Background Severe asthma is a multidimensional disease, with recent evidence supporting the notion that eosinophilic airway inflammation (EAI) is an important driver for exacerbations. In addition EAI has been shown to be associated with airflow limitation in cross sectional studies. However, it remains to be established whether EAI may drive FEV1 decline.
Methods The severe asthma registry at Glenfield hospital, Leicester, was screened for patients with a physician diagnosis of asthma and at least 5 years of longitudinal data recording sputum eosinophils, pre- and post-bronchodilator spirometry, inhaled corticosteroid usage as well as standard demographic indices during stable scheduled follow-up visits. Linear mixed effects models were used to investigate the effect of log sputum eosinophils as a time varying covariate on decline of post bronchodilator FEV1. Models were iteratively compared and refined using standard information criteria. Other fixed effects in the final model were, time and the interaction terms for time * log sputum eosinophils and time *daily dose of inhaled corticosteroids and pack years smoked. Individual variations in the slopes and intercepts of time and time*log sputum eosinophils were considered by adding them iteratively as random effects. A first-order autoregressive correlation structure was used to model covariance of random effects.
Results 92 patients, 46% male with severe asthma were identified from a registry cohort of 686 between 2000 and 2009. The mean (sem) age was 54(12.9) years and age of onset 23 (2.1) years. The mean (range) duration of follow-up and number of visits were 6 years (4.6–10.5), 2.7/year. We found a significant interaction between sputum eosinophils, time and post bronchodilator FEV1. Indicating a net decline (95% CI) of −16.8 mls(25.8–7.8 mls) /annum/log unit increase in sputum eosinophils (F(1, 43.4); p<0.0001). In contrast there was a net decline 95%CI of −0.015 mls (0.029 to 0.0014 mls)/annum/mcg of inhaled beclamethasonediproprionate daily (F(1,726); p=0.031).
Conclusion Eosinophilic airway inflammation is associated with a significant decline in FEV1 in severe asthma.
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