Abstract
BACKGROUND: Small-airways instability resulting in premature airway closure has been recognized as a risk for asthma severity and poor control. Although spirometry has limited sensitivity for detecting small-airways dysfunction, a focus on the air-trapping component of obstruction might identify a risk factor for asthma instability. OBJECTIVE: We sought to use spirometric measurements to identify patterns of airway obstruction in children and define obstruction phenotypes that relate to asthma instability. METHODS: Prebronchodilation and postbronchodilation spirometric data were obtained from 560 children in the Asthma Phenotypes in the Inner City study. An air-trapping obstruction phenotype (A Trpg) was defined as a forced vital capacity (FVC) z score of less than -1.64 or an increase in FVC of 10% of predicted value or greater with bronchodilation. The airflow limitation phenotype (A Limit) had an FEV(1)/FVC z score of less than -1.64 but not A Trpg. The no airflow limitation or air-trapping criteria (None) phenotype had neither A Trpg nor A Limit. The 3 obstruction phenotypes were assessed as predictors of number of exacerbations, asthma severity, and airway lability. RESULTS: Patients with the A Trpg phenotype (14% of the cohort) had more exacerbations during the 12-month study compared with those with the A Limit (P < .03) and None (P < .001) phenotypes. Patients with the A Trpg phenotype also had the highest Composite Asthma Severity Index score, the highest asthma treatment step, the greatest variability in FEV(1) over time, and the greatest sensitivity to methacholine challenge. CONCLUSIONS: A Trpg and A Limit patterns of obstruction, as defined by using routine spirometric measurements, can identify obstruction phenotypes that are indicators of risk for asthma severity and instability.