Neonatal Hypotension: Why Is a Baby’s Blood Pressure Low?

Do you all remember the days when we would arbitrarily decide that a baby had low blood pressures? And then we would start one normal saline bolus, maybe give another normal saline bolus, and then we'd start dopamine. If the baby still had low blood pressures, we'd march up on the dopamine until we reached like 20 mics per kilo per minute. And then if the baby was still hypertensive, we'd start dibutamine. Well, things have changed a little since then. Maybe not a lot, but they have changed little. And that's really what I'm going to be covering in these next series of videos on hypertension, the different ways that we evaluate and manage hypertension in babies now. Hi, I'm Dr. Tala, a board certified neonatologist, and so many people have asked us for these videos. So because it is such a large topic, we are actually going to cover the topic over four videos. In this one, I'm going to be talking mostly about the physiology, why we actually care why a baby would have a low blood pressure. In following videos, we'll go over the definition, the diagnosis of low blood pressures, how do we evaluate a baby with low blood pressures, causes, and then treatment of hypertension. So like I said, in this video, we're going to be going over a little bit of physiology. Why do we even care why a baby has a low blood pressure and bear with me a bit? There's a few equations in it, but if you understand this video, then the rest of hypertension is going to make perfect sense to you. And you will be able to manage your patients better at bedside just because you actually understand what is really happening with the baby. So ultimately we care about a baby's blood pressure because we want to make sure that the baby's blood pressure is sufficient to provide a flow so that the oxygen can get to all the required cells in the body. So as you all know, all the cells in the body, critical organs like the brain, the kidney, the lungs, as well as not as minute by minute critical organs like the kidneys and the gut and the skin, all of them require oxygen to function and to metabolize and to live. So it's critically important that we are constantly able to get oxygen to those cells. And we need good blood flow to those cells to make sure that those cells are getting the oxygen that they need. If the cells are not getting the oxygen they need, then this is defined as shock. Obviously, if they lack oxygen for a short period of time, then shock can be reversible. Whereas if they lack oxygen for prolonged periods of time, then it becomes irreversible and those cells will die. So obviously any time in the body we're worried about cells not getting enough oxygen, we need to be doing something about it before it becomes irreversible. So one of the key issues here is, is there enough blood flow going to the rest of the body? And a blood pressure is a key determinant of whether there is enough flow going or not. But obviously there are other factors involved of whether the cells are getting enough oxygen. For example, say you have a good blood pressure and you do have good flow, but the babies are super anemic. Well, if a baby is very anemic, then by definition, the baby's blood is unable to carry the oxygen it needs to the cells in the body. Remember that oxygen has very poor solubility in blood. So the contribution from the oxygen that's actually dissolved in blood or what we call the PaO2 is very negligible. So if you're extremely anemic, it doesn't matter if you're being put in like a hundred percent oxygen and you've got a PaO2 of 600, those cells are still not going to get the oxygen they need. Another situation could be is if you've got good flow and you've got a good hematocrit, but the baby for whatever reason is very hypoxemic. Maybe the baby has very bad lung disease. So even with a good blood pressure and a good hemoglobin, if the baby is hypoxemic, then obviously you're not going to get the oxygen to the cells that the cells need. So basically getting enough oxygen to the cells in the body is going to be dependent on three major things. One, the flow of the blood going to the cells. Two, the hemoglobin or what is your oxygen carrying capacity? And three, how well is that hemoglobin actually carrying the oxygen? So what is your saturations like? And this brings us to the equation for oxygen delivery. So oxygen delivery is flow times how much oxygen the blood is actually carrying. Or more mathematically, this is DO2 equals CO, which basically stands for flow. We'll come back to that, multiplied by 1.34 times hemoglobin times the saturation plus 0.003 times your PaO2. So let's break this down a bit. So your oxygen delivery is dependent on the flow and here the flow is represented by CO, which is the cardiac output. So the cardiac output is what's actually determining the flow. The middle part of the equation is basically the oxygen carrying capacity of the blood and it's dependent on how much hemoglobin you have and how well saturated that hemoglobin is. The coefficient 1.34 comes into it because each gram of fully saturated hemoglobin can carry 1.34 ml of oxygen. So that is the vast majority of the oxygen carrying capacity of blood. The flow times that middle portion. You can see that right at the end it's plus PaO2 times 0.003. So the amount of dissolved oxygen in blood times 0.003. This is a very minimal amount of oxygen that's contributing to oxygen delivery to the cells. Okay, so let's go back to cardiac output. What is the cardiac output? Well, basically it's how much blood the heart is pumping in a minute. And normally the cardiac output is measured in milliliters per minute, which as you all know, is the same as a flow. We measure a flow in the volume per minute. So looking back at that equation, obviously if the heart is barely pumping anything out, then the blood flow is going to go down. And based on that equation, obviously the oxygen delivery to the tissues is going to plummet as well. And I'm not going to go into this a lot further now, but also realize that there are a lot of factors that determine the cardiac output of a heart. A preload, so the amount of blood that goes into the heart, obviously is going to affect how much blood out that the heart pumps. If there's a high preload and a lot of blood going into the heart, then obviously the heart is going to pump out more blood. So a higher preload will affect the cardiac output. The afterload or the pressure that the heart has to pump against is also going to affect the cardiac output. If the pressure is really high, then the heart is not going to be able to pump as hard and that will decrease the cardiac output or decrease the flow. If the squeeze on the heart is really good, so the heart is really contractile and squeezing really well, then obviously that will increase the cardiac output. If the heart beats faster in a minute, then that's going to increase the cardiac output. So all other things being the same, if the heart is beating at 160 times per minute versus a hundred times per minute, then you're going to get a much higher cardiac output and therefore flow if the heart is beating at 160 times a minute. So where does blood pressure come into this? Well, obviously the cardiac output is a very big factor that determines a baby's blood pressure. If the heart is full and squeezing really well, then obviously that is going to affect the blood pressure. It's going to be a higher blood pressure if the heart is squeezing well and full of blood. But as you all know, there is another huge component that also affects blood pressure and that is the systemic vascular resistance. So the systemic vascular resistance is basically the resistance in all the vascular bed in the system. So everything that is not like the pulmonary vascular bed. This is very logical as well. If the blood vessels in the periphery are all squeezed super tightly and there's the same amount of blood going through them, then obviously that is going to increase the pressure. If those blood vessels are all dilated and the same amount of blood is going through them, then the pressure is going to go down a lot. Other factors can affect the systemic vascular resistance as well. For example, just how big that vascular bed is can affect the systemic vascular resistance. So for example, if the baby has a huge AVM, an arteriovenous malformation, and so a lot of the blood is actually being distributed into that, or a baby has a large PDA and a lot of the extra blood is also going into the pulmonary circulation, then obviously we are now distributing the same amount of blood in a much bigger space. That in itself is going to lower the systemic vascular resistance. And then the third way that the systemic vascular resistance can be affected is by the amount of volume that's going into that vascular bed. So obviously if the volume of blood that's in that vascular bled is much lower, then the pressure is going to be lower. And that could be because the baby bled out. It could be because the baby had a lot of dehydration from insensible losses, like a little baby lost a lot of fluid through its skin. Or it could be because the blood vessels themselves have become really leaky. For example, in sepsis or in neck, when those blood vessels have – when there's a large inflammatory response and a lot of fluid starts leaking out through those blood vessels, then obviously there's going to be less blood within the vessels and so the pressure is going to go down. So many things can also affect the systemic vascular resistance. So realize that the overall blood pressure is affected by two major components. The first one is the cardiac output. How well the heart is squeezing, how much blood flow the heart is producing. And the second factor is the systemic vascular resistance. And the mean arterial pressure is about, mathematically, the cardiac output multiplied by the systemic vascular resistance. So realize that the cardiac output is part of the oxygen delivery equation and it's also a part of the blood pressure equation. So what we're really doing is using that blood pressure as a proxy marker for the oxygen delivery to the tissues. It's not ideal but we can't measure directly the oxygen delivery to the tissues and so we are conveniently using our blood pressures as kind of a marker of how good that flow is. And I'll mention this briefly now but I'll talk about it more later. The different blood pressure measurements, the systolic blood pressure and the diastolic blood pressure, can give you a good idea about what is actually affecting that blood pressure in the first place. The systolic blood pressure is a good surrogate for the cardiac output or the contractility of the heart. So if anything, the systolic blood pressure is a good marker for what the flow is like in the baby's body because like we said, the cardiac output is really what's determining the flow of the blood towards the tissues. The diastolic blood pressure is a good surrogate marker for what's happening in the systemic vascular resistance. So if you have a low diastolic blood pressure, then you probably have low systemic vascular resistance. So whether the blood vessels are all really dilated or whether they're really leaky and all the fluid has leaked out. So you can see that the mean arterial blood pressure, which basically is the weighted average between the systolic and the diastolic, really doesn't give us as much information as either of those two other parameters. Even though, ironically, we're all used to using the mean arterial pressure the most in the unit and at bedside. So logically, going back to the blood pressure equation, which is blood pressure equals cardiac output times systemic vascular resistance, a baby can have a low blood pressure because their cardiac output is low, or they can have a low blood pressure because their systemic vascular resistance is low, or they can have a low blood pressure because both the cardiac output as well as the systemic vascular resistance are both low. Also realize that you may have adequate oxygen delivery to the tissues, even if you do have a low blood pressure. So for example, you could have a good or even elevated cardiac output, but you have decreased systemic vascular resistance. And so your blood pressure looks lower, but you're still getting the cardiac output and the oxygen delivery that you need. This would be a situation where the blood pressure does not correlate with hyperperfusion. The reverse could be true as well. You could have what looks like a reasonable blood pressure, but nowhere near enough oxygen actually reaching the tissues. So you can see that blood pressure is not an absolutely perfect marker for tissue perfusion. So basically, we're trying to use blood pressure as a proxy for whether these cells are getting the oxygen they need. And it's definitely not ideal, but it's kind of what we have. And so we have to use as much information in addition to the blood pressure to help us make that decision. So we will be discussing all of that in the coming videos. If you have reached this far, then please like this video and now go watch the next video on the definition as well as the diagnosis of low blood pressures. Thank you for being here.