Diffusion capacity and CT measures of emphysema and airway wall thickness – relation to arterial oxygen tension in COPD patients

BACKGROUND: Decreased diffusing capacity of the lung for carbon monoxide (DLCO) is associated with emphysema. DLCO is also related to decreased arterial oxygen tension (PaO(2)), but there are limited data on associations between PaO(2) and computed tomography (CT) derived measures of emphysema and airway wall thickness. OBJECTIVE: To examine whether CT measures of emphysema and airway wall thickness are associated with level of arterial oxygen tension beyond that provided by measurements of diffusion capacity and spirometry. METHODS: The study sample consisted of 271 smoking or ex-smoking COPD patients from the Bergen COPD Cohort Study examined in 2007–2008. Emphysema was assessed as percent of low-attenuation areas<−950 Hounsfield units (%LAA), and airway wall thickness as standardised measure at an internal perimeter of 10 mm (AWT-Pi10). Multiple linear regression models were fitted with PaO(2) as the outcome variable, and %LAA, AWT-Pi10, DLCO and carbon monoxide transfer coefficient (KCO) as main explanatory variables. The models were adjusted for sex, age, smoking status, and haemoglobin concentration, as well as forced expiratory volume in one second (FEV(1)). RESULTS: Sixty two per cent of the subjects were men, mean (SD) age was 64 (7) years, mean (SD) FEV(1) in percent predicted was 50 (15)%, and mean PaO(2) (SD) was 9.3 (1.1) kPa. The adjusted regression coefficient (CI) for PaO(2) was –0.32 (−0.04–(−0.019)) per 10% increase in %LAA (p<0.01). When diffusion capacity and FEV(1) were added to the model, respectively, the association lost its statistical significance. No relationship between airway wall thickness and PaO(2) was found. CONCLUSION: CT assessment of airway wall thickness is not associated with arterial oxygen tension in COPD patients. Emphysema score measured by chest CT, is related to decreased PaO(2), but cannot replace measurements of diffusion capacity in the clinical evaluation of hypoxaemia.