1. The oscillation method for measuring total respiratory resistance (Rrs) is a simple method of assessing airway dimensions which can be applied in epidemiological surveys and potentially might be useful for detecting mild airway disease in smokers. However, it is not known whether abnormalities in Rrs are only present when there are also abnormalities in simple spirometric tests.

2. We have compared values of Rrs and its frequency-dependence (fR) using the oscillation technique applied over the frequency range 6-26 Hz in 42 healthy, non-asthmatic men who were never-smokers (aged 26-61 years) and in 41 male cigarette smokers (aged 32-64 years). The results were compared with those for spirometry and the single-breath N2 test which are the most commonly used techniques in epidemiological surveys for detecting the effects of smoking on the lungs.

3. There was a strong trend for Rrs (especially at lower oscillation frequency) and fR to increase with increasing age in smokers. Increases in Rrs and fR were usually present when forced expiratory volume in 1 s was less than 80% of predicted and the forced expiratory volume in 1 s/vital capacity ratio was less than 65%, but abnormal fR was present in some smokers whose spirometry was within conventional normal limits.

4. Abnormalities in Rrs and fR were weakly associated with abnormality of the single-breath N2 manoeuvre.

5. Abnormal fR is normally attributed to uneven narrowing of intrathoracic airways; however, in smokers it was associated with an increase in Rrs at 6 Hz, so we cannot exclude that some of the observed abnormal fR was due to increased dissipation of the applied pressure in the cheeks and extrathoracic airway rather than to in-homogeneities within the lungs.

6. We conclude that the oscillation technique detects abnormalities indicating airway narrowing in some smokers whose spirometry is within normal limits. Hence the technique could be useful in screening programmes aiming to detect early lung damage. The prognostic significance of the additional information provided by measuring Rrs needs to be further assessed.

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