Hi. I’m Mark from ACLS Certification Institute. In today’s video series, we’re talking about arterial blood gases—how to look at and interpret an ABG. We’re going to look at the different components of the arterial blood gas and what they mean. Then we’re going to review the steps for looking at and interpreting systematically an arterial blood gas. Let’s start at the beginning.
First, most important, pH. What is it? The pH stands for parts of hydrogen or percentage of hydrogen. That’s all it means. It’s the amount of hydrogen floating around in your body. If you have too much hydrogen, your pH will shift and you’ll become acidotic. If you don’t have enough hydrogen, you can become alkaline. So, we’re talking about the acid base. The pH reflects the overall acid-base balance of the patient, and the body wants to maintain a normal pH of 7.35 to 7.45—that’s a normal pH range and that’s where the body wants to be. At 7.40, perfect, that’s exactly where we want to be. If the pH drops, the patient will become acidotic. They can develop acidosis. The way I remember this is years ago I was watching the original Batman movie with Michael Keaton and Jack Nicholson. (It was a great movie.) At one point in the movie, the Joker falls into this vat of acid. I’m watching the movie and it dawns on me, when the pH falls, it falls into acid. You can only fall into a vat of acid. You can’t fall up into a vat of acid. When the pH falls, they’re becoming acidotic. That’s how I remember it. Do with that what you want. I don’t know. That’s how I remember when the pH falls, the patient becomes acidotic. Now, normal pH range is 7.35 to 7.45 and we’re looking for 7.40; that’s right in the middle. Having said that, our goal in acid-base management is to correct the pH, fix the pH. The pH is the mack daddy. It is the big kahuna. We want to correct that pH. That’s our goal in acid-base management.
Next, pCO2. First of all, what is it? For the purposes of interpreting ABGs, it’s a respiratory acid. That’s what it is. It’s a respiratory acid. Now, before I start getting emails from all over the world from science geeks (who I love and we need you) and they go, “Well, technically, it’s a gas and it mixes with water and forms carbonic acid,” we get all that and I understand the science behind it, but today we’re talking about interpretation. Actually, if you understand the interpretation a little more, if any of those equations were giving you a problem, this may help, so you can figure those formulas out a little better. Normal pCO2 range is between 35 and 45. If the patient starts to accumulate more of this respiratory acid and starts building up more of this respiratory acid in their body and their pCO2 is climbing because of an increasing amount of respiratory acid in the body, they may develop, yep, respiratory acidosis. That’s it. They have a lot of a respiratory acid in their body. That’s what the pCO2 is, so they’ll develop a respiratory acidosis. If the pCO2 is between 35 and 45, it’s normal. It’s balanced. The acid base in the pCO2 is balanced. It’s fine. Should the pCO2 drop lower than normal then we’ve shifted the acid base the other way and they could develop respiratory alkalosis. When looking at a blood gas, first look at the component and ask yourself what is that. What is the pCO2? It’s a respiratory acid, so if I have an accumulation or an elevated pCO2 or an elevated amount of this respiratory acid, I will develop respiratory acidosis. If it’s normal, between 35 and 45, it’s balanced. If it drops below that, it shifts the other way to alkalosis. First ask yourself what is it.
Next, bicarb. What is it? Bicarb is a metabolic base. It’s an alkali, an antacid, the other side of acidity. Remember the acid base. You have acid and you have base. Bicarb is a metabolic base. If I start to accumulate more of this metabolic base and my levels of this metabolic base start to increase, I will develop metabolic alkalosis because that’s what it is. It’s a metabolic base. Normal bicarb level is between 22 and 26, so if my bicarb is rising because I’m accumulating more of this metabolic base, my levels of this metabolic base are increasing and my bicarb numbers are going up, I have metabolic alkalosis because that’s what it is.
Next, let’s look at the pO2. When you’re looking at an ABG and you’re assessing for acid-base status, look at the pO2. We really don’t calculate that in, but we look at the pO2. What’s important to remember about the pO2 is that normal levels are between 85 and 100; however, the pO2 should be about 5 times the FiO2 for the percentage of oxygen you’re administering the patient. A normal pO2 of 85 to 100 on room air oxygen, which is 21%, (20, 40, 60, 80, 100), is perfect; it should be right around 100. However, if your patient’s on a ventilator and they’re on 100% FiO2 and you come back with a pO2 of 100, something ain’t stirring in the Kool-Aid, something is wrong. This patient is not oxygenating well. If he’s at 100% oxygen (1, 2, 3, 4, 5), his pO2 should be right around 500, but it’s only 100 on the ABG. They’re not oxygenating. We need to find out why they’re not oxygenating and we need to correct that.
How do we read an ABG? What are the steps to reading and interpreting an arterial blood gas? First, start with the pH. Why? That’s the mack daddy. That represents the body’s overall acid-base status. We’re going to start at 7.40, right in the middle. Right in the middle is where the body wants to be. That’s our goal. We look at our pH. In this case, it’s 7.36. It still fits within the normal range between 7.35 and 7.45, but it’s not 7.40. It’s deviated from perfect. We don’t want deviation from perfect. Which way is it deviating? Is it reflecting more acidosis or more alkalosis from that 7.40? We have 7.36, so it’s leaning more toward the acidic side. It’s lower. The pH is lower than 7.40, so it’s leaning toward acid. Our next step is to look at the pCO2 and the bicarb and see do either of those reflect an acidosis. First we look at our pCO2. It’s 57, well above the normal range of 35 to 45, so we found our culprit. We’ve accumulated this respiratory acid and we have respiratory acidosis. We see that we have an elevated pCO2, which is a respiratory acid, so we found our acidosis, but now let’s look at the bicarb. Bicarb is 31. Wow, normal range is 22 to 26, so the bicarb is elevated, which reflects a metabolic alkalosis because that’s what it is.
Now we get into compensation. Remember I said before that our goal in acid-base management is to correct that pH. In this case, the body’s already doing that through the process of compensation. You have the metabolic and the respiratory. One will compensate for the other to try to bring that pH to normal. What we have here is a respiratory acidosis and we have a metabolic alkalosis that is compensating for that. It has managed to bring the pH back into normal range. We call that full compensation. The pH is normal, but there’s a whole lot going on. We have a respiratory acidosis and the metabolic side has compensated with a metabolic alkalosis. This compensation has brought our pH back into a normal range. We call that fully compensated. If the compensation did not bring the pH back into a normal range then we call that a partial compensation.
Let’s take a look at another blood gas. First, we look at the pH. It’s 7.51. Starting from 7.40, which way is this leaning? Is it leaning more acidotic or alkaline? We can only fall into a vat of acid and since it’s going the other way, this is alkalosis. This pH represents alkalosis. Next step, look at the pCO2 and the bicarb to see if either of those reflect alkalosis. Start with the pCO2 first. The pCO2 is 65. It’s 65—that’s an elevated respiratory acid. I have a respiratory acidosis going on here but my pH reflects alkalosis, so I haven’t found it. I found some acidosis, but I’m still looking for that alkalosis. Where is it at? I go to my bicarb. Bicarb is 50, well above the 22 to 26 normal range, so I have a metabolic alkalosis. I found my culprit. There’s the alkalosis. I have a compensated respiratory acidosis but it did not bring the pH back into a normal range, so we would call this partially compensated.
The pCO2 is a respiratory acid. How does the body regulate this amount of respiratory acid in the body? The body takes in oxygen, uses the oxygen, and produces CO2 as a byproduct. It’s the waste. We’re producing all this respiratory gas acid and we’re ventilating it off. That’s what ventilation means, to blow off that acid. When you’re bagging a patient and you’re going, “I’m ventilating the guy,” yeah, you’re putting oxygen into him but it’s him ventilating or exhaling, blowing off that CO2 that manages the level of pCO2 in the body, manages the amount of respiratory acid in the body. If the body’s taking up this oxygen and producing CO2, the patient to breathe, ventilate, blow off at a certain respiratory rate to maintain this normal level. If, for whatever reason, they start breathing less and your patient’s respiratory rate drops, they will not be ventilating as much. They will not be blowing off enough of the CO2, so this CO2 is going to accumulate in their body and develop respiratory acidosis because they’ve accumulated a lot of this respiratory acid. That’s usually the problem. Your patient’s hypoventilating. Address the airway. Address their bagging. Do they need to be bagged? Do they need to be intubated? We need to get a hold of their ventilation so they can blow off the CO2. Now on the other side, you have a patient who’s breathing way too fast. They’re ventilating off all their CO2. They’re blowing it off, so the level of this respiratory acid, CO2, is going to start to drop. They’ll develop respiratory alkalosis. They’re breathing too fast. That’s the problem. Now, you show up at the scene. You’ve got a guy who is half with it. He’s breathing 30 times a minute. You pull out your SOP and you go, “Holy smokes! This guy’s breathing 30 times a minute. This is horrible! I got to put him down. I got to tube him. Somebody hit him with something. Hand me a tube. I got to get a hold of his airway. I got to get his breathing under control.” Do you? Or is that compensatory? Why is he breathing 30 times a minute? It’s his respiratory system compensating for a metabolic acidosis, and he’s breathing fast to develop a respiratory alkalosis to compensate for it. You may not want to stop him breathing 30 times a minute. That’s his body’s natural response to blowing off that CO2, so this is a patient that you may not necessarily want to put down and intubate and start bagging 12 times a minute because his body’s trying to blow off that extra acid. Make sense?
That’s the simple down and dirty on ABG interpretation. I’m Mark for ACLS Certification Institute. Thank you for watching.
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