Background: Fixed airflow obstruction (FAO) has been an issue in the pathophysiology and management of asthma. A previous study found FAO was associated with older age, longer duration of asthma, male sex, higher blood eosinophil counts, more small airway dysfunction, and more exacerbations. However, the association between FAO and smoking history in asthma is not fully understood.

Aims: To assess the difference in clinical characteristics between asthma with and without FAO and asthma-COPD overlap (ACO) patients.

Methods: Subjects included 119 asthma patients without FAO and 51 with FAO and <10 pack-years, and 85 ACO patients with ?10 pack-years smoking history. FAO was defined as FEV1/FVC<0.7.

Results: Patients with FAO were older, had a longer duration of asthma, more impaired spirometry, higher blood eosinophil counts, and more exacerbations than those without FAO. Both patients were female-dominant. Small airway oscillometric parameters, including X5, Fres, and AX values, were significantly greater in patients with FAO than those without FAO. Multivariate logistic regression analysis for factors associated with FAO revealed that the odds ratio for FAO was age (per 10 years), 1.32; Fres, 1.13; and blood eosinophil counts, 1.14. ACO patients were male-dominant and had lower %FEV1, more ex-smokers, and pack-years. However, there was no difference in age, disease duration, oscillometry, blood eosinophil counts, and exacerbations between patients with FAO and ACO patients. 

Conclusions: Except for sex difference, heavy smoking history, and %FEV1, asthma patients with FAO had comparable clinical characteristics, including small airway dysfunction, with ACO patients.