Chest
Volume 135, Issue 1, January 2009, Pages 48-56
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Original Research
Asthma
A Multivariate Analysis of Risk Factors for the Air-Trapping Asthmatic Phenotype as Measured by Quantitative CT Analysis

https://doi.org/10.1378/chest.08-0049Get rights and content

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 ≥ 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 (FEV1/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.

Section snippets

Study Design

As part of SARP, subjects underwent a history, physical examination, allergy skin testing, laboratory tests (including sputum analysis and IgE levels), pulmonary function tests, and fractional exhaled nitric oxide (FeNO) testing; completed questionnaires on demographic factors, medication use, and medical history; and underwent chest MDCT prior to fiberoptic bronchoscopy (bronchoscopy methods are described in the online supplement). All procedures were performed following the SARP protocol.

Results

One hundred twenty SARP subjects were studied (60 with severe persistent asthma, 34 with nonsevere asthma, and 26 normal control subjects). Subject demographics are listed in Table 1, Table 2.

Discussion

This is the first large study of CT-measured air trapping in a range of extensively characterized asthmatic subjects to identify independent risk factors for the air-trapping phenotype, a phenotype associated with the most severe form of asthma. This assessment of air trapping was quantitatively and objectively performed using a histogram based assessment of lung densities (VIDA Diagnostics) based on the density mask but which employs a more sophisticated method for identifying lung boundaries.

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    Grant support was provided by National Institutes of Health HL69149, HL64368, HL69349, HL69170, HL-69155, HL69174, HL69130, HL69167, HL69116, and HL69174–05.

    The authors have no conflicts of interest to disclose.

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml).

    1

    Dr. Busacker is recipient of the American Academy of Allergy, Asthma, and Immunology Strategic Training in Allergy Research (ST*AR) Award.

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