Relationship Between Exhaled Nitric Oxide and Small-Airway Dysfunction in Children with Asthma Using Spirometry and the Impulse Oscillometry System

Purpose: Fractional exhaled nitric oxide (FeNO) is a maker of airway inflammation, and impedance of low frequency in the impulse oscillometry system (IOS) reflects small-airway obstruction. We investigated the association of the FeNO level with IOS parameters and spirometry results in asthma patients.Methods: Fifty-eight children with asthma (60.3%, male), mean age 8.3 years (range, 4.5-16.0 years), were enrolled in the study. Reactance and resistance at 5 Hz with IOS, forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and forced expiratory flow 25%-75%of the vital capacity (FEF25%-75%) with spirometry and FeNO were measured in all patients. The Z-score of spirometry and IOS parameters and the mean level of FeNO were used for correlation and regression analysis.Results: FeNO was not significantly associated with height, age, or other demographic parameters. There was a statistically significant correlation between spirometry results and IOS measurements. The FeNO level was not significantly correlated with IOS variables. After adjusting for height, sex, atopic status, and the use of inhaled corticosteroid, the FeNO level showed significant correlations with Z-score of FEV1/FVC (P= 0.037, adjusted R2= 0.234).Conclusion: FeNO was significantly correlated with Z-scores of FEV1/FVC, but not with IOS variables. Therefore, FeNO may be used to detect whole airway obstruction, but not small-airway obstruction.