Measurement of forced expiratory volume in 1 second (FEV₁) is usually difficult to obtain in children under six years of age because it requires active cooperation. This study evaluates the sensitivity of impulse oscillometry system (IOS) parameters for detecting airway obstruction in comparison with FEV₁.

We studied 174 children who performed the lung function and methacholine challenge tests to diagnose asthma by IOS and spirometry. Children were divided into two subgroups according to their PC₂₀, which is a parameter for bronchial sensitivity. We compared IOS parameters with FEV₁ at the baseline, post-methacholine challenge, and evaluated their correlation.

At the baseline, reactance at 5 Hz (X5) and resistance at 5 Hz (R5) significantly differed between the PC₂₀ positive (PC₂₀≤16 mg/mL) group and PC₂₀ negative (PC₂₀>16 mg/mL) group; however, FEV₁, FEV₁% predicted, FEV₁_Zs (Z score) did not differ. FEV₁ is correlated with X5 (r=0.45, P <0.01) and R5 (r=-0.69, P <0.01). FEV₁_Zs is also correlated with X5_Zs (r=-0.26, P <0.01) and R5_Zs (r=-0.31, P <0.01). After the methacholine challenge test, dose-response slopes in FEV₁ and X5 significantly differed between the two subgroups ( P <0.05).

IOS parameters were more discriminative than FEV1 for detecting decreased baseline lung function between two subgroups and have a good correlation with FEV₁.