Density dependence of expiratory flow—a useful basis for lung function tests?

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Abstract
The aim of this investigation was to study the density dependence of forced expiratory flow and the ability of tests based on density of dependence to disclose smoking-induced small airways dysfunction. The lung function of 212 systematically sampled men (mean age 40.3; range 30-48 years) was studied by recording flow-volume curves during air helium-oxygen (HeOx) breathing and by the single breath N2-test. Density dependence was expressed as the percentage increase in flow from air to HeOx when 50 and when 25% of the vital capacity (VC) remained to be expelled (dMEF50 and dMEF25). The mean dMEF50 in 59 healthy non-smokers was 40% (range +7 to +72) and the mean dMEF25 24% (range -20 to +63). Density dependence apparently decreased with relative lung volume and also with increasing age. Despite the wide variation of density dependence in the reference group, dMEF50 was on average significantly reduced in smokers, and dMEF25 in ex-smokers and subjects with symptoms such as cough and/or sputum production, wheezing unrelated to common colds and dyspnea when walking on level ground. However, only 8% of the smokers exhibited ''abnormal'' density dependence (dMEF25 < (predicted value-2RSD)) as compared to 11 and 33% for the forced expiratory volume in one second (FEV1) and dN2 (alveolar slope). If abnormal dN2 in the smoking group is taken to indicate smoking-induced small airway dysfunction, then the ability of dMEF50 and dMEF25 to disclose such dysfunction is limited.