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 (delta % FEV1 > or = 15%) and negative in 37 patients. Data on Rrs10, Crs, and fn could not be obtained in 7, 8, and 4 subjects, respectively, for technical reasons. The delta % 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 < 0.001). The optimal decision level determined by Receiver Operation Characteristic analysis showed that, compared with the standard method, the head generator improved the specificity of Rrs20 (78 vs 65%) with no change in sensitivity (76% for both). Resonant frequency had larger sensitivity with standard than with head generator (91 vs 53%) but slightly lower specificity (70 vs 78%). Finally, delta %Crs was more specific (72 vs 67%) and more sensitive (68 vs 52%) with standard than with head generator. The overall incidence of false results was lower with the head generator than with the standard generator for resistance and lower with the standard generator than with the head generator for fn and compliance. Thus, the head generator improves the diagnostic power of the forced oscillation resistance in establishing the reversibility of airway obstruction, but parameters derived from the reactance may have better diagnostic value with the standard method.