PART II: Interpretation of Blood Gases -Tala Talks NICU

What the printout is telling you is it's telling you how much acid or how much base needs to be added to the blood to make it pH neutral. Welcome back to Tala Talks NICU. This is part two on blood gases and another couple of housekeeping things from Ariana and Justin. If you could remember to like and subscribe the videos and comment below on any other topics you want us to talk about. And go look for the multiple choice questions under the community tab. They will really reinforce everything that you've been learning in these videos. Okay, great. So in this video we're going to go over three different topics. The first one is the bicarbonate level. What is normal and what sort of information that having the bicarbonate level in the gas can give us. The second is the base deficit or the base excess and what we can know or learn from that, from the gas. And then the third is kind of moving onto more about the generalities about the gases. So what we're actually gonna be talking about is the differences between arterial, capillary and venous blood gases. So watch on. So number one, the bicarbonate level. The bicarbonate or the chemical symbol for it is HCO3 minus is the main buffer or the main alkali or base in blood. It is normally displayed after the PaO2 level, the PO2 level or the oxygen that's dissolved in blood and it is a calculated value. So it's not a measured value. So as a buffer, if the body has more acid in its blood then the bicarbonate will be used to neutralize that acid. Or another way of saying that is if there is more acid in the blood then the bicarbonate will be consumed and the bicarbonate level will be lower. So a low bicarbonate level means by definition that there is a metabolic acidosis. So an acidosis that is not originally because of breathing issues. Different disease processes unrelated to breathing could result in increased acidity. For example, a PDA, sepsis, dehydration, anemia, the list goes on and on. The bicarbonate level can go down because it may be responsible for neutralizing the acidity in the blood but it could also go down because the kidneys may be inappropriately dumping it out. Now, baby kidneys, especially the kidneys of premature infants aren't very refined. They're pretty idiotic and they often unnecessarily waste the bicarbonate ions. So that results in a lower bicarbonate level in babies as compared to adults. In adults, the normal bicarbonate level is somewhere between 22 and 26 milliequivalents per liter. In babies, that number is somewhere between 20 and 24. Honestly, in preemie babies, especially the micro preemies, they can dump that bicarb really, really freely and they can end up with bicarbonate levels in the mid-teens. So again, to reiterate, if there is a low bicarbonate level, whether it's low because the bicarbonate was used by neutralizing acid in the blood or whether it's low because the kidneys were dumping it, then by definition, that is considered metabolic acidosis. If the bicarb is less than 18, it is considered metabolic acidosis. On the other end of the spectrum, if the bicarbonate level is high, above 30, and we'll talk about the different disease processes that can result in a high bicarbonate level, then that is considered metabolic alkalosis. So less than 18 is metabolic acidosis, above 30 is metabolic alkalosis. Let's move on to point number two, or the base deficit or the base excess. Hopefully, you will have learned something about this from the HIE video, because if you remember, the base deficit is an absolutely crucial number in deciding whether you want to try to cool the baby or not. And if you remember, having a base deficit less than minus 16, so minus 17 or minus 19, immediately puts you into the range where you should definitely be thinking about cooling a baby. The base deficit, which happens in metabolic acidosis, or the base excess, which happens in metabolic alkalosis, is also a calculated or derived number. And what that little machine, what the printout is telling you, is it's telling you how much acid or how much base needs to be added to the blood to make it pH neutral or to make it 7.4. So if you have a base deficit of 10, it's telling you that there needs to be 10 units of the base to be added to make the blood neutral or to make it a pH of 7.4. So a negative number is a base deficit, because by definition, that means that the body is lacking a base. And a positive number means that there is a base excess, meaning that we have too much base, or bicarbonate is the most common base in blood, so we have too much base, so we have a base excess. We generally accept a base of minus three to plus three. So a base deficit of more than minus three is considered a metabolic acidosis, so minus four, minus five, minus six. And a base excess of more than plus three, so plus four, plus 12, plus 15, is considered a metabolic alkalosis. Now let's move on to point three. We've now covered all the important numbers that are displayed on a blood gas. So now I just wanna go over some generalities about blood gases. And the first thing that I want to talk about now is the differences between arterial, capillary, and venous blood gases. As you all know, the arterial blood carries oxygen from the lungs towards the cells. The cells, which are then getting the blood from the capillary blood, are using up all that oxygen, producing carbon dioxide, putting it back into the capillaries. Then the capillaries dump their blood into the venous system. So logically, you'd expect the highest oxygenation level, so the highest PaO2, in the arterial blood, and the lowest carbon dioxide in the arterial blood. And then you would expect the lowest oxygen level in the venous blood, because all the oxygen's being used by the cells, and the highest carbon dioxide level in the venous blood, because the cells that have given it to the capillaries, that have then given it to the veins, have given up all the carbon dioxide. The capillaries are gonna be somewhere between the arteries and the veins, with the various gases. Some authorities suggest that the difference in the carbon dioxide level between the artery, the capillaries, and the veins is about five millimeters of mercury. So for example, if you have a PaCO2 of 45 in the artery, then if you get a capillary blood gas, it will be 50, and if you get a venous blood gas, it will be 55. And logically, because now you all know this, the higher the carbon dioxide, the more the respiratory acidosis. So because the carbon dioxide is highest in the venous blood, then there will be more acidity in the venous blood, or the pH will be lower. So you would expect the pH to be the lowest in the venous blood, highest in the arterial blood, and the capillary blood will be somewhere in between. That's what should logically and theoretically be happening. However, a recent study showed that there really wasn't a significant difference in pH and PCO2s between arterial, venous, and capillary blood, which is fantastic for us in the unit, because it means that if you're drawing blood for something else, if you're getting a blood culture, then you can get a VBG and have a pretty good idea of what's going on. Or if you're not even getting a culture and you just wanna do a capillary stick, then that will also give us a pretty good idea of what's going on. There's a couple of exceptions to this. The first, obviously, is when you want to check the oxygen content in blood. Go back and look at the first video, but we really emphasize that only the arterial sample can give you an adequate idea of what's going on with the oxygen status in the body. The second thing is, is that clinically, we've all seen some aberrations to this. So I'm sure you've all seen plenty of times when you've got a capillary blood gas and the pH is ridiculously low and the CO2 is really high and the baby doesn't really look that sick. In those cases, maybe the perfusion was off. Maybe somebody squeezed too hard to get the capillary blood sample out. In those cases, go ahead and repeat an arterial sample, especially if it's a very unexpected gas. And as I'm sure you've all seen before, sometimes the capillary samples can be very off. So that was part two of the blood gases. Please go back and watch the first one and hopefully go watch the next one, which we'll be releasing soon. And the next one will be on putting all of these values together and how we actually interpret the blood gas as a whole. In the meantime, please remember to like and subscribe and to answer those multiple choice questions. Thank you so much.