title: Diffusion capacity and CT measures of emphysema and airway wall thickness - relation to arterial oxygen tension in COPD patients.
abstract: BACKGROUND
  Decreased diffusing capacity of the lung for carbon monoxide (DLCO) is associated with emphysema. DLCO is also related to decreased arterial oxygen tension (PaO2), but there are limited data on associations between PaO2 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 PaO2 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 (FEV1).
  
  
  RESULTS
  Sixty two per cent of the subjects were men, mean (SD) age was 64 (7) years, mean (SD) FEV1 in percent predicted was 50 (15)%, and mean PaO2 (SD) was 9.3 (1.1) kPa. The adjusted regression coefficient (CI) for PaO2 was -0.32 (-0.04-(-0.019)) per 10% increase in %LAA (p<0.01). When diffusion capacity and FEV1 were added to the model, respectively, the association lost its statistical significance. No relationship between airway wall thickness and PaO2 was found.
  
  
  CONCLUSIONS
  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 PaO2, but cannot replace measurements of diffusion capacity in the clinical evaluation of hypoxaemia.
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway; eiru@mac.com.
authors.forename: Eirunn Waatevik
authors.initials: EW
authors.lastname: Saure
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway.
authors.forename: Per Sigvald
authors.initials: PS
authors.lastname: Bakke
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway.|Department of Thoracic Medicine, Haukeland University Hospital, Bergen, Norway.
authors.forename: Tomas Mikal
authors.initials: TM
authors.lastname: Lind Eagan
authors.affiliation: Department of Heart Disease, Haukeland University Hospital, Bergen, Norway.
authors.forename: Marianne
authors.initials: M
authors.lastname: Aanerud
authors.affiliation: Internal Medicine, Pulmonary Division, University of Utah, Salt Lake City, Utah, USA.
authors.forename: Robert Leroy
authors.initials: RL
authors.lastname: Jensen
authors.affiliation: Department of Thoracic Medicine, Haukeland University Hospital, Bergen, Norway.
authors.forename: Thomas Blix
authors.initials: TB
authors.lastname: Grydeland
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway.
authors.forename: Ane
authors.initials: A
authors.lastname: Johannessen
authors.affiliation: Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway.
authors.forename: Roy Miodini
authors.initials: RM
authors.lastname: Nilsen
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway.
authors.forename: Einar
authors.initials: E
authors.lastname: Thorsen
authors.affiliation: Department of Clinical Science, Pulmonary Division, University of Bergen, Bergen, Norway.
authors.forename: Jon Andrew
authors.initials: JA
authors.lastname: Hardie
keywords: COPD; airway wall thickness; arterial oxygen tension; computed tomography; diffusion capacity; emphysema