Norm of Methacholine Challenge Test (Bronchial Challenge Test)
Usage of Methacholine Challenge Test (Bronchial Challenge Test)
Most useful in excluding the diagnosis of asthma. Use in diagnosing asthma is most effective when pretest probability of asthma is 30%–70%. (American Thoracic Society, July, 1999;). Often used after a negative exercise challenge test when asthma is suspected. This test with altered cutoff points of 10% or 15% may be used in chronic asthma to monitor response to therapy.
Description of Methacholine Challenge Test (Bronchial Challenge Test)
The methacholine challenge test involves measurement of lung volumes before and after inhalation of methacholine chloride, a bronchial constrictor. This test is useful in demonstrating bronchial hyperreactivity (BHR), which is a characteristic of asthma. Clients with symptoms suggestive of asthma often have normal resting pulmonary function test results but are more sensitive to the bronchoconstrictive effects of methacholine than healthy people. The test may be performed with tidal breathing and/or with deep inhalation, and it uses the lowest concentration of methacholine needed to achieve a 20% reduction in FEV1 (also known as provocation concentration 20 or PC20).
Professional Considerations of Methacholine Challenge Test (Bronchial Challenge Test)
Consent form IS required.
Bronchospasm and its potential complications. This risk is greater in small children than in adults.
Not appropriate for those under school-age. Also contraindicated when severe airflow limitation is present (e.g., FEV1 <50% predicted or <1 L) or in clients with a myocardial infarction or stroke within the prior 3 months, in clients with known aortic aneurysm, and in clients with uncontrolled hypertension. Relative contraindications include nursing mothers, pregnant clients, clients taking cholinesterase inhibitors for myasthenia gravis, clients with moderate airflow limitation (e.g., FEV1 <60% predicted or <1.5 L), and in those clients unable to perform spirometry using proper technique.
- Verify with physician and instruct client to avoid the following medications before the test: short-acting inhaled bronchodilators (8 hours), medium-acting inhaled bronchodilators (24–48 hours), oral bronchodilators (24–48 hours), cromolyn sodium, nedocromil, hydroxyzine (3 days), cetirizine (3 days), and leukotriene modifiers (24 hours).
- Instruct client to avoid caffeine or chocolate the day of the test.
- Just before beginning the procedure, take a “time out” to verify the correct client, procedure, and site.
- Obtain nebulizer and methacholine chloride. Prepare increasing concentrations of methacholine. Vials should be at room temperature for testing.
- a. For the 2-minute tidal breathing protocol, use the following concentrations: 0.031 mg/mL, 0.0625 mg/mL, 0.125 mg/mL, 0.5 mg/mL, 1 mg/mL, 2 mg/mL, 4 mg/mL, 8 mg/mL, 16 mg/mL.
- b. For the five-breath dosimeter protocol, use the following concentrations: 0.0625 mg/mL, 0.25 mg/mL, 1 mg/mL, 4 mg/mL, 16 mg/mL.
- Verify that resuscitation equipment (including oxygen, nebulizer, sphygmomanometer, pulse oximeter) and personnel able to manage severe bronchospasm are immediately available. Medications to treat bronchospasm should also be immediately available and include epinephrine, subcutaneous atropine, albuterol, and ipratropium.
- Inform client that cough or mild chest tightness may be experienced and that occasionally severe breathing problems occur, but that equipment and personnel prepared to handle these complications are readily available.
- Position the client in a seated position.
- Baseline spirometry is performed and target FEV1 is calculated. The target would indicate a 20% fall in FEV1.
- Two-minute tidal breathing protocol:
- a. A nose clip is applied and the client breathes the lowest of five concentrations of methacholine through a nebulizer for 2 minutes and then the nebulizer is removed.
- b. The FEV is remeasured in 30 seconds and again in 90 seconds after stopping the nebulizer. The highest FEV1 is recorded.
- Five-breath dosimeter protocol:
- a. The client inhales through the nebulizer slowly and deeply for five breaths. Each inhalation should take about 5 seconds.
- b. The FEV is remeasured in 30 seconds and again in 90 seconds after the end of the fifth inhalation. The highest FEV1 is recorded.
- If the FEV does not fall at least 20% from baseline, the test is repeated with a higher dose of methacholine. Twofold increases in concentration are used for the 2-minute tidal breathing method and fourfold increases are used for the five-breath dosimeter method. These steps may be repeated as needed to achieve a 20% fall from baseline FEV, until the highest concentration of methacholine has been reached.
- If the FEV falls at least 20% from baseline, the test is stopped. Record client symptoms. Then administer inhaled albuterol and repeat the test.
- Failure to achieve a 20% reduction in the FEV is considered a negative test.
- Monitor respiratory status.
Client and Family Teaching
- Symptoms suggestive of asthma include cough, chest tightness, and dyspnea.
Factors That Affect Results
- Inability to follow directions and comply with instructions yields invalid results.
- Inhaled heparin may have an inhibitory role on methacholine bronchial challenge, possibly through a direct effect on smooth muscle.
- Clients with mild airway hyperresponsiveness may demonstrate false-negative results with the deep inhalation method of this test.
- Inhaling the methacholine too quickly will reduce measured PC20.
- False-negative tests occur in asthmatic clients with a PC20 greater than 8–25 mg/mL.
- This test has a positive predictive value of 60%–88% and a negative predictive value of 100% to rule out asthma.
- Some experimental work is being conducted to evaluate the usefulness of a skin-prick test with methacholine in shortening the MCT. One study found that a negative skin-prick test reduced the chances of low-to-moderate risk clients having asthma by 10–20 fold.
- Histamine is sometimes used in place of methacholine for the bronchial challenge test, but is associated with increased side effects.