Relationships between Specific Airway Resistance and Forced Expiratory Flows in Asthmatic Children

BACKGROUND: The earliest changes associated with airflow obstruction in asthmatic children are a proportionally greater reduction in FEF(50%) than in FEV(1) using spirometry, and an increase in specific airway resistance (sRaw) using body plethysmography. Consequently, we hypothesized that sRaw could be better linked to FEF(50%) than to FEV(1). The first aim was to assess the relationships between forced expiratory flows and sRaw in a large group of asthmatic children in a transversal study. We then performed a longitudinal study in order to determine whether sRaw of preschool children could predict subsequent impairment of forced expiratory flows at school age. METHODOLOGY: Pulmonary function tests (sRaw and forced expiratory flows) of 2193 asthmatic children were selected for a transversal analysis, while 365 children were retrospectively selected for longitudinal assessment from preschool to school age. PRINCIPAL FINDINGS: The transversal data showed that sRaw is differently related to FEF(50%) (−1/sRaw) and to FEV(1) (near linearly). These results were further explained by a simple one-compartment lung model, which justified the shape of the observed relationships. As hypothesized, sRaw correlated more strongly to FEF(50%) than to FEV(1) (r = −0.64 versus −0.39, respectively; p<0.001). In the longitudinal part of the study, sRaw at preschool age correlated with subsequent FEF(50%) (% predicted) (−0.31, 95% CI, −0.40 to −0.22), but weakly with subsequent FEV(1) (% predicted) (−0.09, 95% CI, −0.20 to 0). CONCLUSION: Specific Raw is more strongly related to FEF(50%) than to FEV(1) and could be used in preschool children to predict subsequent mild airflow limitation.