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Bronchiolitis pathophysiology

Created by Matthew McPheeters.

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Video transcript

- [Voiceover] Often times, I've found that the easiest way to understand the presentation of a disease, is to first have a good understanding of the pathology of that disease. So in this video, we're going to be talking about the pathophysiology of bronchiolitis. Now, pathologically speaking, in its most simple form, bronchiolitis is just inflammation of the bronchioles, and that inflammation of the bronchioles is demonstrated here in this diagram. And bronchiolitis is a very common respiratory tract infection especially in children under the age of two years old, and it's caused by a viral infection. And the most common virus that causes bronchiolitis is RSV. And RSV accounts for more than 75 percent of the cases of bronchiolitis, but there are a couple of other viruses that can cause the disease, and there are two that are important to note. And those are metapneumovirus, which I'll abbreviate MPV, as well as parainfluenza virus. So bronchiolitis really has two pathologic stages. It has the early stage, which effects the upper airway, and it has a later stage, that effects the lower airway. And in the early stage of bronchiolitis, RSV is taken up by the upper airway here in blue, and it causes infection of the epithelium, and the epithelium is the cells that line this cavity. And it subsequently develops inflammation, and it's this inflammation of the upper airway that's responsible for the early signs and symptoms of bronchiolitis which are things like nasal congestion, and a runny nose. Now bronchiolitis doesn't just stop here, because if it just caused inflammation of the upper airway, it would just be the common cold, which is another name for a viral upper airway infection. And in bronchiolitis, the virus actually makes its way down the respiratory tract, down the airways here, and it causes infection and subsequent inflammation in the bronchioles. Now let me bring in a diagram of a bronchiole here to give you a better idea of what's going on. So bronchiolitis is inflammation of the bronchioles, so there's swelling of this epithelial layer in the bronchiole. So you can see that the swelling from the inflammation due to this infection has caused the airway to get narrower. And I'm going to draw that in over here as well. And there's one other process that I want to talk about that can happen. And what happens with inflammation is that sometimes fluid can be secreted from the inflammatory cells and it kind of just collects, and gets almost gunked up in the bronchiole. And this is known as a mucus plug. And you can see that this mucus plug causes a further narrowing of the airway, and it's this inflammation and subsequent narrowing of the airways that results in the classic lower airway signs and symptoms of bronchiolitis. And these are wheezing, and air trapping. Now in order to understand wheezing and air trapping, let me describe the pathological processes in a little bit more detail. Now in this first diagram, let's go through the mechanism of wheezing. And you can see that this inflammation here of this bronchiole has caused the airway to become narrower. And in order for a certain amount of air to travel through that bronchiole in the same amount of time as if it wasn't narrowed, that air is going to have to travel a lot faster. And this air travels faster as it goes in, and then also as it goes out. Now the sound of this air rushing through the bronchiole, can be heard over the chest with a stethoscope, and the sound it makes is known as wheezing. And wheezing is described as a high pitched musical sound, and it's mostly heard during expiration. But what exactly does that sound like? Well, let me show you... (wheezing noises) All right, so the pathologic mechanism behind wheezing is relatively straightforward, it's just air rushing through a narrowed space. But air trapping, another characteristic sign of bronchiolitis is not quite so straightforward. All right, so I mentioned that the epithelium of the bronchiole becomes inflamed and infected. And over time, some of these inflamed cells can die. And let me just draw these in as kind of dying cells. And these dying cells actually fall back into the alveoli, and they just form this mass of dead tissue. And this mass of dead tissue is also known as necrotic tissue. And what happens is when the child breathes in, the air that's coming in through the bronchiole pushes that necrotic mass deeper into the airway into the alveoli, and that's okay because the air is able to come around it, and the alveoli expands as it normally does and it fills with air. However, as soon as the child tries to breathe out, that necrotic mass is then pushed back up against the airway and it blocks the airway. And the air isn't able to exit, and so the alveoli stays enlarged. And then the child takes another breath in, and the same thing happens, the necrotic mass is pushed deeper away from the airway into the alveoli, and air enters the alveoli, and the alveoli starts to expand and then the child tries to breathe out, and once again the necrotic mass goes back and it plugs that airway. And so you can see that over time, this necrotic mass in a sense, turns into a one way ball valve because there's air to enter the alveolus, but it can't get out and it causes the alveolus to expand. And this happens in alveoli all over the lungs, and as a result, the lungs become hyper expanded, and this phenomenon is known as air trapping. And if it gets really bad, it can actually present as a physical exam finding in that the child's chest appears larger than it normally would, and it's also visible on chest x-rays, in the form of hyper expanded lung fields. And as this happens, the alveoli is still trying to exchange oxygen between the environment and the blood. But because of this one-way phenomenon, the old air isn't able to get out of the alveoli, and you get this mixing effect. And so you have a decreased oxygen concentration in the alveoli, and so not as much oxygen is able to be exchanged and transported into the blood. And this decrease in oxygen in the blood is known as hypoxemia. So as this oxygen level in the blood decreases, the body has two main mechanisms to compensate for it, and so the child will increase his or her heart rate, as well as his or her respiratory rate. And this makes sense, because if the blood has a decreased oxygen concentration, in order for the tissues to get the same amount of oxygen over the same amount of time, more blood has to go through that tissue, and so the heart rate is increased. And also, the respiratory rate is increased, to try and get as much oxygen into the blood as possible. But unfortunately, increasing one's heart rate, as well as one's respiratory rate, consumes a lot of energy. And this increased utilization of energy may cause a child to become fatigued. And this combination of hypoxemia and fatigue, is not a very good combination. And because of this, the brain actually starts to shut down different parts of the body in order to conserve the energy and oxygen it has for the most vital functions. And this process is known as lethargy. And so a child who is lethargic is not just fatigued and sleepy, they are also not really arousable. If you try and wake them up, they might kind of open their eyes, but they fall right back asleep. And any child that has one of these symptoms, should probably be seen by a doctor, because they may require treatment in the hospital.