Neonatal Hypotension: Causes & Assessment in Preterm Babies

Hello, I'm Dr. Tala, a board-certified neonatologist, and today we are on part three of our series of videos on hypertension. So if you haven't seen the first two, then I highly encourage you to go back and watch those. And in those, we covered why we even care about low blood pressures, how do we define low blood pressures, and also how do we measure them in the unit. So today is part three, and we are going to cover factors that contribute to hypertension in babies, as well as how do we assess the hypertensive infant. So let's start with reasons why a premature baby may have a low blood pressure. Well, the first one is just immaturity of the heart. Really, every part of the baby's heart and cardiac muscle is more immature. The myofibrils are smaller. They are worse organized. They're not well organized. They've got less intracellular calcium in them. And so even if they do function adequately some of the time with any increased stress, they're unable to organize themselves to increase the cardiac output. Obviously in an older patient under any stress, our heart is capable of beating a lot stronger. The preemie heart is not as good at that. In many cases, this is compounded by the transition from intrauterine to extra uterine life. Remember in utero the heart just had to pump against this pretty low resistance placenta so not a lot of pressure there so it was pretty easy for the heart to pump and then when the cord is cut suddenly you've got this increase in systemic resistance and the heart now has to pump with a lot more energy against this higher pressure system so again if the heart is struggling to increase its cardiac output, then it's going to have a lot of problems if it suddenly sees a much higher pressure or a higher afterload that the heart now has to pump against. The PDA or the patent ductus arteriosus. As you all know, in term babies, the PDA normally closes within 12 to 24 hours of life. Whereas in preemie babies, the body doesn't receive the signals to get this ductus to close. So the ductus often stays open for a long time. And as the pressures in the lungs drop, then this PDA can now give a shortcut for blood coming out of the left side of the heart and taking a shortcut to the lungs. So blood coming out of the aorta that should all be going to the systemic circulation now sees this shortcut going to the lungs and more blood goes to the lungs. That means that even more blood is coming back to the heart. So what can often happen with a PDA is that the systolic blood pressure can be maintained. But because that ductus is open all the time, even during diastole, we can often see a lower diastole or a diastolic steal because some of that blood during diastole is then also going to the lungs. So with a PDA, we have an increased risk of a lower systemic vascular resistance which as we keep saying is normally manifested by a lower diastolic blood pressure HIE or hypoxic ischemic encephalopathy if a baby is stressed out in utero so during the actual delivery process if the baby isn't getting the oxygen that it needs then the fetus or the baby just before it's born starts releasing all these vasoactive drugs. For example, these neuroendocrine substances, including catecholamines, renin, angiotensin, and vasopressin. And these will all increase the systemic vascular resistance. And so after the baby is born, that systemic vascular resistance might actually be higher. So you might actually see an increased diastolic blood pressure. The problem is, though, is that If the baby's heart also took a hit as the baby was being born and the heart itself didn't get the oxygen that it needed, then the heart itself might go into myocardial dysfunction. So what do we end up having? We end up having a heart that doesn't pump well on top of the fact that this heart now has to pump even harder against this increased systemic vascular resistance. So this is a situation where we might see a lower systolic blood pressure, but a higher diastolic blood pressure. And the combination of these two in a situation like that is very bad. Even if your arterial blood pressure is okay, just because it really does imply that you're going to end up with way worse perfusion. Positive pressure ventilation. If a baby is on a ventilator or is having a lot of positive pressure application, then if this pressure is too high, then it can really increase the intrathoracic pressure and as the intrathoracic pressure goes up this may decrease the blood going back to the heart or decrease the venous return to the heart. So obviously if less blood is going back to the heart then the heart doesn't have that preload available to actually be able to pump that blood out to the body. So again this could affect the baby's blood pressure. A similar thing happens if a baby ends up with a tension pneumothorax. Obviously if the baby has a huge pneumothorax with increased pressures in the lungs or just outside the lungs then again that will push on the venous return and that will stop blood going back to the heart so kind of a similar mechanism of having these low blood pressures all related to just breathing issues in the baby sepsis and an inflammatory response so whenever there is a large inflammatory response, whether this is a bacterial infection or a viral infection, or whether it's not whether the baby just had a really invasive surgery, then a lot of inflammatory mediators are released like interleukins and tumor necrosis factor. And what these end up doing is that they can end up making the capillary beds and the systemic vascular circulation a lot leakier. So a lot of that fluid can just leak out from there, which obviously in itself is going to decrease the systemic vascular resistance. On top of that, we can often end up with dilation of these beds as well. So often with dilation of all these, the vascular bed in the periphery, you can also end up with a decreased systemic vascular resistance. So often this translates as having decreased diastolic blood pressure. Sometimes, especially in the early stages of sepsis, the heart will try to make up for the fact that the blood pressure is lower and it will try to increase the cardiac output to try to get more of that blood to the peripheries. And so at the beginning, your blood pressure might be okay because you've got a good cardiac output. Eventually with end-stage sepsis, you're also going to have a decrease in the cardiac output as well. F, relative adrenal insufficiency. The adrenal glands, just like every other part of a preterm baby are also pretty immature. So even in a stable baby, if the adrenal glands are able to produce the steroids and mineral corticoids and the glucocorticoids that the baby needs to maintain its blood pressure, with any amount of stress, the adrenal glands should then be able to produce even more steroids. And a lot of times these babies have a relative adrenal insufficiency. So they're okay most of the time, but with any extra stress, they're not able to produce that extra amount of steroids that they need. This is a problem, and sometimes we have to replace it. G, babies may end up hypovolemic from blood loss. So this could be in the delivery room or before the baby is born, like in a fetal maternal hemorrhage. It could be in an abruption, a placental abruption. It could also be after the baby is born, like a severe IVH or a severe subgaleal bleed. So all of these things are obviously going to predispose a baby to having hypertension because the baby's lost so much blood. And H, congenital heart disease. Remember, before a baby is born, we don't know if all the organs in the body were made anatomically correctly. So you may be assuming that the baby has hypertension from something else, but it really could be that there is a congenital heart disease that you haven't actually diagnosed yet. So even though congenital heart disease is a very rare cause of hypertension in a baby, it is the most common cause of cardiogenic shock in a baby where the heart is actually failing. So it is something that we do always need to consider in any baby with hypertension. Often a baby can have low blood pressure and decreased perfusion as a combination of more than one of these. And we'll come back to all of these different disease states in the fourth video where we discuss the management and treatment of babies with hypertension. Two, how do we assess the hypertensive infant? So really, when we are assessing a baby with low blood pressures, there's two things that we want to get to the bottom of. The first one is, is there something on the exam that actually explains why this baby might have a borderline low blood pressure? And two, are we not just worried about the low blood pressure? Are there also markers of decreased perfusion on this baby that would make us more likely to want to treat this low blood pressure? So let's go through those. So before we talk about whether we think that the baby genuinely has low perfusion or not, let's talk about things that you might be looking for on the exam to kind of help you diagnose whether there is something causing a low blood pressure or not. So just looking at the baby, does the baby have any signs of sepsis? Does the baby have abdominal distension? Are you worried about neck? Has the baby bled out from somewhere? Has the umbilical venous catheter dislodged or the umbilical arterial catheter dislodged and the baby is actually bleeding? Does the baby have a pneumothorax? Does the baby have a PDA, a murmur suggestive of a PDA? Is the rest of the exam consistent with an HIE? Obviously, all of these things should come into account when you're evaluating a baby with a low blood pressure. Like we keep alluding to, looking at the systolic and the diastolic blood pressures will also really help you determine exactly what is kind of the underlying pathophysiology. so like we keep saying 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 you can kind of see what's going on with both of those to be like is this a problem with a heart or is this a problem with a vascular bed so as part of your evaluation definitely look at the systolic and the diastolic blood pressure two things that i also want to bring up and that is the wide pulse pressure and the narrow pulse pressure. So we said in the previous video that a wide pulse pressure could be more indicative of a PDA or a sepsis type picture, whereas a narrow pulse pressure might indicate that the cardiac output is not good. So you've got a lower systolic blood pressure and a higher diastolic blood pressure. So this could happen in a baby with HIE and PPHN. So definitely a big part of your examination should be looking at both systolic, diastolic, as well as the pulse pressures. So maybe by looking at the baby, you'll have kind of like a faint idea of what is actually going on that might be causing a low blood pressure. So now the next thing that we want to do is evaluate the baby to see whether this low blood pressure or borderline blood pressure is actually affecting the perfusion. And we have many different parameters that we can look at to determine that. So let's start with the physical exam. Obviously, you can learn a lot about the perfusion status of the baby by looking at the physical exam. If the body doesn't have enough oxygen, then what the body is going to do is try to divert the perfusion and the oxygen that it does have to critical organs. So the blood is going to be sent preferentially to the brain, to the heart, to the lungs to make sure that the baby is going to stay alive. Whereas the blood will be shunted away from less critical organs. So it's going to be shunted away from the skin, from the kidneys and from the gut. So when we examine the baby, if blood has been shunted away from the skin, then that skin is going to look more pale. So there might be pallor. And we could also feel that the extremities are cooler just because there isn't as much blood going there. And we might see a delayed cap refill time. So a capillary refill time above four seconds. And often we check that on the baby's torso. Because less blood flow is going to the gut, you may see more symptoms of poor feeds, whether the baby's having more emesis or there's more abdominal distension. If the perfusion gets really bad, then that's when the brain starts getting affected. And so then you might see that the baby is getting more agitated or the baby could become really lethargic or just could become very floppy as well. So normally by the time you're seeing neurological signs, then this is kind of really bad because the blood flow is now affecting the brain as well. And this is a great table kind of going through all the different signs and symptoms that you might see if there is decreased perfusion. As you can see here, one of these is an increased heart rate or tachycardia. As we've already discussed, one of the things that a baby can do to increase its cardiac output, which is the blood flow per minute, is to increase how many times the heart beats per minute. So to increase the heart rate. And so tachycardia may be a sign of decreased perfusion and the heart is desperately trying to increase its cardiac output. Also realize that a lot of things may increase the baby's heart rate. So you really do have to know the baby's baseline heart rate, but also make sure that there isn't anything else going on. Is the baby suddenly a lot hotter or dehydrated for another reason? Is the baby on any other medications or anything that would also increase the heart rate. But yes, looking at the heart rate as well as heart rate variability is a very good sign of what's going on with the baby's perfusion. B, the urine output. Like we said, the kidneys are not a critical organ to keep the baby alive. And so if blood is being shunted towards critical organs, then the kidneys are not going to get that blood and oxygen. So often if there is decreased perfusion, then the kidneys are not getting the oxygen, they're not going to make as much urine. Remember though, in the first 24 hours of life, babies really have to urinate once. So it's difficult to kind of gauge whether they've got decreased urine output in the first 24 hours of life. After that, after day two or three or whatever, if a baby is peeing less than 1.5 mLs per kilo per hour, then this is concerning. This is especially concerning dependent on the baby's baseline. If the baby had been peeing four mLs per kilo per hour, and then that suddenly drops down, then yes, this could be a sign of decreased perfusion. Realize, though, that this is normally a pretty late sign of decreased perfusion. So if you're not going to do anything until the baby literally stops peeing, then you've probably already missed a very good window. C, what about biomarkers that you could get? Well, by definition, if the cells are not able to aerobically metabolize, so the cells don't have the oxygen that they need, then they start anaerobic metabolism. And in anaerobic metabolism, a byproduct of that is lactic acid. So lactate will build up if the cells are not getting enough oxygen. So an increase of lactate is also a sign that the cells are not perfusing adequately. Remember, though, that you need good blood flow to get that lactate from the cells back to where you're actually getting the blood sample from. So sometimes even if that lactate is building up you might not see it because it's still stuck around the cells but generally increased lactate is a sign that you've got inadequate perfusion obviously this also goes along with having an increased base deficit so if you have increased metabolic acidosis on your gas then yes by definition the cells are not getting the oxygen that they need and you're going to see a higher base deficit whether it's minus seven minus eight whatever the baby's baseline is it's just going to get worse. D, perfusion index. And this is the number that you see on the oxygen saturation probes, on the oxygen saturation probes when it's giving the perfusion index monitor. And basically what that is, is it's measuring the difference between the pulsatile flow and the non-pulsatile flow in the extremity. And based on that, it is creating a perfusion index, which should correlate with the cardiac output and with the SVC flow. So on paper, the perfusion index is a great way to see whether the extremities are adequately perfusing. In reality, as we all know, there are so many different things that actually affect the perfusion index in babies, mostly the way that the probe is positioned, how long the probe's been on, the movement of the baby. So yes, the perfusion index can be helpful if it is actually working, but often it's just one more thing that we're looking at amongst other factors that are helping us to determine the baby's perfusion. NEARS or near infrared spectroscopy. So about 15, 20 years ago, we all got extremely excited about this because we were all like, okay, this is going to be the best possible way to figure out whether a baby is adequately perfusing or not. This is going to be, it's non-invasive. It's just a probe that we can put on the baby and it will give us a much better idea of actually what's happening with the oxygen balance in the baby. In reality, that hasn't really panned out. In theory, it's great. What NEARS is actually measuring is the oxygenation in the tissue, which really is about 75% venous blood. So the logic is, is that if more oxygen is being extracted from that venous blood, which you would expect if not enough oxygen is reaching the body, then that saturation would go down. So instead of seeing a venous saturation of 60, you'd see a venous saturation of 40, which would imply that a lot more oxygen needs to be extracted. So we are in an oxygen deficit. So on paper, NIRS should be very helpful, especially for example, if we can do the cerebral NIRS to see exactly how much, what is the blood oxygen demand and supply going to the brain. In reality, there is a lot of inter as well as intrapatient variability with the NIRS. So it's very difficult to actually extract information from it. The times that it is helpful is when we do have the NIRS probe on for a prolonged period of time so we can really see trends in what's going on with a patient and whether that perfusion is really being affected. And F, an echo or an ultrasound of the heart. And this is something that should really be emphasized because in any evaluation or management of hypertension, we should be obtaining an echo to give us a lot more information as to what is actually going on with a patient. And maybe in the last 10 years, this is something that we've begun to emphasize the most, the importance of a targeted neonatal echo in all babies that do have hypertension to really try to figure out what is going on with the baby. There are now entire programs dedicated to training people in using targeted neonatal echoes so that they can go through all the different aspects and parameters of what's going on with the heart to really help us give the best information possible. And obviously, I'm not going to go through all of that now, but I just want to hit on some high points that we can learn by getting an echo. So one of the first things that we should be establishing is that the baby doesn't have a congenital heart disease, that we're not missing a tricuspid atresia or a total anomalous pulmonary venous return or something when we actually got a baby with hypertension. So structurally, you're looking at the heart, whether the baby anatomically has an abnormal heart, or if something else weird is going on. Maybe there's a tamponade around the heart. Maybe there's a cardiomyopathy. Maybe the baby has an arrhythmia or something. And we can catch all of on the echo. But even in the structurally normal heart, we can also get a lot more information about what's going on. For example, how is the filling of the heart? Does there seem to be enough blood in there? Or does it look like it's relatively empty? So does the baby need more blood? How hard is the heart actually working? Does it look like it's having to work very hard against increased pressures? For example, can we see all the signs of having persistent pulmonary hypertension on the echo? Or is the heart having to work against high systemic vascular resistance? What's the actual function like of the heart? Is there a good squeeze or is it just kind of like a really pathetic squeeze that's barely moving the left ventricle? As a field, TNE has absolutely exploded over the last decade, meaning that we have a lot more parameters that we can use to really help us guide exactly what that patient needs under that specific situation. so basically we're putting all of these factors together the physical exam the biochemical markers the echo everything else going on with this baby to determine what we should do to actually manage a baby with hypertension so in the next video that is what we are going to be covering how do we actually manage and treat a baby with hypertension and probably with low perfusion as well so if you've reached this far then please like this video and now go watch the next one thank you so much for being here