Small Airway Obstruction in Chronic Obstructive Pulmonary Disease: Potential Parameters for Early Detection

The impulse oscillometry (IOS) is recognized as a complementary method to spirometry in the diagnostics of obstructive pulmonary disorders. The IOS enables to measure total respiratory resistance (R5) and proximal respiratory resistance (R20), with the R5-R20 difference reflecting small airway resistance. This study seeks to evaluate the usefulness of R5-R20, maximal mid-expiratory flow (MMEF)_, and forced expiratory volume in 3 s/forced vital capacity ratio (FEV₃/FVC), in the assessment of small airway obstruction in chronic obstructive pulmonary disease (COPD). One hundred and six COPD patients and 43 control subjects, aged over 55, were investigated. Spirometry and IOS were used to assess pulmonary function. Sensitivity, specificity, positive (PPV) and negative predictive values (NPV) were evaluated. The findings demonstrate significant reductions in FEV₃/FVC and MMEF, and an increase in R5-R20 difference in COPD patients; the changes that depended on the severity of airway obstruction. The sensitivity of R5-R20 in reflecting the MMEF was 84%, specificity 44.2%, PPV 72.4%, and NPV 61.3%. We conclude that the R5-R20 difference is superior to spirometry in the assessment of small bronchi obstruction. A high sensitivity of R5-R20 in reflecting the MMEF makes the IOS method particularly useful for detection of mild lung injury, while a high specificity of the spirometric FEV₃/FVC ratio makes it useful to exclude obstruction of small airways. Both methods are thus complimentary.