Measuring and imaging small airways dysfunction in asthma

Asthma is a chronic inflammatory disorder of the airways causing typical symptoms, and the diagnosis is supported by evidence of airflow obstruction which is variable, reversible or inducible. However, standard assessment of lung function with spirometry does not measure dysfunction in small airways which are < 2 mm in diameter towards the periphery of the lung. These airways make only a small contribution to airway resistance under normal circumstances. Nevertheless, there is mounting evidence that pathology and dysfunction in these small airways are implicated in the pathogenesis and natural history of asthma. Using forced oscillation and the multibreath nitrogen washout techniques, uneven ventilation (ventilation heterogeneity) due to small airways dysfunction has been shown to be an important marker of asthma disease activity, even in the absence of abnormalities in standard spirometric measurements. Recent advances in imaging research, particularly with hyperpolarised gas magnetic resonance imaging, have also given insights into the significance and dynamic nature of ventilation heterogeneity in asthma. The challenge is to integrate these new physiological and imaging insights to further our understanding of asthma and facilitate potential new treatments.