Clinical Interpretation of Airway Response to a Bronchodilator: Epidemiologic Considerations

Abstract

Airways responsiveness to a bronchodilator is frequently measured to assist in determining the cause of respiratory symptoms. Clinically, > 15% improvement in the FEV1 is often used to define the "increased" response indicative of asthma. However, unlike other tests of lung function, reference standard derived from "healthy" members of a general population sample have never been reported. As part of a heath survey carried out on Alberta, Canada, 2,609 subjects completed a standardized respiratory symptom questionnaire and had FEV1 measured before and 20 min after inhaling terbutaline sulfate via a 750-ml spacer device. Among symptomatic never-smoking subjects with FEV1 > 80% of predicted, the upper 95th percentile of bronchodilator response (BDR), when expressed as 100 .times. (FEV1 postBDR-FEV1 preBDR)/predicted baseline FEV1 averaged 9%. This value remained remarkably stable across gender, age (7 to 75 yr), and height groups, and deviated to 6% only when baseline FEV1 was > 120% of predicted. Consistent with other respiratory function variables, in which the upper of normal is often defined as the upper 95th percentile, our population-derived reference values provide a conceptual definition of BDR that can easily be applied to define "increased" response in the clinical setting.