Differences between asthma exacerbations and poor asthma control

Background Increased variation in peak expiratory flow (PEF) is characteris tic of poorly controlled asthma, and measurement of diurnal variability of PEF has been recommended for assessment of asthma severity, including durin g 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 contr olled asthma were examined for all exacerbations that occurred after PEF re ached 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 impro ved to 53% (stable asthma) with inhaled corticosteroid treatment. 40 exacer bations occurred in 26 patients over 2-16 months; 38 (95%) of exacerbations were associated with symptoms of clinical respiratory infection. During ex acerbations, consecutive PEF values fell linearly over several days then im proved linearly. However, diurnal variability during exacerbations (7.7%) w as not significantly higher than during stable asthma (5.4%, p = 0.1). PEF data were consistent with impaired response to inhaled beta(2)-agonist duri ng exacerbations but not during poorly controlled asthma. Interpretation Asthmatics remain vulnerable to exacerbations during clinica l respiratory infections, even after asthma is brought under control. Calcu lation of diurnal variability may fail to detect important changes in lung function. PEF variation is strikingly different during exacerbations compar ed with poor asthma control, suggesting differences in beta(2)-adrenoceptor function between these conditions.