Asthma is a chronic inflammatory airway disorder known to involve the peripheral airways. Current guidelines state that diagnosis and treatment should be guided by symptoms and spirometry. FEV₁ is, however, a poor marker of small airway function and correlates poorly with asthma control and airway inflammation.

To assess the contribution of small airway dysfunction and inflammation in paediatric asthma. A secondary aim was to study the associations between small airway dysfunction, airway inflammation and airway hyper-responsiveness (AHR).

Small airway function was measured as LCI, S_cond and S_acin, evaluated with the SF₆ multiple breath inert gas washout (MBW) technique, in 47 asthmatic children and 74 healthy controls. Exhaled NO at multiple exhalation flow rates including 50 mL/s (FENO₅₀) was assessed to calculate alveolar NO and bronchial NO flux (a surrogate for airway inflammation). AHR was evaluated with isocapnoic dry air hyperventilation challenge in the asthmatic children.

S_cond was elevated in 31 (66%) and S_acin in 18 (38%) of the asthmatic subjects. LCI was increased and FEV₁ decreased in 7 (15%) of the subjects with asthma. Individuals with AHR had higher S_cond (p = 0.001) and FENO₅₀ (p < 0.0001) than those without AHR. The levels of FENO₅₀ and bronchial NO flux were elevated in asthmatic subjects compared with healthy controls. In asthma, S_cond correlated with FENO₅₀ (r_s = 0.42, p = 0.003) and alveolar NO (r_s = 0.40, p = 0.011), and S_acin correlated with alveolar NO (r_s = 0.40, p = 0.015).

Dysfunction of small conducting airways is associated with airway inflammation and hyper-responsiveness in paediatric asthma.