Small airway dysfunction by impulse oscillometry in asthmatic patients with normal forced expiratory volume in the 1st second values

Small airways are relevant to the pathophysiology of asthma. We investigated whether in asthmatic patients with normal forced expiratory volume in the 1st second (FEV₁) values, impulse oscillometry system (IOS), as a measure of small airway function, contributed additional information to spirometry either at baseline or after bronchodilator, and whether it was related to the disease control. The fall in resistance from 5 to 20 Hz (R5‐R20) and reactance at 5 Hz (X5) by IOS and spirometry measures of small airway function (forced expiratory flow at 25‐75% [FEF_25‐75] and forced vital capacity/slow inspiratory vital capacity [FVC/SVC]) at baseline and after 400 micrograms of salbutamol were prospectively measured in 33 asthmatic patients (18 women; age range, 18‐66 years). Disease control was assessed by the Asthma Control Test (ACT). R5‐R20 but not X5 values were significantly related to FEF_25‐75 and FVC/SVC values (p < 0.05 for both correlations). When the bronchodilator response was assessed, no correlation was found among IOS and spirometry changes. ACT scores were related to R5‐R20, FEF_25‐75, and FVC/SVC values (p < 0.01 for all correlations). In asthmatic patients with normal FEV₁ values, R5‐R20 values were related to spirometry measures of small airway function. However, when the bronchodilator response was assessed, IOS and spirometry provided quite different results. Moreover, small airway dysfunction, as assessed by IOS and spirometry, was associated with poor disease control and history of asthma exacerbations. The results of this study confirm the value of IOS, as an investigative tool, and suggest that in asthmatic patients with normal FEV₁ values and poor disease control, small airway function should be investigated.