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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The application of the forced oscillation technique to assess reversibility of airflow obstruction was compared with that of indices of forced expiration and plethysmographic airway resistance (Raw). In 125 patients with airflow obstruction, we measured total respiratory resistance (Rrs) and reactance (Xrs), Raw and specific airway conductance (sGaw), maximal flow-volume curves and forced expiratory volume in 1 s (FEV1), before and 30 min after 2 x 20 micrograms salbutamol by MDI. Salbutamol induced significant change in mean value of all measured indices. The changes in impedance data consisted of decrease in mean value and of negative frequency dependence of Rrs, an increase in Xrs with slight decrease of its positive frequency dependence. Multivariate analysis of differences between pre- and postbronchodilator values showed that the single indices with the greatest sensitivity to detect the effect of salbutamol were, in decreasing order, (1) in relative change (% baseline value): Raw, Rrs at 6 Hz (Rrs6), forced vital capacity (FVC), FEV1, and (2) in absolute change: FVC, sGaw or Raw, Rrs6, FEV1, maximal expiratory flow (MEF50). The effect of salbutamol was described best in (1) by a combination of Raw and FVC and in (2) by sGaw and FVC. For individual detection of bronchodilator effect, threshold values were calculated from mean reproducibility of the three baseline values of the various indices, attempting to estimate whether response to a bronchodilator is statistically significant. The greatest number of significant responses were observed for Raw, sGaw, FEV1, and FVC in that succession, Rrs6 being markedly less sensitive. This discrepancy is due to the lack of Rrs6 response to bronchodilators in patients with severe airway obstruction.(ABSTRACT TRUNCATED AT 250 WORDS)

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