Association of Small Airway Functional Indices With Respiratory Symptoms and Comorbidity in Asthmatics: A National Cross-Sectional Study.

Abstract

Background: Small airway dysfunction (SAD) and airway inflammation are vital in asthma exacerbations. Type 2 inflammation (T2), mediated by cytokines from T helper 2 cell (Th2) such as interleukin (IL)-4, IL-5, and IL-13, is a potential mechanism underlying SAD. Research on small airway function in asthma is limited. We aimed to explore the correlation between small airway function and respiratory symptoms and comorbidity in T2 and non-T2 asthma.

Methods: Derived from the National Health and Nutrition Examination Survey (NHANES), our study encompassed 2,420 asthma patients aged 6 - 79 years, including pulmonary function (PF) data such as forced expiratory flow between 25% and 75% of forced vital capacity (FEF_25-75), forced expiratory volume in 1 second (FEV₁), forced expiratory volume in 3 seconds (FEV₃), forced expiratory volume in 6 seconds (FEV₆), and forced vital capacity (FVC). To evaluate the small airway function, we calculated z-scores for FEF_25-75, FEF_25-75/FVC, FEV₁/FEV₆, and FEV₃/FEV₆. Logistic regression determined the adjusted odds ratios (aORs) for symptoms and comorbidity.

Results: FEF_25-75, FEV₁/FEV₆, and FEV₃/FEV₆ correlated with asthmatic symptoms. FEF_25-75 had the strongest association with wheezing or whistling attacks. An increase of 1 standard deviations (SD) in FEF_25-75 reduced recurrent wheezing (aOR: 0.70; 95% confidence intervals (95% CIs): 0.65 - 0.76) and severe attacks (aOR: 0.67; 95% CI: 0.62 - 0.94). These indices were also linked to dry cough and hay fever, particularly FEV₃/FEV₆ reducing hay fever risk (aOR: 0.70; 95% CI: 0.55 - 0.91) in non-T2 asthma. FEF_25-75/FVC related to persistent (aOR: 0.78; 95% CI: 0.72 - 0.84) and severe attacks (aOR: 1.14; 95% CI: 1.08 - 1.22) in non-T2 groups. Lower indices combined with T2 exposure raised severe attack risk.

Conclusions: In this nationwide study, small airway function correlated with symptom onset, especially in T2 asthma. Small airway injury differed between T2 and non-T2 asthma. Prospective research is needed to establish reference values.