Overview of Methacholine Challenge Test
Asthma can be tough to diagnose because its symptoms aren’t unique to the disease. A helpful way to test for asthma involves checking how sensitive the airways are, and this testing is called “bronchoprovocation”. This process also allows healthcare professionals to measure the severity of airway problems in those who have been diagnosed with asthma.
The most common type of bronchoprovocation testing is the “Methacholine challenge”. In this test, a substance called methacholine is used to make the airways constrict, or get smaller. Methacholine is similar to a natural substance in our body called acetylcholine, but methacholine acts for a longer period.
In a “direct bronchial challenge”, substances are used to chemically stimulate the muscles in the airways. The level of these substances is increased gradually while keeping an eye on any reactions in the airways. The reactions are checked by monitoring any reduction in the amount of air a person can forcefully breathe out in one second, which is known as the FEV1.
The severity of the reaction is calculated based on the dosage (PD20) or concentration (PC20) of the substance that caused a 20% decrease in FEV1. Among all the substances used for bronchoprovocation testing, methacholine is preferred because it doesn’t cause many side effects that affect the whole body.
Anatomy and Physiology of Methacholine Challenge Test
Methacholine is a substance that triggers certain types of receptors in our body called muscarinic receptors. When it interacts with these receptors, present in the muscles of our airways, it causes them to contract or tighten, a process called bronchoconstriction. This action can somewhat narrow down the airways, making breathing more difficult.
Interestingly, methacholine doesn’t only affect airway muscles; it also encourages the cells that produce mucus in our airways to secrete more of it. This means it can increase mucus production in our airways.
Compared to a similar substance called acetylcholine, methacholine’s effects last longer. That makes it quite useful for a type of medical testing called bronchial challenge testing. This test measures how reactive or sensitive our airways are, which can be crucial in diagnosing certain breathing disorders.
Why do People Need Methacholine Challenge Test
The tests for asthma are usually recommended if you’re showing symptoms similar to asthma, or to check how effective your treatment has been. Although these tests can accurately detect asthma for many people, they are not perfect. This is because similar symptoms can be caused by other conditions like allergies, a lung condition called chronic obstructive pulmonary disease (COPD), bronchitis, and cystic fibrosis, leading to what’s called a “false positive” result.
In other words, the test might suggest you have asthma when you actually have one of these other conditions. So, the real value of these tests is in ruling out asthma – that means if you’re showing signs of having asthma and the test comes back negative, it’s highly likely that your symptoms are not due to asthma. This is particularly useful in cases where you might have other health issues, such as problems with your vocal cords, obstructions in your central airway due to tumors or polyps, or certain cases related to your work environment.
When a Person Should Avoid Methacholine Challenge Test
There are situations when a person may not be able to undergo a specific inhalation test, due to various health-related reasons. Here are some examples:
If a person’s FEV1 (Forced Expiratory Volume, a measurement of the amount of air a person can forcefully and quickly exhale in a single second) is less than 60% of what’s expected or less than 1.5 liters, it might stall their ability to take the test.
Similarly, if a person often fails to perform quality spirometry (a test to measure one’s lung function), this test could also pose challenges for them.
The test might also be too risky if a person had a heart attack in the past three months, suffers from uncontrolled high blood pressure, has an aortic aneurysm (a bulge in the wall of the main blood vessel that runs from your heart through your chest and abdomen), or if they recently had eye surgery.
It’s also important to note that the test could be risky for people who are at risk of increased pressure in the brain.
The test is usually safe for young patients with small airway sizes by using ways to measure oxygen saturation levels, including methods like pulse oximetry (which measures oxygen level in the blood) and measuring the pressure of oxygen directly through the skin.
Lastly, a drug called Methacholine used in the test falls under pregnancy category C – meaning its effects on pregnancy and breastfeeding mothers are still unknown, and whether it can create abnormalities in the fetus is yet to be studied. Hence, it’s crucial to consult your doctor about it if you’re expecting or nursing.
Equipment used for Methacholine Challenge Test
The equipment needed for a methacholine challenge test includes:
* A spirometer, which is a device used to measure how well your lungs are working
* A nebulizer, a machine that turns liquid medicine into a mist for you to breathe in. This is used to give you an aerosolized form of methacholine
* Various strengths of methacholine are also needed. Methacholine is a drug that makes your airways narrower, helping to see how your lungs react during the test.
Who is needed to perform Methacholine Challenge Test?
In the methacholine challenge test, which is a lung function test, the right team of medical professionals is very important. This is because the patient needs to breathe in a certain way for the test to produce reliable results. They will show you how to do special breathing exercises (spirometry), and measure your lung’s ability to hold and discharge air (FEV1). It’s important that you can repeat these exercises exactly, otherwise the test might not give useful results.
Also, because this test could potentially trigger sudden tightening of your airways (acute bronchospasm), it’s important that there are staff members present who know how to handle that. They will be ready with special equipment used for sudden emergencies (resuscitation equipment) as well as medications to help open up your airways (bronchodilators) should such a situation arise.
Preparing for Methacholine Challenge Test
Before a certain medical test, it’s important for the patient to stop taking some medications that can affect the results. These medications may make the airways in your lungs (bronchi) less likely to have a quick and abnormal reaction to substances (bronchial hyperresponsiveness). To fully clear these medications from the body so they don’t affect the test, it may take several cycles of the medications breaking down in your body (half-lives).
These medications include medicines that you inhale to help open up your airways (bronchodilators), which you would need to stop 6 to 48 hours or even one week before the test depending on the type; albuterol, a pill that helps relax the muscles around your airways, which should be stopped 12 to 24 hours before the test; inhaled and oral glucocorticoids, steroids that reduce inflammation in the airways, these may need to be stopped two to three weeks before the test; leukotriene modifying agents, another type of medicine to control inflammation, discontinued 48 hours before the test; theophylline, a medicine that helps to open up airways, stopped 12 to 24 hours before the test; cromolyn, a medicine that stops certain cells in your body from releasing substances that can cause asthma symptoms, stopped 8 hours before the test; and anti-histamines, medication that are usually used for allergies but can also affect this test due to their effect on a different nervous system (anticholinergic effect).
During the test, a substance called methacholine is given in increasing amounts or concentrations through a device that creates a mist for you to inhale (nebulizer), and at each step, a breathing test (spirometry) is performed. This creates what is called a dose-response curve, which helps doctors understand how your airways are responding.
It is important that before starting the test, ensure that emergency resuscitation medicines and equipment are available. Trained personnel should be present who can handle urgent medical situations like severe tightening of the airways (severe bronchospasm) or a sudden stop in effective blood flow to the body (cardiac arrest).
How is Methacholine Challenge Test performed
In simpler terms, this technique requires a person to adhere to certain breathing methods. The common method involves normal or relaxed breathing for about one minute. An alternative method uses deep inhales and exhales — like you’d do in a yawn — followed by holding your breath. However, normal, relaxed breathing has been found to be more effective in producing accurate results. To limit inaccurate results, doctors might also ask you to avoid taking in a full breath while doing the deep breathing method.
The person undergoes a test using a medication called methacholine. This medication is used in different strengths, from very diluted to more concentrated. The test begins with spirometry (a simple breathing test) to establish a baseline or starting point. Then, the person inhales methacholine in its most diluted form through a nebulizer — a device that changes liquid medication into a mist for easy inhalation. The doctor measures how much air a person can forcefully exhale in one second (a parameter called FEV1) at 30 seconds and then at 90 seconds after inhalation.
If the person being tested is suspected to have vocal cord dysfunction, the doctor might instruct him or her to take full, deep breaths, and the breathing will be analyzed during both inhale and exhale. Otherwise, the time for exhalation can be reduced from six seconds to about two seconds if the focus is only on FEV1. This process continues, with the concentration of methacholine increasing step by step, until FEV1 falls by more than 20%, or another parameter called specific airway conductance (SGaw) reaches 35 to 40%. Based on these results, the effective dose of methacholine (PD20 or PC20) that leads to a 20% drop in FEV1 is then determined.
A test is considered positive if PD20 is 200 mcg or less or if PC20 is 8 mg/mL or less. If SGaw is used, a positive test is indicated by 100 mcg or 4 mg/mL or less. A test is considered negative if PD20 is more than 400 mcg or if PC20 is more than 16 mg/mL.
In addition to this, the extent to which the airways react to an irritant (termed bronchial hyperresponsiveness) can be categorized based on PC20 results. The categories are normal (more than 16 mg/mL), borderline (4.0 to 16 mg/mL), mild (1.0 to 4.0 mg/mL), or moderate to severe (less than 1.0 mg/mL).
What Else Should I Know About Methacholine Challenge Test?
Methacholine challenge testing is a method used by doctors to check if a patient has asthma, especially when the diagnosis is uncertain. This test is very effective in confirming that a patient does not have asthma. However, if the test result is positive, it suggests the patient has asthma but it’s not conclusive. Sometimes, other conditions can lead to a false positive result. For example, about 7% of healthy people may test positive due to having sensitive airways, which could also hint that they might develop asthma in the future. Hence, these people should be monitored over time.
If a patient has symptoms similar to asthma but tests negative, other conditions should be considered. These conditions, like issues with the vocal cords, asthma triggered by specific workplaces, or blockages in the main air passages (such as a tumor or foreign body), can mimic asthma. A special breathing test can help to identify if a problem with the vocal cords is the cause.
If a patient’s symptoms seem like asthma and they test positive, doctors will want to watch their response to asthma treatment closely. In some cases, the initial breathing tests show blockage in the airway, which can add complexity to the diagnosis because the test may correlate strongly with this baseline blockage. However, if the patient’s breathing dramatically improves after taking a bronchodilator (a medicine that helps open up airways in the lungs) by 12% or more, doctors can likely confirm an asthma diagnosis without needing the methacholine challenge test.
