Prevalence of small airways dysfunction in asthma with- and without-fixed airflow obstruction and chronic obstructive pulmonary disease

Prapaporn Pornsuriyasak,¹ Supattra Khiawwan,¹ Sasivimol Rattanasiri,² Nattawut Unwanatham,² Tananchai Petnak¹

¹ Division of Pulmonary and Critical Care, Department of Medicine, Faculty of Medicine, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand
² Section for Clinical Epidemiology and Biostatistics, Ramathibodi Hospital, Mahidol University, Bangkok, Thailand

Abstract

Background: Small airways dysfunction (SAD) is not uncommon in asthma without fixed airflow obstruction (FAO).
Objectives: We aimed to determine if SAD in non-FAO asthma is different from FAO-asthma and COPD.
Methods: Cases of obstructive airway diseases who underwent spirometry, plethysmography, and impulse oscillometry [resistance at 5 Hz (R₅) and at 20 Hz (R₂₀), peripheral resistance (R₅-R₂₀), and reactance area (AX)] were reviewed, and classified as; 1) COPD, 2) FAO-asthma, and 3) non-FAO asthma. FAO was defined as post-bronchodilator (post-BD) FEV₁/FVC < 0.7. SAD was considered if 1) RV/TLC ≥ 40%, or 2) post-BD R₅-R₂₀ ≥ 0.075 kPa.L^-1s.
Results: A total of 73 patients (22 COPD, 24 FAO-asthma, and 27 non-FAO asthma) were analysed. RV/TLC ratio was higher in FAO-asthma and COPD (45 ± 5% and 42 ± 8%) than in non-FAO asthma (32 ± 8%), p < 0.001. Post-BD values of R₅-R₂₀ and AX (median; range) were higher in FAO-asthma (0.17; 0.08, 0.47, 13.24; 6.52, 82.11) than in non-FAO asthma (0.11; 0.03, 0.23, 8.63; 2.40, 22.02), p = 0.007 and p = 0.017, respectively. The prevalence of SAD among diagnosis group by RV/TLC criterion was different (95%, 59%, and 15% in FAO-asthma, COPD, and non-FAO asthma, p < 0.001), but those were not observed by R₅-R₂₀ criterion (95%, 68%, and 77%, p = 0.052).
Conclusion: SAD in non-FAO asthma was less prevalent than FAO-asthma and COPD.
Key words: air trapping, asthma, COPD, fixed airflow obstruction, impulse oscillometry, small airways