ABG, VBG, and Comparison Explained: Complete Breakdown + Clinical Comparison

Added: 2026-05-21

[00:00:01]All right, everybody. Welcome back to another episode here at WhiteBoard Medicine. We appreciate you checking it out. Hope everybody's having a good day today. Today's episode is one that is critically important, foundational. It is on a test that we do all the time, multiple times every single clinical shift. And that is the arterial blood gas versus the venous blood gas. Which one actually matters in clinical practice? The reality here is that we send blood gases all the time. Does it need to be an arterial blood gas? Could a venous blood gas suffice? When do you really need that ABG? When is that VBG enough? And when can relying on one actually mislead you? In this episode, we're going to break this down. We're going to first dive into arterial blood gases. We're going to talk about all things ABG. We'll then dive into venous blood gases and do the same, all things VBG. And we will end with a segment comparing the two. We're going to talk about how to sample these, what do they mean, what's measured, what's calculated, what is the difference between a VBG and an ABG, and some really key clinical scenarios that might affect this. So, by the end of this episode, we hope you'll have a clear framework to decide ABG or VBG, either both, what is the most appropriate blood test for that patient. And if you want a full study guide, practice questions, medical education, post mini courses, clinical reviews, we are so excited. We have been growing our Patreon community like mad. It's probably becoming one of the biggest collections of emergency critical care medical education content out there. So, we would love, love, love for you to check it out. It's linked in the episode description as well as the pinned comment. Um we are, you know, a bunch of emergency critical care nerds.

[00:01:52]And if you have a similar interest and you want to hop on board, we'd love to have you. So, definitely check that out.

[00:01:57]No further ado though, let's dive in.

[00:02:00]Hey everybody and welcome back to another video. Today, we're going to be doing a video that is more tailored towards, you know, healthcare trainees or practitioners or professionals, rather than the the general public.

[00:02:14]Although, we always encourage everybody who's curious to, you know, watch the videos and check them out. But, we're going to be doing kind of introduction to the arterial blood gas or understanding the ABG. We're going to come out with a series of acid-base videos in terms of understanding acid-base. And this is kind of the start of that.

[00:02:34]So, this will be a quicker video just kind of on the foundational topics on sampling, calculating, you know, what they're good for, what they're used for, all that kind of stuff. So, to start this video, understanding the ABG is critical for all nurses, respiratory therapists, advanced practice providers, docs, and more, right? The ABG is a critical component of the evaluation of many different types of patients and can provide lots of great information.

[00:03:02]What is a blood gas? Well, you're able to do a blood gas on pretty much any place where there is blood. This is typically the veins, venous, the arteries, arterial, or the capillaries, which are between the arteries and the veins. And you can do a sample blood gas, right? You're getting blood and you're checking the gases in that blood on any of these areas. In this video, we're talking about arterial blood gases, but just for kind of further understanding, we drew here kind of an artery in the red going into a capillary in the purple and a vein in the blue.

[00:03:35]And you can sample, you know, an arterial blood gas from the artery, a capillary gas from the capillary, or a venous gas from the vein. So, you know, this would be arterial blood gas, ABG, capillary blood gas, CBG, or venous blood gas, VBG.

[00:03:52]And you do these blood gases to look at things like carbon dioxide or CO2, oxygen or O2, and then the acid-base status or pH. And this can give you a lot of information about how well the patient's breathing and how well their kidneys are working and so on and so forth.

[00:04:06]Just a little intro to how gases work in the blood vessels. This is a red blood cell in the artery carrying oxygenated blood. So, you have oxygen on the red blood cell. You also have oxygen floating around in the blood, which is the partial pressure of oxygen, and carbon dioxide floating around in the blood. It goes through the capillary where oxygen goes out into the tissues.

[00:04:27]Carbon dioxide comes from the tissues into the blood, and then it goes into the vein to be delivered back to the lungs. And there's less oxygen in the red blood cells, less oxygen floating around in the venous blood, and then more carbon dioxide.

[00:04:41]All right. So, sampling. How do we sample? How do we get an ABG?

[00:04:46]Well, you can actually sample any artery theoretically, but most often the arteries you're sampling are the radial artery, which runs in the wrist, or the femoral artery, which runs in the groin.

[00:04:56]Far and away the radial artery is the artery kind of of choice. So, how do you do it? Well, you start with something called the Allen's test. And the Allen's test is to assess for adequate circulatory flow uh not circulatory, collateral flow through the wrist. And what we mean by that is the wrist actually has um and specifically the hand has two different arterial blood sources. All right. One is the radial artery which runs kind of towards and along the thumb. All right.

[00:05:28]And we'll put an R here for radial. The other one is actually the ulnar artery, um which runs near the pinky. And we'll put a U here for ulnar. And both of these perfuse the hand with kind of branches of arteries that come out. So, the Allen's test is essentially you raise the arm above your head, clench the hand in a fist, and press down. You occlude both of these arteries. You press down on both of them. The hand should kind of get to be a pale color. You'll then unclench the fist, and only release the pressure off the ulnar artery while keeping pressure on the radial artery. And what you want is for the hand to pink up again. And that suggests that you're getting enough blood flow through the ulnar artery to keep the hand well perfused um with enough blood. And that means if you were to, you know, injure the radial artery, that the hand would still get enough blood flow. If the hand doesn't pink up, it means that the ulnar artery isn't giving the hand enough blood flow, and then that's very high risk cuz if you were to damage the radial artery during the ABG draw, there would not be enough blood flow to the hand, and you could have ischemia, or lack of blood flow to that hand.

[00:06:37]So, start with the Allen's test to gauge for collateral flow through the ulnar artery. You'll then palpate the radial pulse in the wrist, right around this region. Remember, by the thumb there.

[00:06:47]All right? Once you have a good radial pulse, and we're not going to go through the details of exactly how to do the poke. We can. Let us know in the comments if you want us to make that video. You um poke with a blood gas syringe, which is a certain type of syringe, that artery. And you're always poking in this direction, towards the body.

[00:07:05]All right? You had success.

[00:07:07]You did a good poke. You got the ABG.

[00:07:10]You run that blood sample at a blood gas lab. This separate from the main lab, typically, um cuz you use a blood gas machine, and that it's rapid results, all right? In 5 to 15 minutes. In fact, sometimes you can do something like a hemoglobin or a blood count, or like stat electrolytes off a blood gas. It's not as accurate as if you were to do kind of the serum electrolytes, but it comes back a lot faster, 5 to 15 minutes um in that machine. So, it's a stat rapid blood gas test.

[00:07:36]So, once you get it back, what does it tell you, and how do you interpret it?

[00:07:41]So, there's a few things that are measured, right? That they measure, and there's a few things that are actually derived or calculated from the sample.

[00:07:49]The pH is measured, and that's the acid-base status of the blood. Um and the normal values for that are 7.35 to 7.45. We kind of use 7.4 as a general normal. It also measures the PaO2.

[00:08:04]This is the partial pressure of oxygen in the blood vessel, and that's 75 to 100 mm of mercury. So, the partial pressure, the PaO2, is how much of this oxygen is floating around in the blood.

[00:08:16]It's not the oxygen saturation on the red blood cells. It's how much oxygen is floating around in the blood.

[00:08:22]All right? It also measures the PaCO2, or the partial pressure of carbon dioxide, which is 35 to 45, which we often use 40 as kind of a normal, um millimeters of mercury. And again, that is the partial pressure, so it's the amount of free carbon dioxide floating around in the blood.

[00:08:41]A few things that are calculated. So, these are kind of derived from the ABG blood gas. You know, they're not free measurements, meaning there can be some air. The bicarbonate levels are calculated, and that's 22 to 26. We often use kind of 24 as the generic normal. Base excess or deficit, that's how much extra acid or base is in the blood, and that's -4 to +2.

[00:09:04]And then the SaO2, or the amount of arterial oxygen saturation. That is how much oxygen is actually on the red blood cells themselves. How much oxygen is saturating those red blood cells. All right? And they put the normal in a lot of references as 95 to 100%, although, you know, people with COPD and stuff kind of greater than 88% can be considered normal. And this should correlate with the oxygen saturation that you see on the actual monitor.

[00:09:30]All right? So, these are all calculated values, whereas these are measured values. So, these are the much more important values that we look at, cuz they're directly measured from the blood gas.

[00:09:40]All right. And the last part of this video is going to be an introduction to calculating acid-base status. So, there's a lot to this and we're actually going to come out with an acid-base series of videos identifying compensated versus uncompensated mixed metabolic respiratory combinations of, you know, anion gap metabolic acidosis versus non-anion gap metabolic acidosis.

[00:10:05]Uh but, we're not going to focus on any of these in this. We're just going to do the um introductory kind of simple identifying an acidosis versus alkalosis and knowing whether it's primary metabolic or primary respiratory. And to do this, we drew out just kind of a little hierarchy here. So, first you look at the pH.

[00:10:24]All right. If the pH is between 7.35 and 7.45, um that is a normal pH. You're not acidemic or alkalemic. All right.

[00:10:35]If it is not, you can use kind of the generic 7.4. So, if it is a less than 7.4, you're acidotic.

[00:10:43]And then you need to look at one other thing, either the PCO2 or the bicarb.

[00:10:46]For this, we chose the PCO2.

[00:10:49]So, if your pH is less than 7.40 and your PCO2 is less than 40, that's a primary metabolic acidosis.

[00:10:58]If your pH is less than 7.4 and your PCO2 is greater than 40, that's a primary respiratory acidosis.

[00:11:07]Contrarily, if your pH is greater than 7.4, that's an alkalosis.

[00:11:12]Again, then you look at your PCO2. If your PCO2 is a less than 40, that is a respiratory alkalosis. If your PCO2 is more than 40, that's a metabolic alkalosis. So, just some examples, you know, let's say our pH is 7.20, our PCO2 is 90, and our bicarb again, for the sake of this will be, you know, we'll just say it's 28.

[00:11:39]All right?

[00:11:41]We aren't even looking at bicarb right now, but what we look at would be your pH, right? So, 7.40 p 7.20, it's less than, so it's an acidosis. So, A for acidosis, then look at our pCO2, it's less than 40 or more than 40, it's more than 40, so it is a respiratory acidosis. All right, this would be like a COPD exacerbation.

[00:12:03]Contrarily, let's say, you know, your pH is 7.50 and your pCO2 is 70 and your bicarb in this instance will be I don't choose anything, 19. Again, we're not looking at bicarb to determine these. Um your pH is more than 7.4, it's 7.50. Your pCO2 is more than 40, so it's a primary metabolic alkalosis. All right, so it is an alkalosis and it is metabolic.

[00:12:35]All right, does that make sense? We're going to go into this in much, much more detail in future videos, so subscribe and follow along if you're interested.

[00:12:41]We just wanted to kind of do an introduction to metabolic versus respiratory acidosis versus alkalosis.

[00:12:47]So, for these examples we used the pCO2 as the second marker we looked at after the pH. But, you can replace this with the bicarb if you wanted to. So, we essentially have the same little chart over here and instead of the pCO2 here, we'll write bicarb HCO3 HCO3 HCO3 and a normal for this we'll say is 24.

[00:13:11]Right? So, if you are acidotic, your pH is less than 7.40 and your bicarb is low, bicarb is a base, right? So, if your bicarb is low, that means that your acidosis is metabolic because your base is low, right? Whereas your bicarb is high with an acidosis, that means that it is respiratory. So, it's the opposite of when we looked at the PCO2.

[00:13:41]Same thing here. So, you're alkalotic.

[00:13:42]So, if your bicarb is high, bicarb's a base, so if your pH is higher basic and you're alkalotic, it's going to be metabolic in origin. And contrarily, if you have an alkalosis and your bicarb is low, that would mean that you, you know, are maybe compensating and it's respiratory in origin.

[00:14:04]All right, respiratory.

[00:14:06]So, the things to think about here, whether you look at bicarb, you know, in this area, or whether you look at PCO2, the things to think about are what does this bicarb or PCO2 being low or high mean? So, bicarb being a base, right? We'll say that's a base. So, if the bicarb is low when you have less base and you're acidotic, that's metabolic because your low bicarb is causing that, right? As you can see here. Whereas if your pH is low and you're acidotic, but your bicarb is high, that is conflicting with the pH, right?

[00:14:43]Because if you have more base, more bicarb, you would expect the pH to be high and basic. So, that would mean that what your bicarb is doing is it is compensating for a respiratory acidosis.

[00:14:57]All right? And we don't want to leave you on a cliffhanger, but we're going to talk a lot about compensation coming up in future videos. So, we'll probably leave this this there. All right? But we I guess here, we can do a few more examples, just um these examples over here, but using bicarb rather than um uh PCO2. So, just like we did over there, let's say the pH is 7.20, right? That is less than 7.4, so you have an acidosis.

[00:15:26]And your PCO2, which we're going to ignore in this case, is s- 70.

[00:15:34]And your bicarb is let's call it Actually, let's do this different. The PCO2 is going to be 50.

[00:15:45]And your bicarb is 14. So, if we go down here, pH is 7.40, 7.2 less than acidosis. All right. Bicarb of 24. 14 is less than, so this is a metabolic acidosis because the amount of bicarb, or the amount of base, is low, meaning the low bicarb is causing the acidosis.

[00:16:03]Whereas, if your pH is 7.20, your PCO2 is 70.

[00:16:11]And your bicarb is let's call it um 30.

[00:16:18]You still go down the same chart, right?

[00:16:19]pH 7.40, 7.2 is less than, so it's an acidosis. Your bicarb is 30, so it's more than 24, so it's respiratory because the amount of base is high, but the pH is still low, meaning your bicarb is compensating for primary respiratory acidosis. And again, we'll get into that more. So, you can just follow these charts, but you just need to choose one, bicarb or PCO2, and then you just follow acidosis, alkalosis, high, low.

[00:16:47]All right. And then we get into compensation, maybe we'll go back do the tic-tac-toe diagram or some of those other things. Let us know.

[00:16:53]Um Hey, everybody, and welcome back to another video. Today, we're going to be talking about venous blood gases. So, not arterial blood gases, venous blood gases. And I want to explain what venous blood gases are, how to read them, um what they mean, and how they translate to arterial blood gases. So, a venous blood gas is a sample that is obviously taken from the venous system, the veins.

[00:17:17]Uh you can do this in a few different ways. So, you can get a peripheral sample. You know, you often see this done from the arm.

[00:17:25]And this is, you know, more termed a venipuncture, right? Puncturing the veni, the vein.

[00:17:30]You can also do this from a central sample.

[00:17:37]And this is from like a central venous catheter, a central line.

[00:17:42]And then you actually can also do a mixed venous sample from a pulmonary artery catheter, which we're not going to go into detail on, pulmonary artery catheter.

[00:17:56]So, these are the three main ways to sample the venous blood. And why would you want to do a venous blood gas or a VBG, as they're more commonly referred to?

[00:18:08]Well, honestly, um there's two different reasons. The first and primary reason is cuz we actually can convert the pH, the PCO2, and the bicarb from a VBG to an ABG. So, a VBG can be a surrogate for ABGs in the pH, PCO2, and bicarb. Um ABGs, right, are harder to get. You got to poke the artery. It can be uncomfortable for patients. It can be harder to actually get the arterial blood. Um so, a VBG is a sub and in ABG for these three things, the pH, the PCO2, and the bicarb.

[00:18:50]The other reason is that actually the um SVO2 on a VBG, which we're going to get into, is a a marker, you know, a surrogate used for oxygen delivery and consumption, which we'll touch on, although not go into great detail on in this lecture.

[00:19:12]All right. So, what does a venous blood gas actually measure?

[00:19:19]Let's do blue. So, a venous blood gas measures a few different things.

[00:19:24]All right.

[00:19:26]It measures the PvO2.

[00:19:30]And this is the venous oxygen tension.

[00:19:34]Oxygen tension.

[00:19:38]All right. What is oxygen tension? Well, the oxygen tension is actually the amount of oxygen dissolved in the bloodstream. So, this is the amount of oxygen dissolved floating around in the blood. Not attached to hemoglobin, but the actual dissolved oxygen in the blood.

[00:19:57]All right.

[00:19:58]It can also measure the carbon dioxide tension, or the PvCO2.

[00:20:05]Right? And this is going to be the venous CO2 tension.

[00:20:10]Measures the pH, right? Which is the acidity of the blood.

[00:20:16]Oop, let me mute my computer. It measures the SvO2.

[00:20:22]Which is the oxyhemoglobin saturation.

[00:20:26]Sorry about that. Oxyhemoglobin saturation. So, this is the amount of oxygen actually attached to the hemoglobin, right? Not just the oxygen dissolved in the blood. And then it measures the HCO3.

[00:20:41]Which is the serum bicarb.

[00:20:45]Bicarbonate, bicarb. Oop, there's an R there. Concentration, which I tend to do with brackets for concentration. So, with all this in mind, I think a a question that if we haven't directly answered so far, we we've indirectly touched on is why get a venous blood gas. And the reason is twofold. One is it's because venous blood gases can be converted to be reflective of an arterial blood gas, that arterial blood gas can give you information. So, the pH, the acid-base relationship in the blood of a venous blood gas is pH is similar to an arterial blood gas's pH, and we're going to get into how exactly they're different and how to convert them. Same thing with the carbon dioxide levels. Um if you're worried that someone is retaining carbon dioxide, or you need to verify their carbon dioxide levels for their acid-base status, the venous blood gas carbon dioxide is reflective of the arterial blood gas carbon dioxide. And again, we're going to get into how exactly those translate.

[00:21:37]The second different reason is is that um the SvO2, as we mentioned, can be a surrogate marker for oxygen delivery and consumption, which we will just very briefly touch on at the end, and it can be useful for determining shock states and resuscitation uh for patients in shock. So, there's kind of the two separate reasons. One is because venous blood gas is the pH, PCO2, and bicarb are reflective of an arterial blood gas, and those can tell us things about the acid-base status and whether someone's retaining carbon dioxide or not. And then two is for this oxyhemoglobin saturation in the venous blood, which is a surrogate for O2 delivery and consumption.

[00:22:13]So, what does all this mean, right? For that, I want to kind of draw a picture that is going to be one that, you know, isn't uh to form here.

[00:22:31]So, this is going to be our standing picture for a blood vessel, right? So, we have an artery, which goes into an arteriole, right, which is a smaller caliber, which goes into a capillary, which goes into a venule, which goes into a vein.

[00:22:57]And for our purposes within the blood, right, we have blood cells, right? And these blood cells carry uh our hemoglobin.

[00:23:10]That doesn't look that great, but that's hemoglobin.

[00:23:17]And our hemoglobin is going to be saturated with oxygen in the arteries, right?

[00:23:25]And then floating around in here, we actually have oxygen tension, right? These dissolved O2 molecules. We also have carbon dioxide tension, which will be green.

[00:23:37]These dissolved CO2 molecules.

[00:23:39]And all of this is flowing, right? Towards the capillary.

[00:23:44]And the capillary has tissue, right?

[00:23:49]Outside of it.

[00:23:50]And as these red blood cells go through the capillary and as this dissolved oxygen and the dissolved carbon dioxide go through, things happen, right? So, you get oxygen that flows out from both the hemoglobin as well as the uh oxygen tension. And then these things keep flowing through.

[00:24:24]And on the other side, you have your red blood cells, right?

[00:24:29]That have their hemoglobin.

[00:24:33]That now most likely have a less oxygen attached to it because it gave some to the tissue in the capillary. And then you have your O2 tension, right? Your venous oxygen tension, which is going to be less than your arterial oxygen tension. And your venous carbon dioxide, which tends to be about the same.

[00:24:52]So, when talking about a VBG or an ABG, you actually structure it in a certain way.

[00:25:04]So, it tends to be you can write it like this in your paper, write the pH, then the PvCO2, then the PvO2, then the bicarb, then the SvO2.

[00:25:20]And that's how you can write it. So, how do we convert an ABG, right? If we stuck a needle in this area and got a sample, compared to a VBG, or if we stuck a needle in this area and got a sample.

[00:25:39]It's actually fairly simple and it's one where a lot of times you don't even need to actively convert them, but there are some um kind of data points to keep in mind.

[00:25:51]So, converting a VBG to an ABG.

[00:26:02]Well, things to note is that your SvO2 can't be converted and your PvO2 can't be converted. So, your oxygen tension and your oxyhemoglobin saturation, right? Because going through this capillary, oxygen comes off the hemoglobin and out of the blood. So, the oxyhemoglobin saturation and the dissolved oxygen in a vein are much different than they are in an artery. So, those two don't equate.

[00:26:26]But the city stays about the same, the carbon dioxide stays about the same, and the amount of bicarb stays about the same. But what is about the same? So, for the pH, it's usually 0.03 to 0.04 lower in the venous blood than the arterial blood.

[00:26:47]All right? For the bicarb, it's usually two to three milli equivalents higher in the veins than in the arteries. That makes sense, right? Because if your bicarb's higher, your pH Um no, I should say, I'm sorry, that if your bicarb is higher, you would expect your pH to be higher, but it's lower.

[00:27:11]So, that's something to keep in mind that those two things don't equate. And then your PCO2 is usually three to eight millimeters of mercury higher than your ABG. And this is the one where it makes sense, right? Because if your PCO2 is higher, your pH is often lower.

[00:27:32]So, these are the big differences. With that being said, though, studies have shown that there's significant variability. So, some patients it'll line up perfectly, other patients it won't. And because of that, we have to be careful. So, if your clinical picture does not fit the venous blood gas you're seeing, get an ABG to verify that they're either similar or not.

[00:28:00]Right? So, verify the similarity.

[00:28:03]Cuz if they're similar, then you uh know that you can go by venous blood gases.

[00:28:06]And if they're very different, then you know you can't act on that venous blood gas. The other caveat to this So, we'll put a star here. Is that in patients that are super hypotensive or have extremes of acid base, extremes of acid base, there's been shown to be as much as a three-time factor increase in variability between VBG's and ABG's.

[00:28:40]So, they're much less VBG's are much less accurate in terms of comparison to ABG's in those that are hypertensive or with extremes of acid base.

[00:28:49]Unfortunately, right? These patients, hypotensive patients in shock and extremes of acid base, are often the patients that you need a blood gas on.

[00:28:57]So, just be careful in these patient populations because your VBG might not actually be convertible to your ABG.

[00:29:05]All right. So, usually you can use a VBG for pH, PCO2 in place of an ABG.

[00:29:10]Although, you have to understand that there is significant variability and if it doesn't add up, then get an ABG to verify that your VBG is comparable.

[00:29:19]In those that are hypotensive in shock or extremes of acid base, it can be even more variable. So, you have to be very careful. And again, it's usually worthwhile maybe getting one ABG and making sure it does add uh it does relate to your VBG before acting on the VBG in these types of situations.

[00:29:33]Okay? I do just want to touch on and this will be kind of probably primarily another video, but I do just want to touch on SVO2 and what that means, right? So, the SVO2, as we talked about, is the oxyhemoglobin saturation.

[00:29:50]Oxyhemoglobin saturation.

[00:29:54]So, how much oxygen is on hemoglobin?

[00:29:57]And SVO2 is part of a VBG that is used that is not just used to, you know, translate to an ABG. It's a part of the VBG that, you know, you actually need a VBG to get and not that um you know, for all these other measures, an ABG's always better, but for this, a VBG is actually what you're trying to get. So, in the arteries, you have an arterial saturation of oxyhemoglobin that's usually 95 to 100%. So, 95 to 100% of hemoglobin is saturated with oxygen in the arterial system.

[00:30:36]You then lose 20 to 40% of oxygen off your hemoglobin when it goes through the capillary which leads an oxyhemoglobin saturation in the veins of 60 to 80%.

[00:30:49]This averages to about losing 25% in the capillaries and having 75% return to the lungs, right? Because that's where the veins are going.

[00:31:02]And this value, if taken centrally, right? Off a central line, um because you want to be sampling from that uh superior vena cava, can be used as a surrogate to measure oxygen delivery and oxygen consumption, which can be useful in terms of managing and a diagnosing shock.

[00:31:31]So, let me know if you guys are interested in us doing a video on that.

[00:31:35]Um I think it's an interesting topic and one that is worthwhile discussing. Um but I just wanted to mention that that marker, the SvO2 in a VBG, if taken centrally, peripherally, you can't use it in this manner, right? Because there's too much variability depending on where you take that sample. It has to be taken centrally off a central line.

[00:31:51]Um and that can be used as a surrogate for oxygen delivery, right? How much oxygen comes out here, and then oxygen consumption, or I should say how much oxygen is here, and then oxygen consumption, how much comes out here, meaning how much is left can be a surrogate for that ratio of delivery to consumption. All right, welcome back everybody to another episode here at WhiteBoard Medicine. We appreciate you checking it out. Today's topic is one that's really foundational for a lot of what we do in emergency critical care medicine with our critically ill patients. And that is the venous versus arterial blood gas difference. So, we're going to be comparing blood gases from the venous system versus the arterial system. We'll talk about what they're best used for, what you can kind of get away with, what you should be thoughtful uh around when you are using a blood gas for one indication or another. We'll also talk about what values are derived, calculated versus directly measured, all that good stuff. So, these are tests that we send on patients pretty much every single day in the clinical arena, and having a good grasp on what is accurate and what might be inaccurate in which clinical circumstances can be incredibly important. So, no further ado, let's dive in. We did just want to let you all know, as always, that if you want to this study guide, this PDF that we are actually going through, you can download this off our Patreon page. Um, we have all of the PDFs and study guides from every episode on there, and there's a ton of other stuff, too. It's probably one of the biggest collections of emergency critical care educational content out there. Full ICU curriculums, um, study guides, practice question banks, mini courses, clinical reviews, medical education posts, all that kind of stuff. We've been super, super excited about how much it has grown, and we've been spending a ton of time trying to make sure that uh, that community and the content on that community is up to snuff and helpful for folks. So, we'd love for you to check it out. If you just want to be a fellow emergency critical care nerd like us and join some like-minded folks, that's another awesome area to do it. So, linked in the episode description, as well as the pinned comment on YouTube. No further ado, though, let's get back to ABGs versus VBGs. So, let's start with why this matters. Well, when we're doing a blood gas, we're not just interpreting numbers, right? We're thinking about how those numbers are generated, right? Are they measured? Are they calculated? If they're calculated, uh, what are they calculated from or on what? Um, because that will help you understand what you can kind of, quote-unquote, trust in that blood gas and what you might misinterpret. Because if you misinterpret something then apply it to that patient's clinical scenario, you're on the risk of going down a tricky pathway. So, the core concept here, an ABG is an arterial blood gas. This is sampled from the arterial system. Often, it's a radial artery that you do a radial artery stick on. Um, you can do it in the femoral artery. Uh, not suggesting you should or shouldn't, uh, but most commonly it is from the radial artery. But, it's an arterial blood gas sample, so the blood is taken from the arterial system. This is the gold standard when we're talking about oxygenation, right? We're going to get into all this in a lot more detail, but if you're looking at the PaO2, the amount of oxygen dissolved in the arterial system, an ABG is what you need to do that. It also gives you a more precise view on gas exchange, right?

[00:34:58]Things like pH and even PaCO2, um, are all going to be, quote unquote, more accurate on an ABG.

[00:35:05]A VBG, on the other hand, is taken from the venous blood, right? You can take this from any vein. If you have an IV that draws back, things like that. So, this is a venous blood.

[00:35:16]And this venous blood will not give you as accurate of a view on things. It is completely inaccurate for some things like the PaO2. Um, it can give you a fairly reliable surrogate for things like acid-base, pH, and ventilation, PCO2. But, there's some caveats here, and we'll go through what those caveats are. So, both rely, when we're sending an ABG or VBG, there's a number of things that it will tell us, right? When we're thinking about these blood gases, the things that the blood gas are going to tell us are going to be pH, PCO2.

[00:35:50]Uh, this will all be on there, whether it's accurate or not, PO2, bicarbonate. And there's some other things, um, that can pop up in there, too, but these are the big four that we often send the blood gas for.

[00:36:01]And some of these are calculated, some of these are directly measured, some of these are accurate on a venous blood gas, some of them are only accurate on arterial blood gas. And that's what we'll get into as the, uh, episode progresses here. So, let's start with what's actually measured versus what is calculated. Remember, measured is going to have a less variables affecting it. So, these can sometimes be more trustworthy, the things that you're directly measuring, assuming the measurements are happening um, in a accurate way. Calculated though, these are right these are just derived from different variables. So, if the variables they're derived from are incorrect, the calculated measure will be incorrect. So, directly measured on blood gases are going to be the pH.

[00:36:45]Okay, this is directly measured in a gas electrode. The PCO2, this is directly measured on electrode as well. And the PO2, this is directly measured on the Clark electrode. So, these are all kind of true primary data. Now, with that being said, that doesn't mean the num let's rephrase that. This means that the numbers, assuming they're being measured accurately, are accurate numbers. But, you have to be thoughtful about what they're measuring, right? So, if you get a venous blood gas, a VBG, the PCO2 is technically a PVCO2, right? A partial pressure dissolved oxygen in the venous system. Same thing with the PO2, that's technically a PVO2, or the partial pressure dissolved oxygen in the venous system. So, this is not to be mistaken with a PAO2, which is the partial pressure dissolved in the arterial system. So, don't mistake directly measured and kind of true primary data as accurate in terms of it showing what you hope it shows. But, it is accurate in the sense that it is a directly measured variable within the type of blood gas you send, right? So, the pH, the PCO2, and the PO2 are all directly measured. They're quote-unquote accurate, assuming they're being measured appropriately, but they represent either the venous values or the arterial values, which can tell you different things, which we'll talk more about.

[00:38:07]The calculated values are interesting, and this is something that folks don't always realize, but these calculated values are derived mathematically. So, essentially they take math equations and the measured values, and then calculate or derive these values. And the things that are derived are the bicarbonate, the base excess, and the oxygen saturation. Um sometimes this is directly measured, but most of the time oxygen saturation is calculated as well.

[00:38:36]But the big one that people don't think about is the bicarbonate. This is actually a calculated value. And the to calculate it they use this equation. So they use an equation bicarbonate equals.03 * the pco2 * 10 to the pH - 6.1 power. So you already see here the fact it's calculated means that the pco2 has to be accurate, the pH has to be accurate, these assumed kind of coefficients of.03 um and 6.1 have to be accurate. So all this kind of limits um maybe limits isn't the right word, but you should be um thoughtful on whether this is an accurate value cuz it's going to be based off of these measured values as well as certain other coefficients and variables which then just introduce a degree of error that wouldn't be present if you were directly measuring it. So the bicarbonate on the blood gas um in our humble opinion, we you know, you can use it, you can think about it, but the uh bicarbonate on a basic metabolic panel is going to be much more accurate cuz it's directly measured, whereas the bicarbonate on a blood gas is a calculated value derived from this long equation that uses both coefficients as well as directly measured values like pco2 and pH.

[00:39:46]All right, so that was just a quick glimpse into what's measured and what's calculated. So measured is pH, pco2, po2. Calculated is bicarbonate, base excess, and O2 saturation. So translation, if the pH or pco2 is off, the bicarbonate is also going to be off because it's calculated using the pH and pco2. So it's not independently measured.

[00:40:08]Well, let's get into the bulk of this video which is going to be the ABG vs. VBG a comparison. This table here um and if you're listening to this in podcast version, we do have a table. We'll go through the table, but if you want to actually see it, um, we obviously link the video in the podcast description, so you can hop over to YouTube if you want.

[00:40:24]Or, if you go to our Patreon page and want to download this study guide, obviously it'll be printed out there and you can annotate it as you see fit. So, just to understand the table, we have, um, columns of parameter, ABG, VBG, Y, and clinical takeaway. And then our variables are pH, PCO2, PO2, bicarbonate, and O2 saturation.

[00:40:44]The thing to note here is the verbiage we're using in the table is often higher or lower. And ABG and VBG, the verbiage is comparing it to the other. So, um, for instance, this first line here, ABG is higher, that means that the ABG value is going to be higher than the VBG value.

[00:41:01]And then VBG is lower, which makes sense. So, um, these are obviously opposite each other. Um, there's no kind of like gold standard that we're comparing both of these two. We're comparing the ABG to the VBG when we use verbiage of higher and lower. So, the first parameter is the pH. And the pH on an ABG is going to be higher than the VBG. And we often consider the ABG pH kind of the more accurate one. Um, so, ABG pH is the more accurate one, and that stands true for a lot of these values. The VBG pH is often.03 lower than the pH on the ABG.

[00:41:38]And the reason for this is because carbon dioxide accumulates in tissues, and as CO2 goes up, our pH goes down. So, when we do a venous blood stick to get a VBG, a venous blood gas, we are pulling that venous blood from the tissues.

[00:41:55]Uh, sorry, we're pulling that venous blood from the, um, uh, blood that has already traveled through the tissues.

[00:42:02]So, if, um, you think about the heart pumping blood, goes into the arterial system into arterioles into capillaries.

[00:42:09]And that blood delivers oxygen as well as other things, but then it also reabsorbs CO2 from those tissues. That then goes into a venule, into a vein, and back to the heart. So, the blood in that vein has already accumulated CO2 from the tissues. And that CO2 is going to decrease the pH a little bit. So, the VBG pH is often 0.03 lower than your ABG pH. Um and for most clinical scenarios, that's usable. You know, it's kind of close enough. Now, do note, and we'll get into this more, that in critically ill patients, this is sometimes more inaccurate.

[00:42:44]So, as patients get sicker, you have to be more careful with this.

[00:42:48]Um but in the run-of-the-mill patient, most peo- people would say that a venous blood gas pH is about accurate. And certainly none of this is intended to be medical advice. All of this is just for educational purposes. Please reference your institutional guidelines and societal guidelines to further understand this and make medical decisions. So, just educational, but most people would say that this is pretty uh similar enough. So, if you're just looking for pH, and the patient is not critically ill, often a venous blood gas will suffice.

[00:43:14]What about the PCO2?

[00:43:16]Well, the PCO2, which is the um amount of carbon dioxide dissolved in the blood, is often 4 to 6 higher in the venous blood. And if you were listening closely, the reason is the same as why the pH is a little bit lower. Because carbon dioxide accumulates in the tissues, and [snorts] that carbon dioxide from the tissues gets into the venous blood. So, the CO2 is going to be slightly higher in the venous blood. And that's also why the pH is slightly lower, cuz higher CO2 drives down the pH.

[00:43:49]>> [snorts] >> So, CO2 in arterial blood gas is going to be a little bit lower than a venous blood gas. And the venous blood gas CO2 is going to be a little bit higher, 4 to 6.

[00:43:57]Okay?

[00:43:58]So, again, this is a good correlation.

[00:44:00]Most people would say that you can use a venous blood gas to uh estimate what the CO2 is, and you don't need an arterial blood gas, but do note again, as patients get sicker, this becomes less accurate, so you have to be more careful in those patients that are critically ill. Um, sometimes what we do is, if a patient's a hard arterial stick, um, we sometimes will get a venous blood gas, but we don't tend to make significant clinical decisions, such as let's intubate this patient for respiratory failure, um, without ensuring their arterial blood gas is, um, similar to the venous blood gas that we got.

[00:44:33]>> [snorts] >> All right, what about P O2? So, we'll put P A O2 here, right? And this is the amount of dissolved oxygen in your arterial system.

[00:44:42]So, this, hopefully this little A written here, um, hopefully will show folks that the venous blood gas is not an accurate measure of the P A O2, because it's venous blood, not arterial blood. All that oxygen, not all, but a lot of that oxygen was extracted from the arterial blood before it got into the venous system. Oxygen's extracted by tissues, um, which makes the venous blood gas not an accurate measure for P A O2. You cannot use it. Um, it's not functional in that sense at all. You need to get an arterial blood gas to determine the P A O2. So, the first two, the pH and the PCO2, venous blood gases tend to be an okay surrogate, but the P A O2, um, you have to get an arterial blood gas. All right, so this is all ABG. You cannot get a VBG to estimate it.

[00:45:28]And then, the other ones in this chart are the bicarb and the O2 sat. Remember, these are calculated values.

[00:45:34]So, they're accurate only as so far as, um, the math equation used to derive them are accurate. And remember, the bicarbonate is calculated using the CO2 and the pH. Um, so, a venous bicarbonate, you know, uses the venous CO2 and venous pH. We already said the venous pH is often 0.03 lower than the arterial pH, and the venous PCO2 is often 4 to 6 higher than the arterial CO2. So, the bicarbonate calculated on a venous blood sample is, you know, accurate-ish, but again, as you're getting further down this road, our patients are getting sicker, these calculated values are sometimes not super useful.

[00:46:15]Okay, and oxygen saturation, same. Now, sometimes co-ox can directly measure it, but venous saturations are not accurate.

[00:46:24]An ABG saturation does tend to be accurate, so you cannot use a VBG um to get an O2 sat on your blood gas. You need an arterial blood gas to do that, or just look at the pulse ox.

[00:46:39]So, in summary here, if we were to write this out for a PAO2 or an SPO2, which on the blood gas is usually written SAO2, you need an arterial blood gas.

[00:46:53]For pH, PCO2, and then the calculated bicarbonate, you can usually do an ABG or VBG with the very important caveat that the pH is going to be about.03 lower than the arterial blood gas, the PCO2 is going to be about four to six higher than the arterial blood gas, and as patients get critically ill, this becomes less accurate, so be very careful and get an arterial blood gas to confirm in the critically ill if there are significant derangements you're worried about.

[00:47:28]So, the physiology we're going to dive into a little bit more here. So, venous versus arterial. Well, let's just draw a little kind of vascular bed here. So, artery goes down into arterial, which goes into capillary, which goes into venu- venule, and vein back to the heart. So, if you're listening to this podcast form, we're just doing a really, you know, terrible drawing cuz we're terrible at drawing, and it's probably not that helpful, but maybe it is.

[00:47:58]>> [snorts] >> So, this is going to be heart here, and then this is going to be heart here, right? Arterial blood pumps from the left side of the heart, and venous blood returns to the right side of the heart.

[00:48:11]Um and then this is capillary.

[00:48:13]Okay, this is artery arterial. This is venule vein. Blood travels in this direction, left side of the heart into artery arterial capillary venule vein right side of the heart.

[00:48:27]And you have all these kind of muscle beds here, all these tissues, and that's what these capillary beds are perfusing, right? So, oxygen and nutrients from the blood go out into the tissues, carbon dioxide and waste from the tissues goes back into the capillary. So, that means there's more CO2 in this venous blood because it reabsorbed from the capillary bed, and there's going to be much much less oxygen in this venous blood um because all that oxygen was uh delivered to the tissues compared to the arterial. So, the tissues consume oxygen, produce carbon dioxide, generate acids. So, the venous blood reflects what's left after metabolism, which is why, as we said, the CO2 is going to be higher in the venous blood, the pH is going to be lower in the venous blood.

[00:49:15]The arterial blood reflects what's being delivered, and that's why this is going to be uh accurate oxygen in the venous blood, whereas in the uh sorry, in the arterial blood.

[00:49:26]Ooh, that was a bad uh misspeak. Let's just say it again. In the arterial blood, the oxygen levels are going to be accurate, so your PaO2 is going to be accurate. In the venous blood, you can't even use the venous blood to get a PO2.

[00:49:40]Um it will not be reflective of your PaO2.

[00:49:44]Now, you can get a you know, there's other uses for oxygen saturations in the venous blood outside of the scope of this episode, but there's something called a central venous gas and SVO2, um which is uh oxygen saturation of the venous blood returning actually to the heart and lungs.

[00:50:03]Um but again, for this episode we won't dive into that. So, just know venous blood fairly good for pH and PCO2, not accurate for PO2 um or SAO2 for the physiologic reasons we talked about. So, when is VBG enough? Just uh knock it home. It's enough for pH often PCO2 often and bicarbonate which is calculated with the caveats being pH is going to be about 0.03 lower, PCO2 is going to be about 4 to 6 higher, and then the bicarbonate is calculated off these values, so you will get a little bit of inaccuracy uh with the second caveat that critical illness makes this even less accurate, so just be full of that and careful.

[00:50:42]ABG is required if you need a PO2 measurement or an accurate oxygen saturation on the gas, or if you're worried that the VBG is not accurate based on critical illness. So, high-yield pitfalls, a quote-unquote normal bicarbonate can be misleading because it's calculated from the pH and PCO2, so if those are wrong, your bicarbonate is wrong.

[00:51:01]Pulse ox vs. ABG saturation mismatch.

[00:51:04]ABG calculates the saturation, pulse ox measures the light absorption. So, there's um shortcomings to each of these.

[00:51:12]For instance, carbon monoxide CO poisoning, the ABG may look normal, um but the patient is hypoxic, right?

[00:51:20]So, there's some Maybe we'll get if uh you have an interest in the um uh clinical conditions that cause a difference between the uh ABG oxygen saturation and the pulse oximeter oxygen saturation, let us know.

[00:51:33]That's probably an interesting episode.

[00:51:35]Um so, if you're interested in that, let us know. We won't uh belabor it here uh because it's probably a whole episode in and of itself. And then VBG reliability depends on perfusion, uh and that's why critical illness can throw this off.

[00:51:46]Poor perfusion is going to lead to altered venous sampling and less predictable relationships between the venous blood and arterial blood. Poor perfusion, right? Those patients in shock, those patients who with critical illness. So, again, these rules I think are reasonable, but you will see a significant discordance between arterial and venous blood in the critically ill at times. Um so, don't put all your marbles in the bag of the venous blood sampling um if it uh shows significant derangements. So, quick mental model here. We're getting close to the end. We do have some practice questions at the end, so stick around for that um as a way to know if you took things with you or not. Um but, measured values tend to be more true because there's less variables. Calculated values uh are an interpretation of that truth.

[00:52:29]Measured values are the PCO2, pH, PO2. All right, calculated values are the bicarbonate, okay, the base excess, and the uh O2 sat.

[00:52:40]So, always ask, am I interpreting a measurement or a calculation? Bottom line here you're here a high-yield summary. Um we're going over the same points um but really just to try to take it home. Measured values are the pH, PCO2, and PO2. Calculated values are the bicarbonate, base excess, and O2 sat.

[00:52:56]VBG's work fairly well for pH and PCO2.

[00:53:00]These are directly measured and they correlate fairly well um although do note in critical illness, this correlation may not be as close. ABG's required when you need oxygenation, the PaO2.

[00:53:11]All right, higher and lower always venous versus arterial in the same patient if you're going to compare them.

[00:53:18]All right, let's get into some practice questions. Oop, we almost showed you the first answer. If you have not done practice questions with us before, uh there's three of them and then the episode will be complete. Uh usually beginner, intermediate, advanced. We'll read the question, read the answer options, and then we'll go right into the answer. So, if you need more time to think about the answer, just pause the episode, okay? So, question one. Which of the following variables is directly measured on a blood gas analyzer? A, bicarbonate. B, base excess. C, PCO2. D, oxygen saturation.

[00:53:47]Pause if you need to. The correct answer is uh C, PCO2 is directly measured via electrode. Bicarbonate, base excess, and oxygen saturation are often calculated.

[00:53:58]Um sometimes you can an O2 sat if using a co-oximeter that is directly measured.

[00:54:03]Okay, question two, intermediate. A A patient has a VBG showing a pH of 7.33, PCO2 of 50. Which of the following is the best interpretation? Severe hypercapnia requiring immediate intubation.

[00:54:15]B, likely mild hypercapnia with a reasonably accurate reflection of arterial carbon dioxide. C, uninterpretable. VBG cannot assess ventilation. Or D, indicates severe hypoxemia. Pause here if you need to.

[00:54:26]The correct answer is B, likely mild hypercapnia with a reasonably accurate reflection of arterial CO2. Right, so VBG CO2 and pH is reasonably accurate.

[00:54:39]The CO2 is typically 4 to 6 higher than it is in the ABG. So, in this question stem, the PCO2 is 50. So, the real PCO2 in the ABG is probably somewhere around 45, which is just mildly higher than the normal that's often written as 40.

[00:54:55]>> [snorts] >> Uh pH, right, is usually.03 lower. So, this pH says 7.33, but in reality, it's probably somewhere around 7.36, which is normal, right? pH greater than 7.35 would be considered normal.

[00:55:10]Um so, this correlates well in moderate ranges, and the VBG is reliable in most cases. Critical illness, though, can sometimes disrupt this. All right, last question. Which of the following scenarios would most likely result in the largest discrepancy between VBG and ABG values? A, stable DKA patient. B, mild COPD exacerbation. C, septic shock with poor perfusion. D, anxiety-induced respiratory alkalosis. Pause if you need to. The correct answer is C, septic shock with poor perfusion. So, remember poor perfusion makes the VBG less reliable cuz tissue extraction and perfusion are related.

[00:55:46]So, in shock states impaired perfusion and altered metabolism can lead to unreliable venous arterial correlation.

[00:55:52]So, you got to be really careful using the VBG in these patients. And if you get a significant >> [snorts] >> finding on the VBG, before you make any huge decisions, often clarify it is true with an ABG.

[00:56:06]All right, that's all we have for you today. Let us know what thoughts, comments, questions you have. After this, we'll have kind of a full showing of our Patreon page on this episode. So, stick around if you want to kind of see what some of it looks like.

[00:56:19]And again, we'd love for you to check out our Patreon community YouTube members community. We've been spending a ton of time trying to grow those communities and have been so so excited that so many people have joined. So, linked in the episode description and the pin comment on YouTube. Either way though, stay well, keep learning. We hope to see you next If I could borrow a quick 60 seconds of your time, I wanted to introduce you to a community we're really excited about building. And that is our Patreon community. In this community, you will get access to high-yield medical education posts, ad-free video study guides, practice questions, mini courses, book chapters, and much more. You will see that by joining this community, you'll get access to the PDFs, the study guides for every single YouTube video. Here's an example of one of those right now. In addition to that, you will get access to mini courses. In these courses, we will have videos, study guides, practice questions, bedside tips and tricks, and 30 practice questions at the end to test your knowledge. In addition to that, we have collections on every major emergency critical care topic that contains videos, medical education posts, practice questions, study guides, all categorized.

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