ABG- MADE EASY!!!

Hello!!! Are you ready for the ABG storm, then lets proceed, I am with you in every word, if you have any doubt shoot a question in the comment box, Happy reading.

Normal ABG values are as follows:

1. pH (acid base balance) = 7.35 to 7.45  
2. CO2 (carbon dioxide) = 35 to 45 
3. HcO3 (bicarbonate) = 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

How to interpret ABGs?

All you have to do is memorize four basic questions and then answer them in order:

A. Is the ABG normal? If all of them are, then you have a normal ABG and you can stop here. 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 7.40 the patient is Alkalotic.
2. Acidotic: If the pH is below 7.40 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) that 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 here would be if the pH matches both the CO2 and HCO3, you have combined acidosis or combined alkalosis.  For example, if the pH is acidotic, CO2 is acidotic and HCO3 is acidotic, you have combined acidosis.  If the pH is alkalotic, CO2 is alkalotic, and HCO3 is alkalotic, you have combined alkalosis.  

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, and 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 ranges, and the value that does not match the pH will be outside its normal range, indicating the body is attempting to get the pH back to normal.  For example, if the pH (7.20) and CO2 (50) are acidotic, the HCO3 should be on the alkalotic side (27). 

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, you click here for some more advanced ABG examples.