis

C. Is is respiratory or metabolic? Since the pH is alkalotic and the CO2 is also alkalotic, you have a respiratory problem

D. Is is compensated or uncompensated? It is uncompensated because the pH is outside the normal range of 7.35 to 7.45.

What you have here is uncompensated respiratory alkalosis.

5.  pH 7.07, CO2 89.3, HcO3 26

A.  Is the ABG normal?  No, all the numbers are out of the normal range

B.  Is it acidosis or alkalosis?  Since the pH is less than 7.40 it is acidotic

C.  Is it respiratory or metabolic?  Since the pH, CO2, and HCO3 are all acidotic, you have a special case called combined acidosis.

D.  Is is compensated or uncompensated? Since the pH is outside the 7.35 to 7.45 range, and the HCO3 is inside its normal range, the ABG is uncompensated.

What you have here is a case of uncompensated combined acidosis. Now, had the HCO3 in this example been on the alkalotic side of its normal range (say 27) this ABG would have been partially uncompensated.

Once you practice these you will be able to do these automatically in your head in only a few seconds just by looking at the numbers. Now you will want to move on to ABG interpretation made easy part II and, once you have oxygenation mastered, now it's time for some practice.

This post was originally published on 8/11/10 at respiratorytherapycave.blogspot.com  It has been read and approved by respiratory therapists, nurses, professors, and physicians. 

Further reading: 

1. 6 easy steps to ABG analysis
2. The ABG lexicon
3. More RT tips
4. ABG interpretation made easy part 2: interpreting level of oxygenation 
5. Learn about the 4-5-6, 7-8-9 rule
6. Oxyhemoglobin Dissociation Curve
7. How to know if your patient is a CO2 retainer
8. Cord Blood Gases made easy
9. The argument against ABGs
10. What is an ABG
11. Respriatory Therapy Formulas
12. A history of the hypoxic drive hoax
13. RT Cave Facebook Page
14. RT Cave on Twitter
15.