DENSITY AND FREQUENCY-DEPENDENCE OF RESISTANCE IN EARLY AIRWAY-OBSTRUCTION

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Abstract
The forced oscillation technique is a noninvasive and effort-independent test used to characterize the mechanical impedance of the respiratory system. The aim of this study was to assess the sensitivity of this method in detecting early airway abnormalities caused by smoking or occupational hazards. Respiratory mechanical parameters by the forced oscillation technique and maximal expiratory flow volume (MEFV) curves were obtained in a group of 82 workers from a gas manufacturing plant. In addition, the variations in the frequency dependence of total respiratory resistance between air breathing and a mixture of helium-oxygen (He-O2) were measured. All parameters were normalized for age, height, and weight. In order to compare the 2 types of testing, multiple linear regressions were performed with smoking or exposure as independent variables on one hand and parameters of the 2 types of testing on the other hand. This statistical procedure showed that 2 parameters of the forced oscillation technique constantly reached higher levels of significance than those of the MEFV parameters in the regression equations describing smoking history and occupational exposure: there were the frequency dependence of resistance and the change in frequency dependence between air and He-O2 breathing. When nonsmokers, ex-smokers, and smokers were considered separately, the degree of frequency dependence of resistance and the change in frequency dependence between air and He-O2 breathing were the only parameters that was significantly different between these 3 groups (one-way analysis of variance). When the workers were separated into "exposed" and "nonexposed" subjects, the extrapolated resistance to zero frequency, the resonant frequency, the degree of frequency dependence of resistance, and the changes in frequency dependence between air and He-O2 breathing were all significantly different. In each case, the MEFV variables showed no significant differences between the groups. Thus, the most informative test in our study was the difference in frequency dependence of resistance between air and He-O2 breathing. These results indicate that the forced oscillation technique affords sufficient sensitivity to distinguish subjects exposed to respiratory irritants at a stage when MEFV parameters are not yet modified.