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Original Research |

The Relationship Between Airflow Obstruction, Emphysema Extent, and Small Airways Function in COPDThe Predictors of Airflow Obstruction in COPD

Sophie C. Timmins, MBBS (Hons 1); Chantale Diba, PhD; Catherine E. Farrow, BSpSc; Robin E. Schoeffel, MHSc; Norbert Berend, MD, FCCP; Cheryl M. Salome, PhD; Gregory G. King, PhD
Author and Funding Information

From the Woolcock Institute of Medical Research (Drs Timmins, Diba, Berend, Salome, and King and Mss Farrow and Schoeffel) and Cooperative Research Centre for Asthma and Airways (Drs Timmins, Diba, Berend, Salome, and King and Ms Farrow), Glebe; Department of Respiratory Medicine (Drs Timmins and King and Ms Schoeffel), Royal North Shore Hospital, St. Leonards; and Sydney Medical School (Drs Timmins, Berend, Salome, and King and Mss Farrow and Schoeffel), University of Sydney, Sydney, NSW, Australia.

Correspondence to: Sophie C. Timmins, MBBS(Hons 1), Airway Physiology Group, The Woolcock Institute of Medical Research, PO Box M77, Missenden Rd, NSW 2050, Australia; e-mail: stimmins@med.usyd.edu.au


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.


Chest. 2012;142(2):312-319. doi:10.1378/chest.11-2169
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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.

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