Challenge Tests (Brochoprovocation)
Asthma challenge tests are also known as “bronchoprovocation.” These tests are done by intentionally exposing your airways to a trigger and using spirometry to measure how sensitive (“hyperresponsive”) the airways are. Airway sensitivity is a sign of asthma. The most common types of challenges are methacholine and exercise. Asthma challenge tests are done in specialized facilities that can handle emergencies. Medications are given at the end of the test to open up the airways.
Methacholine is an inhalable spray. This spray irritates the airways of people with asthma. Breathing it in causes your airways to start narrowing. Methacholine challenge tests are often done if your spirometry results are normal or near-normal, but your symptoms indicate that you have asthma.1
How is the methacholine challenge done?
The methacholine challenge is done together with spirometry. Increasing doses of methacholine are given until your FEV1 (forced expiratory volume) drops by 20%.2 Your FEV1 is the amount of air you can blast out of your lungs in the first second of exhaling, as measured with spirometry.
You do spirometry first to get a starting (baseline) FEV1.2 You breathe in the first dose of methacholine using a nebulizer. Then you repeat spirometry to see how your FEV1 changed. If it has not dropped by 20%, you will be given a slightly higher dose of methacholine. The test is repeated until your FEV1 drops 20%. Once your FEV1 falls by 20%, the test stops. You will be given a rescue inhaler (albuterol) to open the airways back up. Spirometry will be done again after you take albuterol to be sure the airways are opening. If you reach the highest dose of methacholine and your FEV1 has not dropped by 20%, you are considered to have “normal” airway sensitivity.2
What do the results mean?
The main result is the methacholine dose that caused your FEV1 to drop by 20%. This dose is called the PC20. The lower the dose, the more sensitive your airways are.
How are the results used?
The methacholine challenge is best for ruling out asthma.1,3 People whose airways do not narrow after inhaling methacholine are very unlikely to have asthma. If your airways are sensitive, it could be a sign of asthma. However, airway sensitivity is also a symptom of other conditions such as allergic rhinitis, cystic fibrosis, and COPD (chronic obstructive pulmonary disease). Your provider will compare the results of the methacholine challenge with your history of symptoms to figure out your diagnosis.
How do I prepare for a methacholine challenge?
Some medications make your airways less responsive and should not be taken before a methacholine challenge. Your provider will tell you which medications to stop and how long to stop them before the test.2
Exercise is a trigger for the majority of people with asthma and for some people without underlying asthma.4,5 There are a number of ways that exercise triggers asthma symptoms: water loss in the lungs, cold air, and rapid breathing, as well as allergens, irritants, and pollution during outdoor exercise.2,6
An exercise challenge test is designed to trigger water loss in the lungs and is usually done to confirm exercise-induced asthma.2 If an exercise challenge cannot be performed, indirect challenges can be done instead.7
How is the exercise challenge done?
Laboratory tests on a treadmill or stationary bicycle are the preferred way to do an exercise challenge.2 Air temperature and humidity can be controlled in the laboratory. However, the exercise challenge can also be done after a free run or other activity that caused symptoms previously.1
Heart rate is used to estimate exercise intensity.6 In the laboratory, this can be measured with an electrocardiograph or pulse oximeter.2 The goal is to get your heart rate up to about 80% of your maximum for 4 to 6 minutes.1 Some laboratories use more precise ways to measure exercise intensity by how hard you are breathing.
You will be asked to do spirometry before exercising to get a starting FEV1. After exercising, you will repeat spirometry every 5 minutes for 20 to 30 minutes.1,2 Usually, FEV1 is lowest 5 to 10 minutes after you stop exercising. The airways generally open up again, and symptoms go away 20 to 30 minutes later.1 If your airways do not recover on their own within 30 minutes or you are feeling breathless, you will be given rescue medications.2
An alternative to spirometry is measuring PEF (peak expiratory flow) before and after exercising. Because a peak flow meter is portable, it can be used in the environment that caused symptoms. However, this test is less reliable than spirometry.6
What do the results of the exercise challenge mean?
An FEV1 decrease of 10% or more after exercising indicates exercise-induced asthma.2,6
How do I prepare for an exercise challenge?
Wear running shoes and comfortable clothing to the laboratory. Do not eat a heavy meal before the test, and avoid vigorous exercise in the 4 hours prior to the test. Medications that make your airways less responsive should not be taken before an exercise challenge or an indirect challenge.7 Your provider will tell you which medications to stop and how long to stop them before the test.2
What are alternatives to an exercise challenge?
Indirect challenges are alternatives to the exercise challenge.6 In an indirect challenge, you are asked to inhale a mixture that simulates exercise conditions. Two common tests are:
- Eucapnic voluntary hyperpnea challenge
- Mannitol challenge
For a eucapnic voluntary hypernea challenge, you are asked to inhale dry air with added carbon dioxide (CO2).7 You will be asked to do spirometry right after inhaling the mixture, and then at regular intervals for up to 20 minutes. An FEV1 drop of 10% or more is considered abnormal.7
For a mannitol challenge, you are asked to inhale a mannitol spray designed to draw water out of your airways.7 You start with a low dose of mannitol and are asked to do spirometry 1 minute later. If your FEV1 does not drop at least 10%, you will be given a slightly higher dose. This is repeated until your FEV1 drops by 10% or more. That dose is repeated, to see if your FEV1 drops by 15% or more. The test continues until your FEV1 drops by at least 15% or you reach the maximum dosage. The lower the dose that caused the 15% drop, the more sensitive your airways are.