ABG (Arterial Blood Gas) Interpretation-The Basics

In this lesson we’re going to start talking about interpretation of arterial blood gas results. For this lesson, we’re going to look at the patho behind it and what’s really going on with the patient - the WHY behind the interpretation. And we’re going to give you the three steps you will use every time when you interpret a blood gas. In this lesson we’ll interpret based on the patho and then in the next 2 lessons we’re going to give you two tricks, or quicker methods to interpret without having to think about it for too long!

So let’s get started. Like we talked about in the last lesson, there are three main things we can gather from an arterial blood gas: acid-base balance, oxygenation status, and other issues. Acid-base is what we’re going to look at in these next few lessons. Oxygenation involves looking at the PaO2 and SaO2. We will have a whole lesson in this course on understanding what’s going on oxygenation-wise, so make sure you don’t miss that - it’s still super important! When I talk about other issues I’m referring to the information we can gather from lactic acid, base excess, and base deficit. Each one of those has their own lesson as well, so we’ll explore those more later.

So, there are 3 basic steps to interpretation of an arterial blood gas. These will make more sense as we go along, but I want to present them here so that you can keep them in mind as we go through this. The first step is to look at each of the three main values (pH, PaCO2, and HCO3 or bicarb) and determine if they are acidosis, alkalosis, or normal. The second step is to determine the source, either metabolic or respiratory. And the third step is to determine if there is any compensation. So let’s look at each of these steps in more detail.

The first step is always to evaluate each of the values. So for pH, normal is 7.35 to 7.45, lower being acidosis, higher being alkalosis. For PaCO2, normal is 35 to 45, and like I mentioned before I always write it backwards to help me remember - so above 45 is acidosis and below 35 is alkalosis. And for bicarb, normal is 22 to 26, lower being acidosis and higher being alkalosis - remember bicarb equals base. So when you get your results, the very first step is always to figure out what’s abnormal.

Then, we have to figure out what the source is. When we’re interpreting, there are two main sources - Respiratory and Metabolic. You can have an acidosis or an alkalosis for each one of these. So the 4 possible conditions are respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. For respiratory, we look at the CO2 - if that seems to be the causative factor, then it’s a respiratory issue. When we have a respiratory source, it is because of an excess CO2 or a loss of CO2 because of an altered breathing pattern. We breathe off CO2 with every breath - so, if we are breathing faster, we blow off more CO2, or if we are breathing too slow, we will retain more CO2? If CO2 is high, it’s acidic, low is alkaline. For metabolic source, we are going to look at the bicarb. An excess bicarb indicates alkalosis and low bicarb indicates acidosis. The reason we might see a metabolic source is because of an increase in acids or a loss of acids from a metabolic source. There are a LOT of examples, including lactic acidosis or ketoacidosis, or we could see a loss of acids like in vomiting or even an issue with the kidneys not regulating the bicarb like they should - we’ll talk in more detail about these things in their particular lessons. Just know that if the bicarb seems to be the source, then it is a metabolic issue.

So I want to look at some examples really quickly with just the first two steps - to make sure you know how to do that initial basic interpretation. In this lesson and the next two lessons, you’ll find a bunch of practice examples - they are the same in each lesson, but I want you to use a different method each time and figure out what works best for you. In this case, we’re just going to look physiologically at what’s going on to figure out the issue.

So here’s are first example - and I’ve put the normal values up here for us as a reference. So - step 1 is determine whether each thing is acidosis, alkalosis, or normal. pH 7.52 is high, so it’s alkalosis. PaCO2 of 28 is low, so we know that’s alkalosis. Bicarb of 22, that’s normal - even though it’s on the edge of normal, it’s still normal, okay? Alright, so that’s step 1. Step 2 is determine the source. Well, if we know that our pH is showing alkalosis - and that our PaCO2 is also showing alkalosis - we can reasonably assume that the CO2 is the source, right? So this would be a Respiratory Alkalosis. Make sense? Okay, let’s do another one.

Step 1 is evaluate the values. pH 7.29 is low, so that’s acidosis. PaCO2 of 37, that’s normal. Bicarb of 16, that’s low, so it’s acidosis. Step 2 - what’s the source? We see that the pH is showing acidosis, and the bicarb is also showing acidosis. Since the CO2 is normal, we can reasonably assume the bicarb is the source. So, this is a metabolic acidosis. So, now that we’ve done a couple examples of just the first two steps, let’s look at compensation.

What does compensation even mean? Compensation is when one system is trying to fix the problem caused by another. So if there’s a metabolic issue, then we’ll see the respiratory system trying to compensate. There are three main compensatory systems in our bodies. One of them is our buffer systems that are constantly working in our body to make slight changes with the goal of maintaining homeostasis. The three main buffers we have are carbonic acid, sodium bicarbonate and potassium - let me just give you the basics of these. The big thing to remember is that the level of acid is determined by hydrogen ion concentration. More hydrogen ions, more acid. Less hydrogen ions, less acid - or more alkaline. Carbonic acid will help buffer alkaline situations by breaking off a hydrogen ion to increase the level of acids. Sodium bicarb helps acidic conditions by trading sodium for hydrogen and absorbing those extra hydrogen ions. Potassium has the same charge as hydrogen, so they can switch places between the cells and the bloodstream to increase or decrease the level of hydrogen ions in the bloodstream. This means that excess hydrogen ions in the blood stream will cause more potassium to come out of the cells to trade places - so acidosis (excess hydrogen) can lead to hyperkalemia, and vice versa with alkalosis. These buffer systems work all the time to maintain homeostasis, but you can see how in severe cases it can cause more problems. The other two buffer systems we have are the lungs and the kidneys. The lungs can kick in within minutes if necessary to breathe faster or slower to control the CO2 concentration. Remember CO2 is acid, so if we hang onto it we can decrease the pH and if we blow more of it off we can increase the pH. The problem is that the body can only handle these respiratory rate alterations for so long. As far as the kidneys, it takes them much longer to kick in and help with compensation, but once they do they can help for a long time. The kidneys will excrete or retain bicarb to help regulate the pH. In acidosis, it will retain more bicarb because bicarb is a base. In alkalosis, they will excrete more. Okay, so that’s the physiology behind compensation, let’s look at how we would see compensation on an ABG.

To interpret compensation on an ABG, what we will see is that the CO2 and the bicarb are both abnormal, and they are in opposite conditions. So you may see the CO2 representing acidosis and the bicarb representing alkalosis or vice versa. That is the first indication that there is some compensation going on. If we see that, but the pH is still abnormal, we would say that it is partially compensated. If we see that the pH has shifted all the way to normal, we would say that it is fully compensated. Even with full compensation, there is still a clear source, so we need to determine which one is the more likely source. So, let's look at some examples.

Remember, we still use the steps in order. So let's do the first step now. pH of 7.32 is low, so it’s acidosis. PaCO2 of 55 is high, so it’s acidosis. And, a bicarb of 29 is high so it’s alkalosis. Now, don’t skip steps here! Step 2 is determine the source, right? So we see that the pH and the PaCO2 are both showing acidosis, so we can call this a Respiratory Acidosis. NOW we can move onto step 3 - is there compensation? The first indication of compensation is that the PaCO2 and the bicarb are BOTH abnormal and showing opposite conditions - do we see that here? YES! We see that the bicarb is trying to provide more base to help fix this acidotic state. So we know there’s some compensation. The question now is whether it is partially or fully compensated. Well, our pH is still abnormal, so it hasn’t quite fully compensated yet. SO - we would call this a partially compensated respiratory acidosis. Make sense? Let’s do one more.

Step 1 - pH 7.44 is normal. PaCO2 of 52 is high, so it’s acidosis. Bicarb of 35 is high, so it’s alkalosis. Step 2 - what’s my source? Okay - so this is where you have to use your critical thinking skills. My pH is normal - but both my PaCO2 and my bicarb are abnormal so there is SOMETHING wrong, right? So now the question to ask yourself is which side of normal is my pH on? Well, at 7.44, it’s on the alkalosis side of normal, right? So we can reasonably assume that this was a metabolic alkalosis, because my bicarb is the one showing alkalosis. Now, Step 3 - is there compensation? Yes, we’ve already seen that - both the PaCO2 and bicarb are abnormal in opposite conditions. And, since my pH is normal - what we’re seeing is a fully compensated metabolic alkalosis.

So make sure you understand that CO2 shows us the respiratory source and that high CO2 is acid, and that bicarb shows us the metabolic source and that high bicarb is alkaline. Then we need to understand that compensation means the other system is trying to fix the problem. In the next 2 lessons we’re going to show you how to go through this process much faster with two tricks to ABG interpretation, but you’ll see that we still use the same 3 steps. Evaluate the levels, determine the source, and look for compensation.

I hope that made sense to you - again, this is kind of the long way to interpret ABG’s - by looking at the physiology. Check out the practice fill in the blank attached to this lesson and do the problems using this method before you move on to the trick methods. That will help you really figure out which method works best for you. Now, go out and be your best selves today. And, as always, happy nursing!!