Use of specific airway resistance to assess bronchodilator response in children

Abstract

Background and objective: Changes in specific airway resistance (ΔsRaw) after bronchodilation, as measured by plethysmography and FEV 1, are frequently considered to be interchangeable indices of airway obstruction. However, the baseline relationship between these two indices is weak, and the value of ΔsRaw that best predicts FEV1 reversibility in children has yet to be determined. The aim of this study was (i) to establish the sRaw cut-off value that best distinguishes between positive and negative bronchodilator responses, as measured by FEV1 reversibility; (ii) to determine whether the discrepancy between ΔsRaw and ΔFEV1 might be explained by independent correlations between ΔFEV1 and both ΔsRaw (mainly airway obstruction) and ΔFVC (airway closure); and (iii) to assess the effect of height and age on the relationship between ΔsRaw and ΔFEV1. Methods: A retrospective study was performed in 481 children (median age 10.5 years, range 6.1-17.6) with actual or suspected asthma, for whom sRaw and spirometry data were obtained at baseline and after administration of a bronchodilator. Results: The sRaw cut-off value that best predicted FEV1 reversibility was a 42% decrease from baseline (P = 0.0001, area under the curve 0.70, sensitivity 55%, specificity 77%) and was independent of height and age. Changes in FEV 1 were significantly but independently related to ΔsRaw and ΔFVC (index of air trapping) (r = 0.40, P < 0.0001 and r = 0.39, P < 0.0001, respectively). Conclusions: A 42% decrease in sRaw predicted FEV 1 reversibility reasonably well, whereas a smaller decrease in sRaw failed to detect approximately one out of two positive responses detected by FEV1, with no influence of height or age. FEV1 and specific airway resistance (sRaw) are used interchangeably as markers of bronchodilator response. A 42% decrease in sRaw showed good specificity but poor sensitivity for predicting FEV1 reversibility, partly because FEV1 and sRaw have different physiological components. © 2011 Asian Pacific Society of Respirology.