Airflow obstruction in asthma is associated with increased risk of morbidity and exacerbations. A number of reports have identified that small airways obstruction (SAO) may be present in asthma in the presence or absence of fixed airflow obstruction (FAO). We sought to evaluate the prevalence of SAO and its clinical expression in the presence or absence of FAO.
132 adult asthmatics (GINA 1(11), GINA 2-3 (30) GINA 4-5 (89)) were recruited from Glenfield Hospital. All patients underwent impulse oscillometry and spirometry with reversibility. Two markers of SAO were evaluated (R5-R20 and AX). Due to the lack of robust reference equations for predicted impulse oscillometric parameters of SAO, we defined 'abnormal' as a value above the asthmatic mean.
We identified that 23.5 % and 15.2% of patients with asthma had SAO in the presence of a normal post bronchodilator FEV1/FVC >70% - when evaluating the markers AX and R5-R20 respectively. A similar percentage had SAO and FAO for both markers (24.2% for AX and 19.7% for R5-R20). The frequency of asthma exacerbations in the previous year was significantly elevated (3.13/yr (AX), 3.53/yr (R5-R20)) in patients with isolated SAO compared to patients without SAO and spirometric airflow obstruction (1.29/yr (AX), 1.64/yr (R5-R20)) [p=0.0014 ANOVA, p>0.05 intergroup comparison]. There were no differences in eosinophilic airway inflammation between the groups.
We have identified that isolated small airways obstruction is associated with asthma exacerbations independently of eosinophilic airway inflammation. Further studies are required to evaluate whether treating isolated SAO may reduce the frequency of asthma exacerbations.