Pulmonary Function of ACO: What Is the Role of the Forced Oscillation Technique?

ACO patients had an intermediate degree of airflow obstruction, as measured by spirometry, between asthma and COPD patients, although it depended on the studied population. Reversible airflow limitation was more common in ACO patients than in COPD patients. ACO patients had more severe airway obstruction measured by the forced oscillation technique (FOT), expiratory flow limitation, and emphysema than asthma patients. Although the difference between ACO and COPD patients was modest, recursive partitioning analysis revealed that the combined assessment of respiratory system resistance (Rrs) and respiratory system reactance (Xrs) enabled the differentiation between ACO and asthma or COPD with high specificity. Colored 3-dimensional images of ACO patients showed an intermediate pattern between asthma and COPD patients. The annual changes in FOT parameters, especially Xrs, correlated with the annual changes in FEV1 in asthma, ACO, and COPD patients. Reversible airflow limitation measured by spirometry was predicted by the FOT in asthma and ACO patients but not in COPD patients. Most FOT parameters changed in linkage with spirometry after treatment. Although the interpretation of the FOT parameters, especially the difference between Rrs at 5 Hz and 20 Hz (R5−R20), is controversial, there is a trend toward using R5−R20 and Xrs as small airway dysfunction.