A multivariate analysis of risk factors for the air-trapping asthmatic phenotype as measured by quantitative CT analysis

Abstract

Background: Patients with severe asthma have increased physiologically measured air trapping; however, a study using CT measures of air trapping has not been performed. This study was designed to address two hypotheses: (1) air trapping measured by multidetector CT (MDCT) quantitative methodology would be a predictor of a more severe asthma phenotype; and (2) historical, clinical, allergic, or inflammatory risk factors could be identified via multivariate analysis.

Methods: MDCT scanning of a subset of Severe Asthma Research Program subjects was performed at functional residual capacity. Air trapping was defined as >or= 9.66% of the lung tissue < - 850 Hounsfield units (HU). Subjects classified as having air trapping were then compared to subjects without air trapping on clinical and demographic factors using both univariate and multivariate statistical analyses.

Results: Subjects with air trapping were significantly more likely to have a history of asthma-related hospitalizations, ICU visits, and/or mechanical ventilation. Duration of asthma (odds ratio [OR], 1.42; 95% confidence interval [CI], 1.08 to 1.87), history of pneumonia (OR, 8.55; 95% CI, 2.07 to 35.26), high levels of airway neutrophils (OR, 8.67; 95% CI, 2.05 to 36.57), airflow obstruction (FEV(1)/FVC) [OR, 1.61; 95% CI, 1.21 to 2.14], and atopy (OR, 11.54; 95% CI, 1.97 to 67.70) were identified as independent risk factors associated with the air-trapping phenotype.

Conclusions: Quantitative CT-determined air trapping in asthmatic subjects identifies a group of individuals at high risk for severe disease. Several independent risk factors for the presence of this phenotype were identified: perhaps most interestingly, history of pneumonia, neutrophilic inflammation, and atopy.

Figure 1

CT-derived three-dimension display of the lungs, airways and regions of air trapping. Example comparison of two asthmatic subjects falling in the non-severe (upper row) or severe (lower row) categories. In the left column, the lung lobes and airway tree are shown from a ventral view. In the right column, the air trapping is depicted, color coded by lobe and displayed from the dorsal aspect. Software allows one to click on an airway path of interest and airway segment labels are automatically generated. Trapped air defined as voxels within the lung field falling below −850 are highlighted and coded by lobe in the left column. The severe asthma subject has 21.0% of lung less than −850 HU as compared to the non-severe asthma subject with 4.75% of lung less than −850 HU. Images from Pulmonary Workstation 2.0 (Vida Diagnostics, Coralville, Iowa)

Figure 2

Association between air trapping and presence of severe asthma or severe asthma exacerbations.