Understand the Arterial Blood Gas “ABG”! Awesome!

Understand the Arterial Blood Gas “ABG”! Awesome!

September 2, 2019 39 By Jose Scott


– [Dr. Joel] Hey guys, this is Dr. Joel with MedImmersion.com. Thanks for watching. This video is going to be awesome. Remember, all of my videos
are very interactive and I give a lot of extra
links to other videos, other playlists, other learning resources and all of those are located in the upper right-hand corner in
the YouTube card system. These cards work on desktop devices, as well as mobile devices. Also, if you want to save
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when you’re logged in, you will automatically skip over these lengthy introductions on my videos and other people’s videos and you won’t have to watch any YouTube advertisements on my videos. So, it’s a win-win for you, okay? Let’s get started. In this video about the
arterial blood gas lab, the ABG, I’m gonna start
with an introduction to the basic concepts about an ABG, and then, I will talk about the components individually and their normal values, followed by how to interpret the numbers or the interpretation of the ABG. Then, I’ll give you some examples and we’ll finish off with some knowledge challenge questions. All right, let’s do it! So, the first thing I
want to point out, guys, is that an ABG is an
Arterial Blood Gas test. So, why is that important? Basically because most of the time in the clinic or the urgent care or ER, your access point to the patient is an IV, an intravenous line, not an arterial line. So, while you could
pull blood from that IV and send it to the lab, the results that you got back would be
a VBG, not an ABG, okay? And it’s for that reason
that most often times, the ABG is used in critical care settings, like an ICU or somewhere like that where a team has come in
and gotten arterial access, which is more difficult
than venous access. In less critical
settings, like the clinic, the pulse oximetry is
used more frequently, just because it’s less invasive by far, it’s faster, and definitely cheaper. An ABG is important because it gives us an idea about what’s
dissolved in the blood and also, for acid-base homeostasis. This is important because our bodies are extremely sensitive
to swings in the pH. One of the most important things that could happen is
denaturing of proteins. Now, that doesn’t mean that
they’re gonna completely unfold. It just means that they’re not going to function like they should and you’re just gonna feel miserable. It’s for this reason that our pH in our bodies is highly, highly regulated and that’s done by two main systems, the respiratory system
and the renal system, which I cover in this lecture. If you want to learn a little bit more about buffering agents, check out one of the links
in the YouTube cards. The first thing that I want to talk about is the respiratory system and I think it’s pretty easy to understand why the respiratory
system has a big effect on the amounts of gasses
dissolved in my blood and the reason is because hey,
I mean, these are my lungs. They deal with gases all day long, right? But what about the pH? Why do the lungs have an effect on the pH? And the answer to that is
found in carbon dioxide. You see, carbon dioxide,
when dissolved in the blood, is able to transform or
shift to carbonic acid, which is the same thing that
you see in a fizzy soda pop. It’s the fizz, it’s the
dissolved carbon dioxide, which turns into acid
and then can re-express from the liquid in a gas form. So, if I decrease the amount
of carbon dioxide in my blood, then I decrease the
amount of carbonic acid and you can taste that in a soda pop. If a soda pop has been sitting out for a long time and has lost all its fizz, it tastes really too
sweet and the reason is, is because it’s not counterbalanced
by that acidic taste, which kind of neutralizes the sweetness. So, if I were to hyperventilate, (huffing in and out) for a minute or two, I would ventilate off or degas my blood of carbon dioxide and therefore, decrease
the amount of carbonic acid and then therefore, increase the pH or make my blood more alkalotic. The converse is also true. If I stop breathing for a period of time, I can increase the acidity or
decrease the pH of my blood and this is a pretty fast response. If I were to hyperventilate
for a few minutes, you would be able to notice a difference in the pH of my blood. It happens pretty quickly. Now, all of this applies to an ABG with the term, respiratory compensation and what that means is, let’s say that I have a disease process in my body that’s causing my blood to be too acidic. Well, normal physiology,
the respiratory compensation would be for me to breathe faster. This happens automatically. It’s normal physiology. I would breathe faster to degas or ventilate off carbon dioxide and that would decrease
the amount of carbonic acid and bring my pH closer to normal, okay? And again, that’s a fast process. This is normal physiology and it’s called respiratory compensation and we’ll talk a little bit
more about it later on, okay? Let’s move on. The second system that
I want to talk about that’s important for the
ABG is the renal system. The renal system can change the amount of dissolved substances in the blood by how much the nephrons retain or secrete those molecules of interest and in this case, for pH, we’re interested in bicarbonate and hydrogen ions. Bicarbonate is a base and
hydrogen ions are acidic so, if the renal tubular
cells retained bicarb, this is the second bullet point, they held onto bicarb, and they
secreted more hydrogen ions, then the blood would
become more alkalotic. Does that make sense? If we’re holding onto
base and secreting acid, then the blood would
become more alkalotic. And the converse is also true. If we secreted or dumped off all the base into the urine and held onto all the acid, then our blood would become more acidic. Now, opposite to the respiratory system, the renal system is a little bit slower. It takes more time to
change the acidity of urine and to make any meaningful contribution by the production of
different pHs of urine. But, it still makes a huge response and in fact, this is a
more long-lasting response. It’s just a little bit slower
in getting its wheels turning. And this is called the
metabolic compensation. So, you remember how I
just said the previous one was the respiratory compensation? Unfortunately, it can’t be easy. It can’t be renal compensation. We call it metabolic compensation. So, if I have a disease process in my body that is causing my blood to be too acidic, the metabolic compensation,
the normal physiology would be for me to retain bicarb and thus, neutralize the acid or bring the pH closer to normal thus, compensating for
this disease process that was causing the acidity. So, now that I’ve kind of
described the two main systems that have a big effect on the pH and the dissolved substances in the blood, the respiratory system
and the renal system, let me introduce you to how
you start to interpret an ABG and first off, this is the fish bone. This is the shorthand for an ABG and it’s literally just
numbers separated by slashes and you just have to know
what each number means. I’ve given you a, this white
picture here in the middle is how it might appear on your SOAP note. You know, it’s just numbers
separated by slashes. How do you remember
what each number means? That’s what this mnemonic is for. Hot C.O.C.O in BEd. The first H for Hot is for the pH. And then, in C.O.C.O, each of those four letters represents something. So, the first C is for the partial pressure of arterial carbon dioxide. The first O is for partial
pressure of arterial oxygen. And then, bicarb, and then, O2Sat, as in the saturation of hemoglobin. And then, the BE is for
the base excess, okay? Let’s jump into each one of these numbers and talk about what they mean and also, what their normal values are. Starting off with the first value, this is the pH, this is
the direct measurement of the acidity or alkalinity of the blood and the normal values
are between 7.35 and 7.45 and I like the numbers 35 and 45 because you see those
numbers in the normal values of a lot of different laboratory tests. Like, for example, in the BMP, okay? The normal sodium value
is between 135 and 145 or the normal potassium
is between 3.5 and 4.5. So, you can use 35 and 45 over and over again to memorize normal values. And by the way, if you
need to look up a BMP or get a video on that
or the CBC or other labs, go in the upper right-hand
corner and check out the links that I’ve given you in the YouTube cards. Next, the PaCO2, which is the partial pressure of arterial carbon dioxide, the normal value’s between 35 and 45. There’s my favorite numbers again. This is used to assess
the respiratory component of acid/base regulation
and what does that mean? Well, I talked a little bit
about respiratory compensation or the respiratory component
a couple of slides ago. Remember, the carbon dioxide in your blood shifts between carbon
dioxide and carbonic acid. So, when this number goes
up, when the PaCO2 goes up, there is more carbonic acid in your blood. Your blood is more acidic. And when that number goes down, the opposite is also true, okay? So, this is the respiratory component. The PaO2 is the partial
pressure of arterial oxygen and this is an indirect measurement of your patient’s oxygenation status. Now, because this is not
talking about hemoglobin, this is just the amount of oxygen that is dissolved in the serum, the liquid portion of your blood and that is not enough
oxygen to support metabolism. You can’t live on the oxygen that is dissolved in your serum. Hemoglobin, the hemoglobin
and red blood cell system carries way, way more oxygen, tons more. This is just an indirect measurement of how much oxygen is diffusing
with the atmosphere or transferring back and forth, okay? So, this is, it’s kind of
an indirect measurement. I’ll talk a little bit more
about that in a second. The next one is the bicarb. HCO3 is the bicarb. Normal values there between 22 and 26. Remember, this was the renal system or the metabolic component,
the metabolic compensation of acid/base problems in your body. If you are too acidic, your kidneys will automatically retain this bicarb thus, neutralizing or hoping to neutralize the acidic disease process
because bicarb is a base. The next one is oxygen
saturation of hemoglobin and this normal value
is between 95 and 100 and this is actually the oxygen, the main oxygen-carrying
component of your blood. And lastly, base excess,
if it’s positive, right? So, if it’s, because the normal values are between positive two and negative two, so if it’s negative two, then
you have a negative excess, which is more accurate to say a deficit, so it could be a base
excess or a base deficit. And it gives you a more isolated view of the metabolic component
of your acid/base status because bicarb and carbonic acid and CO2 can kind of transform
in between one another, so they are mixed just a little bit. But, this base excess is just purely looking at what component
is metabolic, all right? Let’s step into it a little bit further and start using these values
and putting it all together. Now into interpreting
the ABG and look guys, this is actually a lot
easier than you think. It’s super easy. There’s really only two things
that you need to look at. Two steps. Step one, look at the pH. Is it acidic or is it alkalotic? Let’s say it’s acidic. The pH is too low. Okay, hmm, step two, which one of the two major components that we talked about, the respiratory component
or the metabolic component is responsible for that abnormal pH? I said it was acidic, so what would I expect an acidic PaCO2 to look like? Well, I would expect that
to be a little bit high. If the PaCO2 is high, then it is responsible for the acidic pH and vice versa with the
metabolic component. If the bicarb is too low, well then, I’m gonna suspect that that
is causing this acidic pH. Does that make sense? And then, sometimes
you might see a picture where they’re both abnormal, so then, you just look at which one is worse, okay? And then, you also look at the patient. You know, 99 times out of 100, if you look at the patient
or listen to their history, you’re gonna know what’s happening. So, let’s test these
two steps and practice and I promise you guys, super easy. Let’s do it. Okay, example number one. In this example, in fact,
in all of my examples, I’m not even gonna give you the numbers. Why? Because you don’t need the numbers. You just need to know if it’s high or low. I mean, simplification here. I don’t care. I just want to know is this patient in an alkalotic state or in an acidotic state? And in this example, the pH is high. So, what does that mean? That means this is an alkalosis, right? Next, I look at the respiratory
and the metabolic components and I say, “Well, it looks like
they’re both pointing down, “so which one of those is
causing the alkalosis?” Well, if the carbonic acid is down, that would cause an alkalosis, right? Well, there you go. That means this is a respiratory alkalosis with a metabolic compensation. See how that works? So, there is an issue with the breathing that is causing an alkalotic state and the kidneys have started to compensate by excreting bicarb. Respiratory alkalosis
metabolic compensation. What? Is it really that easy? Yeah, it is! It’s that easy. The other stuff, I mean, who’s gonna pimp you about the PaO2? Who’s gonna pimp you about the hemoglobin? Nobody! They just wanna know can you
read the acid/base disorder. The base excess on the end, that can come into it a little bit later but, I mean, that’s just
gonna kinda follow the pattern that we’ve already talked about. It is that easy. Let’s do some more examples. Okay, using the same steps. Step one, is this alkalotic
or an acidotic state? It is an acidosis or an acidotic state. Which of the two components is causing it? Is it respiratory or metabolic? It is respiratory. Can you see that? Can you see how there’s an increase in CO2 and therefore, an
increase in carbonic acid causing the pH to go down? Respiratory acidosis with
a metabolic compensation. Shoot, now that you’re a pro, I’m just gonna let you
figure this out by yourself. You can pause the video and I’ll give you the answer in just a second. Did you say metabolic alkalosis with a respiratory compensation? If you did, you’re right, good job. And now, the final challenge for the win. Can you do it? This is a metabolic acidosis with a respiratory compensation. Yes, good job, guys. Okay, so now for some real
quick tricks to help you out. Notice how in this
example, all three arrows are pointing in the same direction. Anytime that all three, the pH, the PaCO2, and the bicarb are pointing in the same direction, either up or down, then you’re looking at a metabolic problem or a metabolic component causing
the acidosis or alkalosis. Prove it to yourself. Back up the video and look
at the metabolic alkalosis. All three of the arrows were pointing up. Also, for the respiratory problems, the pH arrow and the other two are going to be pointing
in opposite directions. That will be a respiratory
alkalosis or acidosis. Pretty easy, all right? So, that’s just a little pointer to help you speed up a little bit. All right, guys, congratulations, you made it all the way to the end! This is your first Knowledge
Challenge question. A 45-year-old white female is brought to the emergency department via ambulance where you are working hard as a medical student in one of your rotations. This patient has a weak
pulse and she’s breathing, but she’s not really responsive to you. She’s not perking up. You notice some dried
blood and some track marks and some pitting in her antecubital fossa, which is the front of the elbow where people like to inject drugs. You suspect this is probably
a narcotic overdose. What does the ABG tell you? How about I give you some
arrows to help you out? Again, I’m not going to push memorizing the normal values yet. I just want you to recognize patterns. So, is this an acidosis or an alkalosis? This is an acidosis. What is causing the acidosis? The respiratory component, good. This is a respiratory acidosis because she’s not breathing very much. Her breathing reflex has
been subdued via the drugs. What about the compensation? Well, in this example, I
didn’t put the bicarb above what I consider to be a normal value. I put at the high end, but not above because she hasn’t been
overdosing long enough for her kidneys to compensate. But, you could say that
they’re starting to because that value has risen, okay? Good job. Let’s try the next one. Knowledge Challenge number two. This one’s gonna be a little bit harder because I’m not gonna give you any help. So, this is a 35-year-old black male. He presents to you in the clinic where you’re working as a student. He’s obese, he’s got half of a cigarette behind his right ear and I don’t know, like a
McDonald’s bag in his left hand with it looks like there’s some
fries or something in there. He says he has not felt good all day. Fatigued and nauseous and despite drinking a gallon of orange juice, and he knows he’s getting enough fluid because he’s peeing like a racehorse, he just feels so thirsty and his breath smells like nail polish remover. First of all, what does the ABG tell you? And second of all, what is his diagnosis? So, the ABG tells you that
this is an acidotic state caused by a metabolic process
with respiratory compensation and there’s even some
base excess suggesting again, metabolic issue here. This guy, he could be in
some real trouble here. This looks to me like
diabetic ketoacidosis. He needs some help. He needs some help quick. You need to get him on some fluids and get him some insulin like, now. And if he’s in the clinic,
you need to think about maybe calling an ambulance for
him and getting him to an ER. So, if you picked metabolic
acidosis, you got it. Good job! Hey YouTubers, thanks
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