Frequency dependence of resistance (R5-20) assessed by impulse oscillometry (IOS) is suggested to be a measure of small airways. Small airways involvement during induced bronchoconstriction has been shown to reflect severity of asthma in adults.

Our aim was to evaluate if methacholine (Mch) induced changes in R5-20 are associated with the severity of exercise induced bronchoconstriction (EIB) in young children.

A total of 109 children aged 3–8 years were studied, 95 with obstructive symptoms and 14 in good health, to assess small airways function during a Mch challenge. R5-20 and other IOS resistance and reactance parameters were measured at baseline and after the Mch challenge. In a standardized exercise test, the children were grouped according to the severity of EIB expressed as the percentage increase in resistance at 5 Hz (ΔR5) after exercise, indicating either no EIB (ΔR5 < 40%, n = 84), moderate EIB (ΔR5 40–80%, n = 13) and severe EIB (ΔR5 > 80%, n = 12).

The baseline R5-20 was not associated with the severity of EIB, but during Mch induced bronchoconstriction the change in R5-20 was significantly higher in children with severe EIB (2.61 fold increase) than in children with moderate EIB (1.48) or no EIB (1.74, p = 0.036). No significant associations were found in changes of other IOS parameters. The children with severe EIB also used more short-acting beta-agonists during the past two months than the other two groups (p < 0.001).

Frequency dependence of resistance (R5-20) measured by IOS during the Mch induced bronchoconstriction and more frequent use of beta-agonists are associated with severe EIB in young children.