Neonatal Hypotension Defined: How Low Is Too Low for a Premature Baby?

Hello, I'm Dr. Tala, a board certified neonatologist. And today we are going to be going over part two of our video series on neonatal hypertension. Hopefully you've already watched the first video where we went over the importance of blood pressure or why we even care about it. And really what I discussed in that is that we care about low blood pressure because it may be a marker of low perfusion and low oxygen delivery to the cells. So I guess now you don't have to watch it if you haven't seen it already. So in this video today, we are going to cover how do we measure blood pressures in babies as well as how do we define hypertension? Let's start with one. How do we measure blood pressure in neonates? Well, ideally what we would like is a simple, accurate, painless, continuous, non-invasive method of measuring blood pressures in babies. As you all already know, unfortunately this doesn't really exist. So the gold standard that we use for measuring blood pressures in babies is through an intra-arterial line. So we will either put in an umbilical arterial catheter or a peripheral arterial line in babies. Apart from the fact that these are very invasive, they can have their own issues as well. So a lot of these end up with like the wave dampening and you can't kind of figure out what's going on, especially when the catheters are really little. It doesn't take a lot for that, the vessel to spasm or a little clot there to interrupt your blood pressure reading. You can get little air bubbles in them. Often you have to actually recalibrate the system. And as you all know, we've seen clots that have occurred. This could end up with just the actual line clotting off, not a big deal apart from the fact that it stops working or the clot is actually in the arterial system and can end up with decreased blood supply to wherever that blood supply was going. So we've all seen babies lose digits from a PAL gone bad. There've also been many case reports of clots in umbilical arteries after a UAC that have actually also affected the blood supply to the lower extremities. So these do not come without risk. The good thing is, is that non-invasive blood pressure monitoring has been steadily improving over the last few decades. And now we have very good cuff blood pressure monitoring using oscillometric devices. Basically an oscillometric device detects the amplitude of the pulsation. So the cuff is squeezed to above the pressure of the systolic pressure. And then as it's slowly released, the device starts sensing bigger and bigger pulsations. When it's reaching the maximum pulsation, this is deemed to be the mean arterial pressure. So really what the oscillometric device is measuring is the mean arterial pressure. Then based on algorithmic calculations and other computer stuff, it's then calculating the systolic and the diastolic blood pressures based on the mean arterial pressure. So how accurate is non-invasive blood pressure monitoring? Well, in one study done in Australia, they compared 1,800 paired measurements in non-invasive blood pressure monitoring versus invasive blood pressure monitoring at the same time in babies born at less than 32 weeks gestational age. So if you look at this graph, you can see the x-axis shows the actual mean blood pressure obtained by the umbilical arterial catheter. The y-axis shows how much higher or lower they found the blood pressure based on cuff pressures. Sometimes they're great. So you can see that by all the dots around the zero, but sometimes they're really bad, more than 20 millimeters of mercury below or above the actual real blood pressures. So the authors of this study concluded that if you ever decided that you actually want to treat the hypertension, then you really should be getting intra-arterial access. The International Neonatal Consortium published a review article a few years ago where they analyzed the non-invasive blood pressure monitoring in babies. They brought up all the different ways that blood pressures can be affected. So obviously the state that the baby is in is going to affect the blood pressure. If the baby is extremely active and agitated and screaming its head off, the blood pressure is going to be higher. If the baby is sleeping, then the blood pressure is going to be lower. They concluded the most accurate blood pressure measurement that you are getting from a non-invasive device is the mean arterial blood pressure. This makes sense based on how that oscillometric device is actually working. It's actually measuring the mean blood pressure most accurately. And the systolic and the diastolic are being derived. They also gave recommendations on how the blood pressures should be measured with a cuff. And they recommended that the right upper arm is always used in neonates unless there is some sort of contraindication. They did say that in the first few days of life, using the legs might also be accurate. They also gave recommendations based on several studies about the recommended size of the blood pressure cuff. As you all know, if the wrong size blood pressure cuff is used, then the blood pressure really doesn't mean a lot. It can be way too high or way too low depending on whether it's too big or too small. And so what they recommended based on these studies is the width of the blood pressure cuff or the bladder is 50% of the circumference of the arm. This will give the most accurate non-invasive blood pressure reading. Again, they emphasize that these readings aren't perfect. So if you are genuinely concerned about the baby's blood pressure, then non-invasive monitoring is not ideal. So if you are starting the baby on several presses, then we really do need intra-arterial monitoring. Now let's go to number two. How do we define hypertension? This really is the age old question. And part of the reason that it's been so difficult to tease out what we should or shouldn't do with blood pressures is that there really is no great definition for when is a baby hypertensive. Obviously, based on what we said in the first video, that we really only care about blood pressure in the context of whether it's perfusing the body or not, it's going to be very difficult to actually find that one number that's going to give us all this information. In clinical trials and practice, hypertension is defined as a blood pressure below the fifth to 10th percentile for the age of the patient. But obviously this works in adults. You figure out what the average blood pressure is for every adult. And then if they're less than five to 10th percentile, then this is low. But what is the normal blood pressure in a 390 gram 22 weeker? A 22 week baby is not supposed to be born at all. So we really don't know what the normal blood pressure is supposed to be. Obviously, we do have some graphs measuring out the third percentile for the systolic, the mean, as well as the diastolic blood pressures when babies are born for their gestational ages and hours after birth. But can we really use those to determine whether that is a normal blood pressure and an adequate blood pressure for a baby? One of the most commonly used definitions for hypertension is when the mean blood pressure is less than the gestational age of the baby on that first day of life. The HIP trial or the Hypertension in Premature Infants trial that was published in 2021 basically defined as hypertension as a mean blood pressure, one millimeter mercury less than the gestational age of the baby that the baby had for at least 15 minutes. So say you had a 25 week baby, then the gestational age therefore is 25 weeks. If that baby had a blood pressure of 24 for 15 minutes or more, then that baby was defined as having hypertension in that study. For want of a more accurate description, many studies have also used this definition of hypertension. Honestly, at some point you just need a definition so that you can actually study it. Although a lot of other studies have also said that this may overestimate or even underestimate what the ideal mean arterial pressure is in premature babies. Another study suggested that any blood pressure, any mean arterial blood pressure less than 30 should be considered hypertension. In that small older cohort, basically babies that had blood pressures less than 30 had an increased risk of IVH and death. That mean blood pressure of 30 comes from somewhere. Researchers have suggested that under a mean blood pressure of 30, cerebral autoregulation is lost. So let me explain what cerebral autoregulation is. It's basically this mechanism in the brain that makes sure that the brain is always getting exactly the right amount of blood flow that it needs. So for example, if the pressure in the body goes up, then we don't want all that increased pressure going to the brain. So what are the blood vessels in the brain going to do? They are going to constrict so that not as much blood flow is going to the brain. The opposite is true as well. If the blood pressure goes down, then the blood vessels in the brain are going to dilate to try to make sure that we're still getting perfusion going to the brain. So cerebral autoregulation is obviously very important in everybody, especially in little babies when we're really worried about the risk of IVH. Obviously, if the pressure shoots up and we don't have any cerebral autoregulation and the blood vessels are just dilated, then suddenly there's gonna be a lot more blood going to the brain. Same thing if the pressure goes down and the blood vessels are constricted, we're not gonna have any blood going to the brain. So at a mean arterial blood pressure of 30, it was shown that cerebral autoregulation is lost. So at 29, even if the blood pressure is going down, then we are not getting more dilation of the blood vessels in the brain. Having said all of that and having taken care of tiny 22-week babies, it's very difficult to argue that we should always be trying to get a mean arterial blood pressure of 30 in these very little ones. In reality, it's highly unlikely that a blood pressure, let alone a mean arterial blood pressure, is going to account for what is true hypertension and decreased perfusion in a wide variety of gestational ages. And remember that blood pressures in all babies rise significantly in the first 72 hours of life. So with so much variability between babies and within the same baby, it's going to be very difficult to find that one number where we can all agree, yep, that's definitely hypertension. And as would be expected when we have a pretty flimsy definition of hypertension, studies have not consistently shown that having low blood pressure, for whatever definition that is, is definitively going to cause a worse outcome. And the ones that have shown that lower blood pressures are associated with increased risk of IVH or an increased risk of worse outcome, how can we say that it was actually the blood pressure that caused that? It could just be correlational. Whatever caused the baby to have a low blood pressure in the first place might also be the other thing that is causing the baby to have an increased risk of the IVH. So again, very difficult to tease out what we should be doing with the blood pressures. And then there's the complicating factor of actually starting vasoactive drugs. Some studies have suggested that just a baby being on the vasoactive drug is going to worsen the outcome. Again, is this because only the sicker babies were put on the vasoactive drugs and they were more likely to have a bad outcome anyway? So where does this actually leave us? Well, obviously we can't ignore blood pressures altogether. We all have to have some kind of vague idea of where we would expect blood pressures to be for each gestational age. For example, if you have a 25-weeker and the systolic blood pressure is 20, that is not going to be acceptable. In the same way that if you have a 25-weeker with a systolic blood pressure of 100, that is also not acceptable. In both those situations, something seems to be very off in the baby's bodies. So at least it seems that the mean arterial blood pressure, less than the gestational age, is a pretty good starting point. Or the other thing that you could do is you could look at age-appropriate blood pressure graphs for the age and for the gestational age of the baby and for how old the baby is as well, and compare your current blood pressure to the blood pressure on those graphs. But as we keep emphasizing over and over again, the blood pressure is only one aspect of whether the babies are getting the oxygen they need. What we really want to figure out is whether the babies are adequately perfused or not. And I am going to be talking about that a lot more in the next video. The other important thing that I want to add here is that a lot of these definitions have relied on the mean arterial blood pressure. Whereas obviously you realize that we also measure the systolic arterial blood pressure and the diastolic arterial blood pressure. So like we said in the first video, the systolic blood pressure is a good surrogate marker for the cardiac output. And the diastolic blood pressure is a good surrogate marker for the systemic vascular resistance. So it is possible, obviously, logically, that you can have a normal mean arterial pressure just because you've got a low systolic arterial blood pressure, but your diastolic is higher. Or you could have a really low diastolic blood pressure and your systolic is normal. So again, the mean arterial blood pressure does not tell you the full picture. For example, say that you have a 24-week infant and the systolic blood pressure is 35 and the diastolic blood pressure is 16, but the mean arterial blood pressure is 24. So in this case, the mean arterial blood pressure is actually pretty acceptable. But what do we actually have here? We have what's called a wide pulse pressure. So there is a big difference between the systolic blood pressure and the diastolic blood pressure. And what this looks like is that the diastolic blood pressure is on the low side. How do we diagnose a wide pulse pressure? You double the diastolic blood pressure. And if that is still less than the systolic blood pressure, then it is a wide pulse pressure. So in this instance, double 16, it's 32. It's still less than 35. So 35 over 16 is a wide pulse pressure. So in this case, even though the mean arterial blood pressure was okay, I would still worry about this patient because of the low diastolic pressure. And where can we see low diastolic pressures and a wide pulse pressure like that? PDA notoriously has wide pulse pressures as well as sepsis. So again, the mean arterial pressure is normal here, but I would still be worried about this patient. What about the other end of the spectrum? Say we have a baby with a narrow pulse pressure. You may also have a normal mean arterial blood pressure, but what's really happening is that you have a low systolic blood pressure and an elevated diastolic blood pressure. Or in this situation, you could end up having a narrow pulse pressure. So again, the mean arterial pressure in this case doesn't give the full story. Let me give you an example. Say you have a baby, a term baby, a 40-week baby that has bad HIE and PPHN. And say the systolic blood pressure is 40 and the diastolic blood pressure is 35 and the mean arterial blood pressure is 38. So this doesn't look too bad for a 40-week gestational age baby. However, what you've also got is a very narrow pulse pressure. The systolic is pretty low, which already implies a low cardiac output, already very concerning for the perfusion to the body. And the diastolic blood pressure is also elevated. So this looks like there's a very high afterload. The heart isn't being able to pump against these high pressures. This is altogether a very concerning situation that we would want to do something about even if the mean arterial blood pressure is actually okay. So when do we consider it a narrow pulse pressure? Well, in older children and adults, if you subtract systolic minus diastolic and that number is less than 25% of the systolic, then it's considered narrow. That is pretty complicated. In babies, if the difference between the systolic and the diastolic blood pressure is less than 10 millimeters of mercury, then we consider that to be a narrow pulse pressure. So what I'm trying to say is here, it's not just the mean arterial blood pressure, it's also the systolic and the diastolic blood pressures that you should be looking at. And like we said in the first video, really, we only care about blood pressures in the context of perfusion. So in the next video, we're going to be looking at how do we actually evaluate an infant to see whether the blood pressure and the perfusion genuinely are decreased. If you reach this far, then please like this video and then go watch the next one on how do we actually evaluate a baby with hypertension and what causes hypertension in a baby.