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- Now bronchiolitis is a common respiratory tract infection in young children. But what exactly do I mean by this and how does it differ from a common cold? Well, in order discover this, let's break down the word. Bronchiolitis can be broken down into two parts. Bronchial and itis. Now bronchial refers to an anatomic location in the lungs. So to describe this, let me just bring in a cartoon of the respiratory tract. Up here in blue is the oral and nasal cavity in the upper part of the throat. That's our upper respiratory tract. And then as you go down you encounter the airways and the lung tissue itself. And so this is the lower part of the respiratory tract, the lower respiratory tract. And the bronchials, as you go down through the trachea here into the bronchi, these kind of ending branches out here, these are bronchials. So you can imagine that bronchiolitis has something to do with the bronchials. But what does itis mean? Well generally speaking in medicine, any time you see a word that ends in itis, it refers to inflammation. Now what do I mean by inflammation? In order to describe inflammation, let me use an example here. Imagine this guy over here hits his head. Now what's gonna happen over time? Well the area of his head underneath where he got hit, it's gonna kind of bulge up. And you're gonna get a little swelling. So you have some swelling, and it's gonna turn red, and the area's gonna become painful, and even a little bit warm. And those are the characteristics of inflammation. Pain, tenderness, warmth, and redness. And what inflammation is, it's the body's reaction to injury. So in bronchiolitis, you get that same kind of inflammation, but in the bronchials. Let me show you that. So here we have a zoomed-in image of the bronchials here. So in bronchiolitis, you're gonna have inflammation of these. So we now know that bronchiolitis is inflammation of the bronchials, but what causes bronchiolitis? Well bronchiolitis is a viral infection. And the most common virus that causes bronchiolitis is known as respiratory syncytial virus. But that's a little bit difficult to say. So we're just gonna call it RSV. Now RSV causes bronchiolitis by entering the respiratory tract. And it can enter the respiratory tract either from someone who's infected who coughs and then coughs the virus out into the air and then this guy over here will inhale it, or that virus can be sitting on something and this person may touch it with their hand and get the virus on their hand, then touch their mouth, and that is the way they get the virus into their respiratory tract. And the first part of bronchiolitis is actually an infection and inflammation of the upper airway. So even though bronchiolitis is inflammation of the bronchials, it starts out as this infection and inflammation of the upper airway. And this makes sense because the first signs and symptoms of bronchiolitis are actually a runny nose and nasal congestion. Now at this point if you're thinking bronchiolitis just kinda looks like the common cold, you'd be right because the common cold is a viral infection of this upper airway, just like what we just happened. But in bronchiolitis, the virus actually makes its way down from the upper respiratory tract down into the bronchials, and it causes the infection down here. And so that's what differentiates bronchiolitis from the common cold. And it's this infection and subsequent inflammation of the bronchials that produces a few more symptoms that are a little more characteristic of bronchiolitis. And these symptoms are cough and wheezing. Now a cough can occur in an upper airway infection because you have some secretions from the infection that fall down the airway and that irritates the airway so you cough it up, but wheezing only happens if you have disease in this lower airway. So wheezing is a very important symptom for bronchiolitis 'cause it lets you know that something's going on in the lower respiratory tract. Well these symptoms don't really seem that bad, so what's the big deal with bronchiolitis? The big deal is that sometimes in bronchiolitis the child's lungs have a hard time getting oxygen, and that can cause some problems. So let me show you how that happens. Now imagine that this part right here is where oxygen in the air is exchanged in the body. And in order to get there, the air is gonna have to travel through the bronchials. But in bronchiolitis, sometimes this inflammation gets so bad that these bronchials are blocked. So the air can't get into the lung tissue itself, and you have no oxygen there. And if the oxygen in the air can't reach the lung tissue, it can't get into the blood. And this is known as hypoxemia. And the child's body does two things to try and compensate for this low oxygen. And that's to increase his or her heart rate and increase his or her breathing rate. And this increase in heart rate and breathing takes up a lot of energy. And over time the child can get kind of tired and gets a little bit sleepy. And sleepiness is a sign in bronchiolitis that the child's getting really sick. So any time any of these symptoms like increased heart rate or a child just having a hard time breathing or becomes really sleepy, any time those are present it's probably time to call the doctor because the child may require treatment in the hospital. So now that we know that bronchiolitis is inflammation of the bronchials that's caused by viral infection, what are some of the risk factors for developing bronchiolitis? Well since bronchiolitis is a viral infection, the most common risk factors have to do with being in contact with the virus. So being around other children that are sick is a big risk factor. So I'm gonna just label that as sick contacts. And another risk factor that has to do with exposure to the virus is season. And RSV is most common in the fall and winter months. Now this is a little misleading because the reason we think RSV is most common in the fall and winter is because children tend to be inside and in kind of more closed spaces, so they're in closer contact with the sick contacts. So this may actually be just a representation of sick contacts, but it's good to remember that children are most commonly infected with RSV in the fall and winter. But what are some of the other risk factors? Well since the RSV infection that leads to bronchiolitis first starts in this upper airway, if a child's immune system isn't working properly that infection is more likely to start here. So I'm gonna put poor immune system. Now there's also some risk factors that have to do more with the lower respiratory tract. The first one I want to talk about is age, because bronchiolitis is most common in children under the age of two years. Now the next one is prematurity. So any child who's born premature, especially less than 35 weeks gestation, their lungs just aren't quite as developed as children who were born mature. And part of this is that their airways are just narrower. And if you imagine that this inflammation is occurring in the airways and the bronchials, that's gonna decrease the size of the airway and air is gonna have a harder time getting through. So if you have a narrower airway to start with, less inflammation causes this blockage of air flow and results in hypoxemia, like we talked about earlier. And another risk factor that affects the lower respiratory tract is pre-existing lung disease. And the last one I want to mention is smoke exposure. And smoke exposure causes small amounts of injury to the lower airways and lower respiratory tract here. So children exposed to smoke will already have a little bit of inflammation. And this pre-existing low level of inflammation due to second-hand smoke exposure makes them more susceptible to infection of this area. So to recap, bronchiolitis is a very common respiratory tract infection in children, especially under the age of two years old. And if you can remember to break down the word, you can remember what's going on. Remember, it's inflammation, or itis, of the bronchials, which are the end part of the airway before we get to the lung tissue. And this inflammation is caused by viral infection, mostly commonly RSV. It presents with runny nose, nasal congestion, cough, and wheezing.