A technique is described allowing one to determine simultaneously the resistance and reactance of the total respiratory system for various frequencies. During spontaneous breathing, regularly recurring impulses are produced at the mouth by means of a loud speaker. A Fourier analysis of the mouth pressure and flow signals yields mean resistance and reactance values, over 16 s, for all harmonics of 2 Hz up to 30 Hz. The values are in good agreement with those obtained in the absence of breathing and those determined by means of the forced oscillation technique and by body plethysmography. The reproducibility of the measurements is satisfactory (coefficient of variation: 11.6%).

Using a forced oscillation technique, we measured the resistance (Rrs) and reactance (Xrs) of the respiratory system between 2 and 32 Hz at three different lung volumes in 15 healthy subjects and 7 patients with chronic obstructive pulmonary disease. Rrs and Xrs were partitioned, by means of a pressure recording in the esophagus, into the resistance and reactance of lung and airways (L) and the chest wall. The measurements were validated by checking the adequacy of the frequency response of the esophagus, by the lack of difference between thoracic and mouth flow, by an estimation of the error introduced by the shunt impedance of the cheeks, and by comparisons with the values of pulmonary compliance and resistance determined in the same subjects with classical techniques. In both healthy subjects and patients, the chest wall has a low resistance that increases somewhat at low lung volumes and behaves functionally as a two-compartment system, with low capacitance at frequencies exceeding 4 Hz. Rrs varies with lung volume and is markedly frequency dependent in patients; both phenomena are due primarily to corresponding variations of RL. In healthy subjects, at and above functional residual capacity (FRC) level, the lungs behave as a one-compartment system, the reactance of which is mainly determined by the gaseous inertance, at least beyond 2 Hz. In patients and in healthy subjects breathing below FRC, the observed frequency dependence of resistance and the simultaneous increase in resonant frequency can be simulated satisfactorily by Mead's two-compartment model, assuming a large increase in peripheral airways resistance.

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We compared dose-response curves obtained with the forced oscillation technique (FOT) and with body plethysmography during bronchoprovocation in children. In 40 stable asthmatic children (age, 5-16 yr) we performed challenges with doubling concentrations of inhaled carbachol (0.15-10 mg/mL) until specific airway resistance SRaw had increased by 100% (PC100SRaw). The FOT-response was assessed by total respiratory system resistance (Rrs, cmH2O.1(-1).s) and reactance (Xrs, cmH2O.1(-1).s) from 8 to 26 Hz, expressed as mean Rrs (Rrs), mean Xrs (Xrs), Rrs at 8 Hz (Rrs8), and mean slope of Rrs (dRrs/df). Dose-response curves were analyzed for threshold concentrations (TC) causing a 3 SD change from baseline and sensitivity indices (SI) defined as differences between baseline and postchallenge values (at PC100SRaw) divided by baseline SD. Median TC of Rrs8, Rrs, Xrs, dRrs/df, and SRaw was 0.21, 0.30, 0.34, 0.41, and 0.42 mg/mL, respectively, indicating a slightly higher sensitivity for FOT. Median SI values of SRaw and Xrs (12.0 and 8.2; difference n.s.) were significantly higher than those of the other parameters. Multiple regression analysis revealed only the absolute change of Xrs (delta Xrs), baseline Rrs and age as significantly (P less than 0.001) correlated with the percentage change of SRaw (delta %SRaw). Best correlation (r = 0.86) with delta %SRaw was found for the function: FOT score = -102.5 X delta Xrs X exp(-0.196 X Rrs + 0.038 X age). Provocative concentrations estimated by this FOT score differed from PC100SRaw by less than one (two) concentration steps in 34 (40) out of 40 children.(ABSTRACT TRUNCATED AT 250 WORDS)