Specificity and Sensitivity of Respiratory Impedance in Assessing Reversibility of Airway Obstruction in Children

Flow in the upper airway wall induces significant error in estimating respiratory impedance by the standard forced oscillation technique in subjects with airway obstruction and may be minimized by oscillating pressure around the subject's head (head generator technique). The aim of this study was to determine whether the latter improves the power of forced oscillations in detecting airway response to bronchodilators in children. Seventy-five children with airway obstruction were studied (ages 5.5 to 15 years old). Fifty-three had asthma and 22, cystic fibrosis. A bronchodilator was administered, and the percent changes in respiratory resistance at 10 Hz (Rrs10), 20 Hz (Rrs20), respiratory compliance (Crs), and resonant frequency (fn) with standard and head generator were compared with the corresponding change in FEV1. The response was positive in 38 (Δ% FEV1 ≥15%) and negative in 37 patients. Data on RrslO, Crs, and fn could not be obtained in 7, 8, and 4 subjects, respectively, for technical reasons. The Δ%Rrs20 was not different between head and standard generator in nonresponders (mean ± SEM: 19.0 ± 4.5, vs −11.8 ± 3.1%), but significantly larger with head than standard generator in responders (−54.1 ± 3.0 vs −26.5 ± 2.4%; p