A significant bronchodilator response is a specific finding from spirometry, a common lung function test that measures how well air moves in and out of the lungs. Spirometry is used to detect and evaluate airflow obstruction. The bronchodilator response component determines if any identified obstruction is reversible, meaning the airways can open up in response to medication. Observing a significant response provides important information about the nature of the airway disease and helps guide treatment decisions.
Understanding the Bronchodilator Challenge
The bronchodilator challenge is a standardized procedure that is part of a complete spirometry assessment. A patient first performs baseline spirometry, which involves taking a deep breath and exhaling as forcefully and quickly as possible into a spirometer. This initial measurement establishes the level of any existing airflow limitation.
After the baseline test, the patient is administered a short-acting bronchodilator medication, typically a short-acting beta-agonist like albuterol. A bronchodilator works by relaxing the smooth muscles surrounding the airways, causing the air passages to widen. The medication is usually delivered through a metered-dose inhaler or a nebulizer.
Following the administration of the medicine, the patient rests for 10 to 15 minutes, allowing the drug to reach its peak effect in the lungs. The patient then repeats the spirometry maneuver to obtain a second set of measurements. Comparing the “pre-bronchodilator” and “post-bronchodilator” results reveals the extent to which the airways are able to open.
The Specific Criteria for a Significant Response
A result is defined as a “significant bronchodilator response” (SBR) only when the improvement in lung function meets two distinct criteria simultaneously. Standard criteria are based on changes in two primary spirometry values: Forced Expiratory Volume in 1 second (\(\text{FEV}_1\)) and Forced Vital Capacity (FVC).
\(\text{FEV}_1\) measures the volume of air a person can forcefully exhale in the first second of the maneuver, indicating airflow mechanics. FVC measures the total volume of air that can be exhaled after a maximal inhalation, reflecting the overall capacity of the lungs.
A significant response requires an improvement in either the \(\text{FEV}_1\) or the FVC that meets both a percentage threshold and an absolute volume threshold.
The improvement must be 12% or greater than the pre-bronchodilator value. Additionally, the absolute increase in volume must be 200 milliliters (mL) or more. Both the percentage increase and the volume increase must be present in the same parameter (\(\text{FEV}_1\) or FVC) for the response to be classified as significant.
Diagnostic Meaning of the Results
The presence of a significant bronchodilator response suggests that the underlying airflow limitation is reversible, which has important implications for diagnosis and treatment. Airway reversibility is a characteristic feature of asthma, where bronchospasm—the tightening of the muscles around the airways—is a primary cause of obstruction. A positive SBR strongly supports an asthma diagnosis, as it shows the constricted airways can be opened with medication.
Conversely, the lack of a significant bronchodilator response often suggests a fixed or irreversible airflow obstruction. This pattern is commonly associated with Chronic Obstructive Pulmonary Disease (COPD). In COPD, the obstruction is caused by structural changes in the lungs and airways, such as damage to the air sacs and chronic inflammation, which prevents full widening of the airways and limits response to medication.
A significant response can occur in patients with COPD, and a lack of response does not entirely rule out asthma. Patients with COPD may show an increase in FVC after bronchodilator use, indicating a reduction in air trapping or hyperinflation, even if the \(\text{FEV}_1\) does not meet the criteria for significance.
The test result is one piece of information that is interpreted alongside a patient’s symptoms, medical history, and other diagnostic tests to reach a definitive conclusion.
The information gained from the bronchodilator challenge helps practitioners tailor a patient’s treatment plan. A significant response suggests that inhaled bronchodilators will be an effective part of long-term management to control symptoms and improve lung function. Even if the response is not significant, demonstrating any degree of reversibility in COPD can indicate a component of the disease that will respond to therapy, potentially leading to improved quality of life and reduced symptoms.