Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide, and the burden of the disorder will continue to increase over the next 20 years.1,2 COPD is characterized by airflow limitation and classification or staging systems so far have largely been based on only the degree of airflow limitation despite the recognized imperfect relation between the extent of airflow limitation and the presence of symptoms.3 Especially in the later phase of the disease, COPD is now considered as a multicomponent disase4–6 and the use of all-cause mortality as a paramount end-point for the evaluation of novel therapies has focused attention on the role of co-morbidities as part of the natural history of the disease process.7