Respiratory resistance by the forced oscillation technique in asthmatic children and cystic fibrosis patients.

Measurement of the total resistance of the respiratory system (Rrs) is an attractive alternative to measurement of forced expiratory volume in one second (FEV1) in young children because it requires minimal co-operation. The purpose of this study was to assess the ability of the forced oscillation technique (FOT) to detect airway obstruction in asthmatic children and in patients with cystic fibrosis (CF). Spirometry and Rrs were recorded in 45 asthmatic children (32 males and 13 females) and in 45 patients with CF (28 males and 17 females). Rrs was measured at 10 Hz with the Siregnost FD5 (Siemens, Germany). The asthmatic children were slightly younger than the patients with CF (10+/-3 vs 14+/-7 yrs), and had milder airway obstruction (FEV1 80+/-19 vs 66+/-27% of predicted). Rrs was significantly higher in the asthmatic children (6.6+/-1.7 cmH2O x L(-1) x s) than in the patients with CF (4.8+/-1.4 cmH2O x L(-1) x s). A normal FEV1 (> or = mean -2SD) was associated with a normal Rrs (< or = mean +2SD) in 17 of the 45 asthmatic children and in 13 of the 45 CF patients. By contrast, a low FEV1 (< mean -2SD) was associated with an increased Rrs (> mean +2SD) in 21 of the 45 asthmatic children, but in only 3 of the 45 CF patients. Thus, FEV1 and Rrs yielded concordant information in asthmatic children much more often (38 out of 45) than in CF patients (16 out of 45) (p < 0.001). In CF, Rrs failed to detect even severe airways obstruction. These findings might be accounted for by the inability of Rrs to reflect peripheral obstruction. We conclude that total respiratory resistance is suitable to assess airways obstruction in asthmatic children but not in cystic fibrosis patients.