Oscillometry and Asthma Control in Patients With and Without Fixed Airflow Obstruction

Asthma is defined by the presence of reversible airflow limitation, yet persistently abnormal spirometry may develop despite appropriate asthma treatment. Fixed airflow obstruction (FAO) describes abnormal postbronchodilator spirometry that is associated with greater symptom burden and disease severity. Respiratory oscillometry measures the mechanics of the entire airway tree, including peripheral airway changes that have been shown to influence asthma symptoms.To evaluate the relationship between abnormal oscillometry following bronchodilator and symptom control in adults with asthma.A prospective cohort of patients with asthma attending an airways clinic completed oscillometry (resistance and reactance), spirometry, and the Asthma Control Test. Postbronchodilator lung function below the lower limit of normal was considered abnormal. Spirometric FAO was defined as FEV1/forced vital capacity below the lower limit of normal. Spearman's rank coefficient and multiple linear regression were performed to assess associations of lung function parameters with Asthma Control Test. The discriminative ability of abnormal lung function to identify poor asthma control was determined using Cohen's kappa.Ninety patients with asthma were included; 48% had spirometric FAO. Only reactance parameters, not spirometry, significantly related to (rs ≥ 0.315; P < .05) and identified asthma control (r2 = 0.236; P < .001). Lung function was more strongly associated with asthma control in patients with FAO compared with those without. Abnormal oscillometry identified an additional 24% of patients with poor asthma control as compared with spirometric FAO.Reactance related to asthma control, independently of spirometric FAO. Abnormal postbronchodilator reactance identified more patients with poor asthma control compared with spirometry. These findings confirm that oscillometry is a relevant lung function test in the clinical assessment of asthma.