Small airway oscillometry indices: Repeatability and bronchodilator responsiveness in young children

The impulse oscillometry (IOS) indices absolute and relative difference between respiratory resistance at 5 and 20 Hz (R5-20 and R5-20%, respectively) and the area under the reactance curve (AX) are postulated to reflect small airway function. Data on their cutoff values to evaluate bronchodilator responsiveness (BDR) or between-visit changes after interventions are limited in young children.

We evaluated the BDR of 103 healthy children aged 2-7 years, who received either salbutamol (n = 84) or placebo (n = 19) in order to determine cutoff values for BDR of R5-20, R5-20%, and AX. We then determined the repeatability within and between two IOS measurements 7-14 days apart in young children aged 4-8 years with asthmatic symptoms (n = 43), including cutoff values for significant between-visit changes.

The investigated IOS parameters showed marked BDR (fifth percentile cutoff of 75-110% of the baseline value) in healthy children, whereas no significant changes were seen after inhalation of placebo. The agreement within the triplicate IOS measurement was excellent (ICC > 0.80), and the agreement of results between visits was good (ICC > 0.60). A change in R5-20, R5-20%, and AX of 0.65, 1.08, and 0.84 z-scores, respectively, would exceed 95% confidence intervals for between-visit variability.

We introduce cutoff values for BDR of R5-20, R5-20%, and AX, and their repeatability indices and cutoff limits for significant between-visit changes. These IOS parameters may show greater variability than the conventional IOS indices during follow-up, but the between-visit agreement remains good, providing potentially useful endpoints for monitoring lung function in young children.