Detailed characterization and impact of small airway dysfunction in school-age asthma.

Background: Small airway dysfunction (SAD) is increasingly recognized as an important feature of pediatric asthma yet typically relies on spirometry-derived FEF _25-75 to detect its presence. Multiple breath washout (MBW) and oscillometry potentially offer improved sensitivity for SAD detection, but their utility in comparison to FEF _25-75 , and correlations with clinical outcomes remains unclear for school-age asthma. We investigated SAD occurrence using these techniques, between-test correlation and links to clinical outcomes in 57 asthmatic children aged 8-18 years.
Methods: MBW and spirometry abnormality were defined as z -scores above/below ± 1.96, generating MBW reference equations from contemporaneous controls ( n  = 69). Abnormal oscillometry was defined as > 97.5 ^th percentile, also from contemporaneous controls ( n  = 146). Individuals with abnormal FEF _25-75 , MBW, or oscillometry were considered to have SAD.
Results: Using these limits of normal, SAD was present on oscillometry in 63% (resistance at 5-20 Hz; R5-R20; >97.5 ^th percentile), on MBW in 54% (S _cond ; z -scores> +1.96) and in spirometry FEF _25-75 in 44% of participants ( z -scores< -1.96). SAD, defined by oscillometry and/or MBW abnormality, occurred in 77%. Among those with abnormal R5-R20, S _cond was abnormal in 71%. Correlations indicated both R5-R20 and S _cond were linked to asthma medication burden, baseline FEV ₁ and reversibility. Additionally, S _cond correlated with F _E NO and magnitude of bronchial hyper-responsiveness. SAD, detected by oscillometry and/or MBW, occurred in almost 80% of school-aged asthmatic children, surpassing FEF _25-75 detection rates.
Conclusions: Discordant oscillometry and MBW abnormality suggests they reflect different aspects of SAD, serving as complementary tools. Key asthma clinical features, like reversibility, had stronger correlation with MBW-derived S _cond than oscillometry-derived R5-R20.