INTERPRETATION OF BRONCHODILATOR RESPONSE IN PATIENTS WITH OBSTRUCTIVE AIRWAYS DISEASE

Background There is no agreement on how a bronchodilator response should be expressed. Ideally, the index used should be able to distinguish asthma from chronic obstructive lung disease and be independent of initial FEV1. Methods Two hundred and seventy four adult (aged 18-60 years) outpatients with obstructive airways disease were studied. Patients were divided into syndrome groups on the basis of a standardised history: asthma (n = 99), asthmatic bronchitis (n = 88), and chronic obstructive lung disease (n = 51); 36 subjects could not be attributed to any subgroup. FEV1 was measured before and 20 minutes after inhalation of 1000-mu-g terbutaline. Different expressions of bronchodilator response (DELTA-FEV1) were compared with respect to their dependence on initial FEV1 and their efficacy in separating subjects with asthma from those with chronic obstructive lung disease. DELTA-FEV1 was expressed as a percentage of initial FEV1 (DELTA-FEV1%init), absolute value (DELTA-FEV1[1]), percentage of predicted FEV1 (DELTA-FEV1%pred), standardised residual (DELTA-SR-FEV1), and percentage of maximal possible increase (DELTA-FEV1%[pred-init]). Results DELTA-FEV1%init was more dependent on initial FEV1 (rho = -0.405) than DELTA-FEV1[1] (r = -0.145), DELTA-FEV1%pred (r = -0.166), and DELTA-SR-FEV1 (r = -0.127). DELTA-FEV1%[pred-init] reached infinity when initial FEV1 approached predicted levels. DELTA-FEV1%pred had a higher likelihood ratio (1.71) for separating patients with asthma from those with chronic obstructive lung disease than other expressions of bronchodilator response. Asthmatic patients had larger mean bronchodilator responses than patients in other subgroups; this difference was largest for DELTA-SR-FEV1 (F = 9-19) and DELTA-FEV1%pred (F = 9.03); it was much smaller for DELTA-FEV1%init (F = 5-89). Despite significant differences in mean response, there was a large overlap of individual responses between diagnostic subgroups. The bronchodilator response was continuously and unimodally distributed for all expressions. Conclusions DELTA-FEV1%pred appears to be the most useful method of expressing bronchodilator response, both for clinical and for research purposes. Reversibility of airways obstruction in response to a bronchodilator is a continuous variable and not a dichotomous trait. Any cut off level of a "positive" bronchodilator response is therefore arbitrary.