Forced oscillatory technique R5-19 values correlate with spirometry FEV1/FVC in severe eosinophilic asthma. An observational, prospective, cohort study

Introduction: Severe asthma affects 3–10 % of asthmatic patients. Biologic therapies can act as disease modifying agents in severe asthma. The prevalence of small airways dysfunction (SAD) increases with asthma severity. Forced Oscillatory Technique (FOT) is a reliable method for studying small airways. The aim of the study was to analyze FOT parameters in a cohort of eosinophilic severe asthma patients naïve of biologic therapy, and to describe the presence of correlation with spirometry data. Variations of FOT parameters after 6 and 12 months from the start of biologic therapy were also prospectively recorded and analyzed. Methods: 47 severe eosinophilic asthma patients were consecutively enrolled. FOT, spirometry data, levels of asthma biomarkers, number of exacerbations, Asthma Control Test (ACT) were determined at baseline (T0: patients naïve of biologic treatment) and after 6 and 12 months. Results: at T0, a significant linear correlation was found between R5-19 and FEV1/FVC (Forced expiratory volume in 1 s/Forced Vital Capacity) values (p = 0.0008). At T0, FOT R5-19 values were more elevated in obstructed patients. The cut-off value of R5-19 that best discriminates the presence of obstruction (FEV1/FVC <0.70) was determined at 0.81 cmH2O/(L/s) (sensitivity 0.58, specificity 0.76, ROC-AUC 0.67). A significant relationship was found between FEV1/FVC and FOT R5-19 also after 6 (p = 0.007) and 12 months (p = 0.027) of biologic therapy. No significant correlations were found between any other FOT parameter and blood eosinophils count, FeNO, number of exacerbations or ACT. Conclusions: FOT R5-19 values correlate with FEV1/FVC and are significantly higher in obstruction. This correlation could be explained by the higher resistances of small airways in obstructed patients.