Introduction and Objectives The airways response to bronchodilators (BDR) has been used as a test to diagnose asthma and to differentiate it from other obstructive pulmonary diseases. The main outcome in assessing BDR is FEV₁, mainly a large airway measure. Measures of small airways are not included in everyday practice for BDR testing, although evidence suggests small airways dysfunction is found in asthma and COPD patients. This systematic review assessed the current evidence on small airways response to short-acting inhaled bronchodilators in asthma and COPD.

Methods The protocol was registered in PROSPERO (CRD42020164140). Electronic medical databases (EMBASE and Medline) were searched using related keywords. Abstracts and full texts were screened independently by two reviewers. Studies that reported the change of physiological small airways function (spirometric, oscillometry, multiple breath washout) and FEV₁ were included. The revised Cochrane risk of bias tool for RCT and the NIH quality assessment tool for cohort and cross-sectional studies were used to evaluate the studies.

Results Of 934 articles identified from the databases search, 13 met the inclusion criteria, with asthma (n=10) and COPD (n=3) patient studies. A total of 1110 participants were included; 911 were asthmatic, 90 COPD and, 109 were controls. Heterogeneity between studies was noted in the (1) diagnostic criteria for asthma or COPD, (2) agreed criteria for demonstrating BDR using standard spirometry, (3) methods used to deliver aerosolised medications and, (4) included measures of small airways function. Using spirometry, MMEF showed higher percentage of change (5.3–47%) in asthma and (3.6–25%) COPD, than FEV₁ which was (3.9–32%) in asthma and (2.8–16.3%) COPD [Abstract P78 figure 1]. The contrary was noted in severe asthma patients. Using oscillometry, BDR was observed with total resistance change of (R5) in asthma patients (-0.16 kPa/L/s) and between (-9.0— -22.4 kPa/L/s) in COPD patients.

Conclusions Small airways function appears to change following BDR, but currently studies are too heterogeneous to recommend their inclusion in clinical practice. More research is needed to form a consensus on how to assess BDR in general and in small airways in specific, and whether this adds utility to the diagnosis and management of airway disease patients.