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 p ressure around the subject's head (head generator technique). The aim of this study was to determine whether the latter improves the power o f forced oscillations in detecting airway response to bronchodilators in children. Seventy-five children with airway obstruction were studie d (ages 5.5 to 15 years old). Fifty-three had asthma and 22, cystic fi brosis. A bronchodilator was administered, and the percent changes in respiratory resistance at 10 Hz (Rrs10), 20 Hz (Rrs20), respiratory co mpliance (Crs), and resonant frequency (fn) with standard and head gen erator were compared with the corresponding change in FEV(1). The resp onse was positive in 38 (Delta% FEV(1) greater than or equal to 15%) a nd negative in 37 patients. Data on Rrs10, Crs, and fn could not be ob tained in 7, 8, and 4 subjects, respectively, for technical reasons. T he Delta%Rrs20 was not different between head and standard generator i n 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 spe cificity of Rrs20 (78 vs 65%) with no change in sensitivity (76% for b oth). Resonant frequency had larger sensitivity with standard than wit h 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 incid ence of false results was lower with the head generator than with the standard generator for resistance and lower with the standard generato r than with the head generator for fn and compliance. Thus, the head g enerator improves the diagnostic power of the forced oscillation resis tance in establishing the reversibility of airway obstruction, but par ameters derived from the reactance may have better diagnostic value wi th the standard method.