Airway reactance predicts static lung hyperinflation in severe asthma.

BACKGROUND AND OBJECTIVES: Static lung hyperinflation (SLH) measured by body plethysmography (Pleth) in asthma is associated with poor outcomes. The severity of SLH may be associated with small airway dysfunction (SAD), which can be measured by impulse oscillometry (IOS). This study aims to determine the correlation between SLH and SAD in patients with severe asthma and the improvement of SLH and SAD in response to treatment.

METHODS: We analyzed data from patients who were enrolled in the Taiwan Severe Asthma Registry, which was a prospective observational cohort. Pleth and IOS were regularly performed. The relationship between spirometric and IOS parameters was determined. Changes in the clinical outcomes in response to treatment were analyzed.

RESULTS: In 107 patients with severe asthma, 83 (77.6%) had SLH by increased residual volume to total lung capacity (RV/TLC) ratio. Most patients were older female with worse pulmonary function and SAD compared with those without SLH. The SAD by increased airway resistance/reactance was significantly correlated with SLH. Airway reactance at 5 Hz (X5) ≤ -0.21 [kPa/(L/s)] detected SLH with the area under the receiver operating characteristic curve of 0.84 (p < 0.0001, sensitivity = 85.2%, and specificity = 83.3%). After 12 months, patients who received add-on biologics treatment had significantly reduced exacerbation, fractional exhaled nitric oxide level, blood eosinophil counts, improved forced expiratory volume in the first second, X5, and a trend of reduced RV/TLC ratio compared with those without biologics treatment.

CONCLUSIONS: In severe asthma, airway reactance X5 could be a novel parameter to assess SLH.