Asthma Diagnostics: The Methacholine Challenge
Like most asthmatics you have probably experienced the ever "fun" methacholine challenge. For those of you asking yourself, what this is, a methacholine challenge also known a "bronchoprovocation" test .They are performed to assess airway "hyperresponsiveness".
Essentially it is used in conjunction, with spirometry and the diagnostic skills of your doctor to rule out asthma. It is considered to better at ruling out a diagnosis, than confirming one. Most patients with asthma are thought to have some degree of bronchial hyperresponsiveness. "However, bronchial hyper responsiveness is also seen in a wide variety of other diseases, including smoking-induced chronic airway obstruction (COPD), congestive heart failure (CHF), cystic fibrosis, bronchitis, and allergic rhinitis."1-3
A methacholine test may not be for everyone.
This test is contraindicated in specific circumstances and patient population. In general, the conditions are ones in which either the quality of the test or the patient may be at risk. This chart from the ATS summarizes some of the them. It is important to note that the level of lung function that at which the test is contraindicated in controversial.
Contraindications for methacholine challenge testing
Severe airflow limitation (FEV, < 50% predicted or < 1 .O L)
Heart attack or stroke in last 3 mo
Uncontrolled hypertension, systolic BP > 200, or diastolic BP > 100 Known aortic aneurysm
Moderate airflow limitation (FEV, < 60% predicted or < 1.5 L)
Inability to perform acceptable-quality spirometry
Current use of cholinesterase inhibitor medication (for myasthenia gravis)4
I have heard that I have to prep for this test, how do I prepare?
Preparing for this test usually involves withholding bronchodialators or medication that can reduce airway hyperresponsiveness. The amount of time that you may withhold these medications, may differ from centre to centre. You should be provided by your care team with specific instructions.
What actually happens during this test?
You are given increasing increments of medication (methacholione or histamine) until your FEV1 decreases by 20%. The test is then stopped and your symptoms are reversed though the administration of a bronchodilator.
I have a love, hate relationship with this particular test. I have fairly hyperresponsive airways and I do not tend to have make it very far. As my FEV1 drops usually by the third round of methacholine. It is a really safe test, although I tend to not feel that well after it. It is important to remember that everyone is different and that you should discuss any concerns you have with your care team.
Has asthma changed your exercise routine?