If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Asthma diagnosis

Created by Amy Fan.

Want to join the conversation?

Video transcript

- Diagnosing asthma can depend on a particular doctor's practice. I've seen it diagnosed with the history, and if the asthma medications work for them, then they have asthma. But to be more methodical about it, let's talk about the gold standard tests that can be done to say, yes, this person has asthma. So, the first gold standard test, I actually find kind of cruel and unusual to do this to people, but it is the best way to tell. We call it the methacholine test. Methacholine. Now, this substance is something you inhale, and then we observe what happens. Methacholine test. So, if we have our normal airway here. The lumen is nice and big, and our smooth muscle is just floating around, not constricting too much. Now, when we give the methacholine, everybody's smooth muscles will contract. Everybody's airway will get smaller. So, for a person without asthma, after the methacholine, they might look like this. You can tell the lumen's a little bit smaller. It's going to be a little bit harder to breathe. If this person has asthma and their lungs are predisposed to really react, then their lumen might be this big, and the smooth muscle is going crazy constricting this airway, making very little room for the air to go through, and you have asthma. So, this is just testing how your lung responds to something that makes it constrict, because asthma just describes the tendency of the airway to react this way. So, if the methacholine can get your lungs to do this, in the small airways, then that is a positive test for asthma. So, how exactly do we know that the lungs have done this? And we can see this through a group of tests, and this is done with or without methacholine. It's a very standard test to do when people have all kinds of respiratory issues. We call it the PFTs or pulmonary function tests. So, pulmonary refers to the lungs. We're testing the function of the lungs. Pulmonary function test. All right. So, what happens is we have our lucky patient. Let's make him orange. We have our lucky patient with hair. I always draw hair on my people. So, Mr. Bob here says, I have trouble breathing. So then, pulmonary function tests. We give him a mouthpiece that he puts in his mouth, and then when he's breathing in or out a machine records everything that happens, the volume of air, the speed of air. Let's call this our PFT machine. So, the real thing is a lot more sophisticated than my rendition here. And I've actually done this test. They thought I might have had asthma when I was a kid, and I had to do this test, and let me tell you, it is not fun. What they do is, they tell you to take a gigantic breath in, as much as you can hold, and then when they tell you to, blow out as hard as you can and for as long as you can, until you feel like there's nothing left in your lungs, and you're going to die. And that's what's necessary to do this function test. Now, my drawing here is very misleading. So, I drew this like that to show you that the breath is rushing out very fast, but the fact is, this mouthpiece catches all of the air. So, the air actually goes in here. I'm going to erase all these things out here. So, just remember that the mouthpiece catches all this air, and pulmonary function test depends on the fact that all the air is isolated into our machine. Okay, there's some math involved here. So, believe it or not, even though you're breathing out for as long as you can, it can last a long time, depending on your lung capacity. Most of the air should come out in the first second of your breathing out. So, we call this the FEV-1. The one stands for the one second. So, the amount you breathe out in the first second divided by the full vital capacity, which is all the air you ever breathe out until you gave up, that's the full vital capacity. The ratio here, one divided by this, should, in a normal lung, be equal to or greater than 80%. So, more than 80% of the air that you eventually can ever breathe out should come out in the first second. This tells us you have a healthy lung, without obstructive disease. But in asthma, I'll put a little A here, in fact, in all obstructive diseases where breathing out is a problem, the FEV-1 divided by the full vital capacity will be less than 80. So, actually some people say less than 75, but you know by this ratio being diminished that something is wrong with this exhale, that less than 80 or 75% of the air came out in the first second. That means the lungs are not elastic enough or for some reason the air's not getting through fast enough when you're putting all that force behind it to breathe out. So, this forced vital capacity test with the ratio is one of a group of tests that's done with this machine. We call this spirometry, and spirometry can be done with the methacholine challenge or without at any time. Mathematically, these numbers here really tell us a lot about how your lungs are behaving when you're exhaling. So, next let's talk about the peak flow test. The peak flow actually is not just used to diagnose asthma. This is used by people who have asthma, to monitor their disease. Peak flow. And this standard is created for each person. So, they know for your lungs what is the 100%. So, knowing your 100%, every time you do the peak flow test and you exhale, the machine can tell at what percentage of your maximum you're at. So, imagine this person already has asthma. Sometimes you can have a little bit of reaction going on, and you can't really feel that out of breath yet. So, this is just like a traffic light. When the machine gives you green, that means your at 80 to 100% of your max. So, you can keep going about your day. You don't need any medication. You're doing fine. And then the next one, of course, like our traffic light, is yellow. So, yellow means you have to be careful. At this point, the person probably has some symptoms. They can feel short of breath. They can be coughing, maybe a runny nose, or they're sick with something else, and this gives you about 50 to 80% of your max. You should probably take some medication to open up the airways a little bit. Try to get back into the green. So, take medication, wait 10, 15 minutes, take it again and see. Try to make sure that you're back in the green. And, of course, red means stop, right? So, red is below 50% of your max, and if you're at below 50%, this person should go to the hospital. They need some more aggressive medication. It's basically an alarm signal, that you should do something about your asthma right now, and you probably don't even need the machine, because if you're at below 50%, this person's going to feel pretty crummy. Their chest will be tight. It'll be hard to breathe. So, gotta go do something about it. Now, there are other tests that might be less specific to our cause, but they're still used. So, sometimes people will get an x-ray, and people with asthma should have a normal x-ray. So, why do we still do it? Because you don't always know that they have asthma. The x-rays give people a chance to look at the lungs, to make sure there's nothing else going on, because symptoms can be similar. So, this person can have pneumonia, can have an infection in the lungs, with fluid and consolidation. That's why they're having shortness of breath. Or they could have a foreign body, and that's why they feel like they can't breathe. There could be fluid. There could be mucus. They could have a pneumothorax, where the lung has partially collapsed. So, all these things can be seen on an x-ray. So, sometimes when the person comes in with shortness of breath, we do an x-ray to rule out other things that perhaps can be treated differently. Sometimes people get a stress test. Similar to a cardiac stress test testing your heart, this one tests your lungs. Because stress tests, some people have stress or exercise-related asthma. So, this person walks on a treadmill, and we see when they're sweating and working hard, does that make their asthma, does that make their breathing worse, because that could point to exercise-related asthma. And lastly, I want to talk about a test called the nitric oxide test. Nitric oxide is a gas. So, it's made of nitrogen and oxygen, and if we wanted to do the chemical symbol of it, nitrogen is an N, and oxygen is an O, so NO is a nitric oxide test. We do this when we think this person has asthma, but we want to see if asthma treatments will be good for him. What the nitric oxide test does, is it tells us if an inhaled corticosteroid, which is a standard treatment for asthma-- I'm trying to draw an inhaler here, so an inhaler you push a button here, and this mist comes out, that the person with asthma will breathe in, and it will help them feel better. So, there are steroids in this inhaler. Just sometimes we don't know if this person would benefit from steroids, or if they would, how much to give. And that's where nitric oxide can tell us. By breathing in nitric oxide, which is something that dilates, dilates the airway, relaxing the smooth muscles, then if the nitric oxide works, then the corticosteroid is more likely to work on them. So, we don't do nitric oxide as a therapy. Nobody's going to be breathing in nitric oxide every day, but in the short term, now what happens when you breathe in nitric oxide can help us predict what would happen when they breathe in the steroid treatment. So, this is diagnostic but also helping us to treat.