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5-10% of asthma cases are poorly controlled chronically and refractory to treatment, and these severe cases account for disproportionate asthma-associated morbidity, mortality, and health care utilization. While persons with severe asthma tend to have more airway obstruction, it is not known whether they represent the severe tail of a unimodal asthma population, or a severe asthma phenotype. We hypothesized that severe asthma has a characteristic physiology of airway obstruction, and we evaluated spirometry, lung volumes, and reversibility during a stable interval in 287 severe and 382 non-severe asthma subjects from the Severe Asthma Research Program. We partitioned airway obstruction into components of air trapping (indicated by FVC) and airflow limitation (indicated by FEV1/FVC). Severe asthma had prominent air trapping, evident as reduced FVC over the entire range of FEV1/FVC. This pattern was confirmed with measures of RV/TLC in a subgroup. In contrast, non-severe asthma did not exhibit prominent air trapping, even at FEV1/FVC
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