Baseline characteristics of the study sample, presented as the mean ± s.d. for continuous variable and as the percentage for categorical variables.

#smokers/ex-smokers/nonsmokers.
*FEV1: Forced expiratory volume in 1 s.
†PaO2: arterial oxygen tension.
§LTDOT: Long-term domiciliary oxygen therapy.
##Exacerbations requiring either hospitalization or treatment with oral antibiotics or oral steroids.
**Associations were tested with t-test and Chi-square.
Baseline characteristics of the study sample, presented as the mean ± s.d. for continuous variable and as the percentage for categorical variables.