Background: The nature and strength of the relationships between specific airway resistance (sRaw) and forced expiratory flows have poorly been described.

Methods and patients: We determined the mathematical relationships between resistance and forced expiratory flows based on a two-compartment lung model. Pulmonary function tests (sRaw_0.5, FEV₁, FVC, FEF_25-75%, FEF_50%) of 2193 asthmatic children were selected from two databases for a transversal analysis. Subsequently, 365 children were selected for longitudinal assessment from preschool to school age.

Results: The mathematical analysis and the observed data demonstrated that sRaw is differently linked with FEF_50% (-1/sRaw) and FEV₁ (near linearly). A statistically higher correlation between sRaw_0.5 and FEF_50% than with FEV₁ was evidenced. In asthmatic children suspected for small airway disease (FEF_25-75% / FEV₁ ratio < 0.80 and FEV₁ > 80% predicted, n=968), the relationship between sRaw and flow rates was linear, a 10% reduction of FEF_{25- 75%} or FEF_50% corresponded to a mean increase in sRaw of ˜ 0.50 kPa.s. In the longitudinal part of the study, sRaw_0.5 at preschool age was better correlated with FEF_50% than FEV₁ at school age.

Conclusion: Specific Raw is better linked to FEF_50% than to FEV₁ and can detect airway obstruction from preschool to school age.