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As we start our Arterial Blood Gas course, I want to start by helping you understand what is included in an ABG and why we use ABGs in our patients. Sometimes ABGs can be very intimidating because there are a lot of values to look at, but hopefully this will help to kind of demystify it for you.
First, and probably the most obvious is that an Arterial Blood Gas is obtained from an artery! That could be a direct stick into the radial, brachial, or even femoral artery, or your patient might actually have an indwelling arterial line that we’re monitoring. Just make sure you know your state and facility policies. For example, in Texas, nurses can obtain arterial samples from direct sticks in the radial artery, but in South Carolina we aren’t allowed – the respiratory therapist has to do it. And usually only providers can do femoral sticks. So just make sure you know. Now, we want to get arterial blood specifically because it is going to give us completely different results than venous blood. Think about it, your blood flows out of your heart and through the lungs to be oxygenated, then gets pumped out to the body in the arteries. So the blood in the arteries is fully oxygenated – it will tell you the full oxygenation capacity of the patient. If we get venous blood, a lot of the oxygen has already been dropped off in the tissues, right? We also get to see our acid-base balance – again in the full capacity of the patient. So what we’re getting are systemic results. It completely eliminates what I call “tissue factors”. In other words, it looks at the patient’s ability to oxygenate, their acid-base balance, BEFORE the tissues get involved. There are so many things we would look at in venous blood that tell us how well the tissues are using the oxygen, etc. – but that’s not what we’re aiming for with a blood gas. It’s like running a diagnostic on a car – we’re not trying to find how much damage has been done, we’re trying to figure out what’s not working or what could go wrong before it actually causes damage. Make sense?
So let’s just review the possible things that can be found on an Arterial Blood Gas. First we have pH, this specifically stands for “Partial Pressure of Hydrogen” and it’s an indicator of the acidity or alkalinity of the blood. Before I move on, I want to clarify what “partial pressure” means – With any other blood level, we think ‘concentration’ right? Well partial pressure is the same thing, except it’s referring to the concentration of a GAS that is dissolved in the blood plasma. Other things are particles like electrolytes, etc., so they’re measured differently. Things like hydrogen, carbon dioxide, and oxygen are gases – so they are measured using ‘partial pressure’. So our normal pH in the blood is 7.35 to 7.45. A higher hydrogen concentration actually correlates to a lower pH number, so anything less than 7.35 is considered acidosis (too much Hydrogen ions in the blood), anything greater than 7.45 is considered alkalosis (too few Hydrogen ions in the blood). Then we have pCO2 or partial pressure of carbon dioxide – normal value is 35 to 45. Now, I always write this one backwards because I want you to immediately start making the connection that greater than 45 is acidosis, less than 35 is alkalosis. So, more CO2 means more acid. This is controlled by our lungs – we breathe out carbon dioxide with every breath. Next is HCO3 or “bicarbonate” or “bicarb”. Normal value is 22-26, and this is controlled by our kidneys – they’re responsible for excreting or holding onto bicarb. Less than 22 is considered acidotic, more than 26 is alkalosis. Think “bicarb” “base”. More bicarb, more base – alkalosis. These three values are where we look for our acid-base balance.
We call also look at oxygenation status. For that we use PaO2 and SaO2. PaO2, is the partial pressure of oxygen in our arteries – that’s what the ‘a’ is for. This is literally how much oxygen is dissolved in our arterial blood. Normal is 80 – 100 mmHg. But I want to make one thing SUPER clear here – this normal value is assuming the patient is on Room Air, which is 21% oxygen. We write that FiO2 (fraction of inspired oxygen). Okay, so if the patient has a PaO2 of 90, but they’re on 100% FiO2, that’s NOT normal. The second oxygenation method we have is the SaO2 or the saturation of oxygen in the arterial blood. This refers to how much of the hemoglobin is fully saturated by oxygen. Remember that each hemoglobin molecule can hold up to 4 oxygen molecules. So what percentage of them are totally full? Normal is 95-100%, BUT this isn’t super reliable – we’re gonna talk in more detail about these two things in the Oxygenation lesson later in this course.
There are a few other things we can receive in an ABG – one is lactic acid- normal value is 0.5-1 mmol/L, and an elevated level indicates that there is some kind of anaerobic metabolism going on – which may mean sepsis or perfusion issues. Another is base excess or base deficit – this measures the presence of extra bases (or alkaline substances) in our blood. Normal is from -2 to 2. A positive number indicates MORE base or base excess, a negative number indicates LESS base or a base deficit. And finally, in many cases you can also obtain electrolyte values from an ABG. Though this isn’t the #1 purpose, it can be a much faster way to grab a potassium level than waiting for a chemistry, so it’s good for you to know that it’s possible!
So, to recap – make sure you understand the purpose for an ABG, which is to get some systemic information about acid-base balance and oxygenation and to kind of run a ‘diagnostic’ on our body. Make sure you know what values are included and what they mean – for the extra ones like lactic acid or electrolytes, sometimes you have to specifically ask for those to be run, so make sure you know what you want ahead of time. Understanding these things is going to form the basis for your ability to interpret an ABG. In the next few lessons, we’re going to look deeper at interpreting an ABG and how to understand what’s really going on with your client.
Okay guys, I hope this little introduction to ABG labs was helpful, I’m excited for you to dive in and learn more about how to interpret them and how to care for patients with abnormalities. Make sure you check out all the resources attached to this lesson, as well as the rest of the course. Now, go out and be your best selves today. And, as always, happy nursing!!