Chest
Volume 142, Issue 2, August 2012, Pages 312-319
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Original Research
COPD
The Relationship Between Airflow Obstruction, Emphysema Extent, and Small Airways Function in COPD

https://doi.org/10.1378/chest.11-2169Get rights and content

Background

The severities of COPD (FEV1 % predicted) and airflow obstruction (FEV1/FVC) are considered to be due to both emphysema and small airways disease. To our knowledge, this has not been previously confirmed by combined measurements of emphysema and of small airway function. We hypothesized that small airways disease and emphysema extent contribute independently to the severity of both COPD and airflow obstruction.

Methods

Twenty-six subjects with COPD underwent measurements with forced oscillation technique (FOT) at 6 Hz and single-breath nitrogen washout. Respiratory system resistance, respiratory system reactance (Xrs), and expiratory flow limitation (EFL) index (measured as mean inspiratory Xrs − expiratory Xrs) were derived from FOT. Closing volume/vital capacity (CV/VC) was derived from the washout. Emphysema extent was measured as low attenuation areas < −910 Hounsfield units, expressed as a percentage of CT scan lung volume from multislice CT scans taken at total lung capacity.

Results

Subjects were aged (mean ± SD) 69.6 ± 8.0 years. Postbronchodilator FEV1 was 64.8 ± 19.8% predicted, and diffusing capacity of lung for carbon monoxide was 50.7 ± 15.8% predicted. Emphysema extent was 22.6% ± 15.0% CT scan volume. CV/VC was 16.9% ± 7.9%; Xrs, −3.72 ± 3.03 cm H2O/L/s; and EFL index, 3.88 ± 3.93 cm H2O/L/s. In multiple regression analyses, FEV1/FVC was predicted by both emphysema and CV/VC (model r2 = 0.54, P < .0001) whereas FEV1 % predicted was predicted by emphysema and EFL index (model r2 = 0.38, P = .0014).

Conclusions

The severities of COPD and airflow obstruction are independently predicted by both small airways disease and emphysema extent.

Section snippets

Subject Characteristics

Subjects were recruited from a volunteer database at Royal North Shore Hospital and advertisements in a local newspaper. Inclusion criteria were age ≥ 40 years, a physician diagnosis of COPD, smoking history of > 10 pack-years, a postbronchodilator FEV1/FVC < 0.7, and symptoms of shortness of breath on exertion or chronic cough for at least 12 months. Subjects were excluded if they had coexistent asthma or other respiratory disease or an exacerbation of COPD within the past 6 weeks defined as

Results

Subject baseline demographic characteristics and lung function data are provided in Table 1. Twenty-six subjects participated in the study and had moderate COPD, although there was a wide range of severities. Dlco was severely impaired, and subjects had mild to moderate hyperinflation (Table 1). Rrs was increased at 173.5 ± 45.1% predicted, and Xrs was increased at 653.05 ± 371.7% predicted. Six subjects were not taking any medications, 17 were taking long-acting muscarinic antagonists, 13 were

Discussion

In this COPD cohort, we found airflow obstruction and FEV1/FVC to be independently predicted by both small airways disease as measured by CV/VC and the extent of emphysema as measured by %LAA on CT scan. The severity of COPD as determined by FEV1 % predicted was independently predicted by EFL measured by FOT (EFL index) and by emphysema. The results of the present study are consistent with the notion that airflow limitation in COPD measured by spirometry is a global measure that has separable

Acknowledgments

Author contributions: Dr Timmins had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Dr Timmins: contributed to the study design, data analysis and interpretation, and writing of the manuscript.

Dr Diba: contributed to the data analysis and interpretation and writing of the manuscript.

Ms Farrow: contributed to the data collection and critical review of the manuscript and final draft.

Ms Schoeffel: contributed

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    Funding/Support: The research was funded by the Australian Lung Foundation Webster Memorial Award and the Cooperative Research Centre for Asthma and Airways Project 2.1.

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.

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