ABGs Part 2 – Compensation

In my last post about ABGs, you learned how to do a basic analysis of uncompensated acid/base disorders. If you need a review, check out that post then come back here. I’ll wait.

Now that you’ve got uncompensated gasses figured out, I’ve got some bad news. Most of the time you won’t be handed a nice clear-cut ABG like that and will, instead, be dealing with compensated and partially compensated acid/base disorders…sorry. The good news is, once you’ve got the basics down you will have no time taking it a level deeper to understand compensation.

Let’s look at partial compensation first.

You take report on a patient with the following blood gas: pH= 7.33, CO2 = 55, Bicarb 30, PaO2 = 80. If you do the tic-tac-toe method here, you’ll end up with a diagram that looks like this:

What are you noticing that’s different about this tic-tac-toe diagram from your uncompensated diagrams? If you’re saying, ‘there’s nothing in the NORMAL column’ then you are correct!  In PARTIALLY COMPENSATED acid/base disorders everything is abnormal…don’t worry, we’ll go step-by-step.

1) What is your pH? Is it acidotic, normal or alkalotic? In partial compensation, your pH will always be abnormal. If it were normal then that would be considered fully compensated (more on this later). So, look at your pH and decide if it is acidotic or alkalotic.

2) In this example, it’s acidotic at 7.33 (getting close to normal, but not quite there yet). Now look at your CO2 and Bicarb. Which one is also acidotic? In this example, it’s the CO2 that is also acidotic so this means we are working with a RESPIRATORY ACIDOSIS.

3) Now look at the Bicarb…if it’s normal then you’d have uncompensated acidosis meaning the kidneys are not altering their production of bicarb to try to change the body’s pH. In this case, the kidneys are producing MORE bicarb than usual to help buffer all that acid. So what we have is an abnormal Bicarb that is alkalotic to try and balance out the acid. So we know we are dealing with some degree of compensation.

4) Is it partially compensated or fully compensated? Look again at the pH…if it is abnormal (which it is), then it is only PARTIALLY COMPENSATED. Making sense?

So in this case, we have PARTIALLY COMPENSATED RESPIRATORY ACIDOSIS.

Let’s try another one:

pH: 7.47, CO2 = 31, Bicarb = 18, PaO2 = 78

In this case your pH and CO2 are both alkalotic which tells you we are dealing with a RESPIRATORY ALKALOSIS. When you see that the Bicarb is also abnormal this clues you in to the fact that it is PARTIALLY COMPENSATED.

OK, one more: pH: 7.26, CO2 30, Bicarb 12, PaO2 86

In this case, you start by determining that your pH is acidotic. Next, you notice that between the Bicarb and CO2, one of them is also acidotic and that’s the Bicarb…so you now know you are dealing with a METABOLIC ACIDOSIS.

Is it compensated or uncompensated? Look at the CO2. Since it is also abnormal (and the opposite of the acidosis), you know the CO2 is trying to compensate for all that base, so it is PARTIALLY COMPENSATED METABOLIC ACIDOSIS.

Now, let’s look at fully compensated acid/base disorders. 

The thing to know about FULLY COMPENSATED acid/base disorders is that the pH will be within the normal range of 7.35 to 7.45. We are going to have to now look a little closer at this number to determine “the lean.” Don’t fret…you’re doing great!

pH: 7.37, CO2 28, Bicarb 17, PaO2 60. When you initially plug these numbers into your tic-tac-toe diagram they look like so:

Hmmm….the pH is normal. Do we even have an acid/base problem here? Now is when we need to look a little closer. Take a peak at the pH. It’s 7.37, which is a perfectly acceptable pH. But, the astute nurse knows these other abnormal values aren’t going to be ignored so s/he looks a little closer at the “lean” of the pH. What is the “lean?” Recall that a normal pH is 7.35 – 7.45, with a pH of 7.40 being right in the middle. We are going to look at where our pH lands on either side of that perfect 7.40…it will either be less than or greater than, leaning to acidic or leaning to acidotic. To be more precise, a pH of 7.35 to 7.39 “leans” toward acidosis, while a pH of 7.41 to 7.45 “leans” toward alkalosis.

So, now let’s draw our diagram with the “lean” taken into consideration.

Because our pH is “leaning” toward acidosis, we have a tic-tac-toe of METABOLIC ACIDOSIS. And because the pH is within the range of normal, we have a FULLY COMPENSATED METABOLIC ACIDOSIS. In this case, the body has adjusted CO2 levels to balance things out.

We’ll do one more, then you’ll have this down solid!

pH = 7.36, CO2 = 55, Bicarb 30, PaO2 60. Be sure to take the “lean” into consideration as you draw your diagram!

Looking at your diagram, you see that you have a FULLY COMPENSATED RESPIRATORY ACIDOSIS.

Now, doesn’t that all just make perfect sense?

Be safe out there!

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