Small Airway Dysfunction in Adult Patients with Bronchial Asthma

<bold>Background:</bold> The lack of standardized criteria makes the noninvasive small airway dysfunction (SAD) assessment challenging. This study aimed to evaluate the prevalence of SAD in bronchial asthma and to assess the consistency of impedance and lung function parameters in estimating SAD. <bold>Methods:</bold> Oscillometry-defined SAD was defined when any of the following parameters were abnormal: R5-19 >ULN, X5<LLN, Fres >18 Hz, and Ax >30 cmH₂O/L/s. Spirometry-defined SAD was defined as MMEF < 60% of predicted. Body plethysmography-defined SAD was defined as RV/TLC> 140% of the predicted value. The agreement between the two criteria for diagnosing SAD was assessed using Cohen’s Kappa (κ). Tidal expiratory flow limitation (EFL_T), i.e., ΔX5 ≥ 0.28 kPa/L/s, was also examined. <bold>Results:</bold> A total 152 consecutive patients with stable bronchial asthma (males: 48.7%) were recruited. The mean age was 42.3±13.3 years, and the FEV₁% predicted was 56.8±20.5. The prevalence of abnormal R5-19, X5, Fres, and Ax was 42.1%, 46.1%, 67.8%, and 32.9%, respectively. The κ values among various impedance parameters ranged from 0.51 to 0.84. The oscillometry, spirometry, and body plethysmography-defined SAD were present in 67.8%, 72.8%, and 57.9%, respectively. The κ value between spirometry and plethysmography-defined SAD was 0.28. The κ values between oscillometry with spirometry and plethysmography-defined SAD were 0.22 and 0.32, respectively. EFL_T was present in 17.8% of patients. <bold>Conclusions:</bold> The prevalence of SAD in bronchial asthma varied from 57.9% to 72.8% depending on the physiological parameters used to define it and their cutoff. The agreement between the impedance parameters and lung function parameters in assessing SAD was fair. EFL_T can occur in patients with bronchial asthma.