Clinical Characteristics of Adult Asthma Associated with Small Airway Dysfunction

Suboptimal asthma control is common despite modern asthma therapy. The degree of peripheral airway involvement remains unclear and poor medication delivery to these regions might be a contributing reason for this failure in obtaining adequate symptom control. A cohort of 196 adults (median (range) age 44 (18-61) years, 109 females, 54 ex-smokers, six current smokers) with physician-diagnosed asthma were recruited from primary care. Subjects were characterized clinically by interviews, questionnaires, skin prick tests (SPT) and blood eosinophil counts. Lung function was assessed by spirometry, impulse oscillometry (IOS) and nitrogen multiple breath washout (N2 MBW). IOS assessed peripheral airway resistance (FDR, frequency dependence of resistance). N2 MBW assessed global ventilation inhomogeneity (LCI, lung clearance index), specific indices of peripheral airway function (Scond × VT and Sacin × VT; VT, tidal volume), and inter-regional inhomogeneity (specific ventilation ratio). Never-smoking healthy cohorts of 158 and 400 adult subjects provided local reference values for IOS and N2 MBW variables, respectively. Peripheral airway dysfunction was detected in 31% (FDR or specific ventilation ratio) to 47% (Scond x VT) of subjects. Risk factors for peripheral airway dysfunction were identified. Among subjects with low FEV1 and either positive smoking history and/or blood eosinophilia (>4.0%), 63% had abnormality across all peripheral airway outcomes, whilst only one subject was completely normal. Abnormal peripheral airway function was present in a large proportion of adult asthmatics at baseline. Reduced FEV1, a positive smoking history, and/or blood eosinophilia identified "a small airway asthma subtype" that might benefit from peripheral airway targeted therapy.