Understanding the relationship between percentage emphysema as assessed by volumetric CT and lung function in patients with chronic obstructive pulmonary disease

Background : COPD is a heterogeneous disease characterized by airway inflammation and emphysema. It is an underestimated disease with delayed diagnosis until airway obstruction is detected on spirometry. Spirometry is the most relied upon method to assess severity and progression of disease. Newer methods like volumetric Computed Tomography measure the percentage emphysema. The aim of the study was to determine the relationship between CT measures of emphysema and lung function in Chronic Obstructive Pulmonary Disease. Methods :65 stable COPD patients were enrolled. COPD was diagnosed by GOLD guidelines 2009 with post bronchodilator FEV1/FVC < 70%. Spirometry performed as per ATS/ERS 2005 guidelines. Volumetric CT was performed using Siemens Somaton Definition AS multi detector CT using low dose radiation protocol (140kVp, 30 mAs). Two airflow obstruction parameters were assessed, post bronchodilator FEV1% and FEV1/FVC. Emphysema was defined by percentage voxels < -950 Hounsfield Units on inspiratory scans. CT measures of emphysema were correlated to FEV1 post and FEV1/FVC post by univariate and linear regression analysis. Results : 78% patients were in moderate to severe COPD category.Mean age 63.9(8.2),mean FEV1 post 58.1(18.9), mean % emphysema was 23.4%. Correlation coefficient of FEV1 post to % emphysema was -0.31, p=0.01 and linear regression explained 9% variability in FEV1 and Mean Lung Density r=0.172,p=0.17. Conclusions : % emphysema showed moderate correlation with lung function in COPD. Analysing expiratory CT findings of air trapping and airway wall thickening may explain the variability in lung function.