I was chatting with some nursing students recently and asked what their most challenging subjects have been so far…someone piped in and said that ABGs were a real pain in the neck… and no wonder! Her professor had suggested they just try to memorize the values. Sorry, but that’s not going to do squat to help you analyze an ABG at the bedside of your critically ill patient in renal failure with COPD and pneumonia.
So, I thought it would be nice to do a little post about ABG analysis, mainly the tic-tac-toe method. Now I was going to link you to the original article, but it’s only available on EBSCO (or other subscription services) so if you have an EBSCO account through your school I highly recommend the article titled, ‘Turn ABGs into Child’s Play” by Doreen Mays and Eileen O’Connor.
Since this brilliant article was written, lots of folks have jumped on the tic-tac-toe bandwagon. If you google it, you’ll find videos and powerpoint slides galore. I haven’t gone and looked at them all, so I can’t tell you if they’re amazing or not. I do know the original article is amazing, so next time you’re at the library or logged in through your university’s site I highly recommend you read it. It will change your life 😉
Of course, you know I’m not going to leave you hanging…so basically, it boils down to this. You know your normal ABG values, right? If not, then you do need to memorize that part and you need to know if the value is acidic, normal or basic/alkalotic. So, let’s review:
pH: 7.35 – 7.45 (lower than 7.35 is acidic, higher than 7.45 is basic)
CO2: 35-45 (lower than 35 is basic, higher than 45 is acidic)
Bicarb: 22-26 (lower than 22 is acidic, higher than 26 is basic)
The next thing you need to understand is that CO2 has to do with the lungs, and Bicarb has to do with the kidneys…so CO2 is respiratory related and Bicarb is metabolic related. Got it?
Ok, so now I’m going to get artsy…forgive me please! Let’s say you have a patient who has a bad pneumonia who suddenly isn’t looking so spiffy. You are super worried so you get an ABG. Your results show the following:
pH = 7.16; Bicarb = 24; CO2 = 59; O2 = 60.
I can tell you right now that’s a pretty crudtastic ABG. Let’s figure out why it’s such bad news. Ready? The first thing you’re going to do is draw a tic-tac-toe grid, like this:
So, did you remember his crudtastic ABG results? Start with the pH…it’s 7.16. Is that acidic or alkalotic? Think about it for as sec and you’ll remember that a low pH is acidic. Good! Write “pH” under the acidic column like so:
The next value we’re going to look at is the Bicarb, which is 24. Is that acidotic or alkalotic? Think about it! Think carefully. Yes, it’s a trick question! That bicarb value is totally normal, so we’re going to write “Bicarb” (or you could write HCO3 if you wanted to be fancy) in the middle column like this:
Actually, you can look at Bicarb or CO2 after your pH, it doesn’t really matter…but this just happens to be how I set up my art show, so we’re doing it this way. Now let’s look at your CO2. It’s 59. Is this acidic or basic/alkalotic? Hmmm….high CO2 is acidotic, you say? Yes, you’re right! So, let’s write CO2 in the acid column just like this:
And then you step back and admire your tic-tac-toe board because guess what? You’ve got three-in-a-row…you win!
Because your three-in-a-row is in the acidic column, you know you’re dealing with an ACIDOSIS. Now, is it respiratory or is it metabolic? Easy. Respiratory acid/base is determined by the CO2, while metabolic acid/base is determined by the Bicarb. So which one is in your column? The CO2, right? That means, this is a RESPIRATORY ACIDOSIS.
The next question you’re going to ask yourself is, “Self, is this compensated, partially compensated or uncompensated respiratory acidosis?” Well, self, it’s a good thing you’re so smart because that’s an easy one for you, too. To determine compensation, we look at the pH first. Is it abnormal? Yes…so we are definitely not compensated. Now we look at the Bicarb…the only number we have left (except for O2, which we’ll get to in a minute). The Bicarb is normal…so what does this mean? This means the kidneys have not yet had a chance to kick in and start buffering that acid. If they had, the Bicarb would be high. But in this case, it’s totally normal, which means your poor patient has some acidic blood coursing through his little ol’ system and his dumb ol’ kidneys haven’t done a darn thing about it. So in this case we have UNCOMPENSATED RESPIRATORY ACIDOSIS.
Now we’re going to take note of his O2 and his clinical presentation (which is as crudtastic as his ABG). This dude needs to be intubated RIGHT AWAY!!! Get the team in there now (and what supplies do you want to have handy? Hmm? We’ll save that for another post.) Want to do another one? Of course you do!
Let’s say you’ve got a patient who’s on a vent, zonked out on paralytics and breathing at a set rate of 16. He’s suspected of taking an overdose of antacids, thinking they were ecstasy pills. So, he’s not the brightest bulb in the pack…welcome to nursing. So, anyway his ABG results are such:
pH = 7.56; Bicarb = 32; CO2 = 37; O2 = 90
So, first draw your tic-tac-toe grid. I’ll wait. Now, let’s look at our pH. Is it acidotic or alkalotic? It’s alkalotic, so let’s write it in:
Your next value is the Bicarb of 32. Where does that go? Yep! You got it…his Bicarb is high so he’s alkalotic.
Even though you have tic-tac-toe already, you still need to analyze the CO2 to see if we’re compensated or not. So, his CO2 is 37. Where’s that go?
So, what do you think? You have a tic-tac-toe with the Bicarb matching up with the pH…so you have a METABOLIC ALKALOSIS. Is it compensated or not? Have the lungs adjusted CO2 to balance things out? Nope. So it’s UNCOMPENSATED METABOLIC ALKALOSIS. Since he’s on paralytics his body isn’t going to breathe at a rate to naturally compensate for the alkalosis…so we need to make those vent adjustments ourselves and turn his rate down…maybe to around 12 or so. We’ll keep an eye on his CO2 and O2 as well…but most folks tolerate a rate of 12 just fine.
So now go watch a few YouTube videos. Let this sink in and then we’ll tackle compensated and partially compensated acid/base disorders. Then we’ll tackle mixed acid/base disorders. Then you’ll be even more amazing than you are right now.
Be safe out there!