Differences between asthma exacerbations and poor asthma control

Background Increased variation in peak expiratory flow (PEF) is characteristic of poorly controlled asthma, and measurement of diurnal variability of PEF has been recommended for assessment of asthma severity, including during exacerbations. We aimed to test whether asthma exacerbations had the same PEF characteristics as poor asthma control. Methods Electronic PEF records from 43 patients with initially poorly controlled asthma were examined for all exacerbations that occurred after PEF reached a plateau with inhaled corticosteroid treatment. Diurnal variability of PEF was compared during exacerbations, run-in (poor asthma control), and the period of stable asthma before each exacerbation. Findings Diurnal variability was 21·3% during poor asthma control and improved to 5·3% (stable asthma) with inhaled corticosteroid treatment. 40 exacerbations occurred in 26 patients over 2–16 months; 38 (95%) of exacerbations were associated with symptoms of clinical respiratory infection. During exacerbations, consecutive PEF values fell linearly over several days then improved linearly. However, diurnal variability during exacerbations (7·7%) was not significantly higher than during stable asthma (5·4%, p=0·1). PEF data were consistent with impaired response to inhaled β2-agonist during exacerbations but not during poorly controlled asthma. Interpretation Asthmatics remain vulnerable to exacerbations during clinical respiratory infections, even after asthma is brought under control. Calculation of diurnal variability may fail to detect important changes in lung function. PEF variation is strikingly different during exacerbations compared with poor asthma control, suggesting differences in β2-adrenoceptor function between these conditions.