Inclusion Criteria:  meeting the diagnosis of COPD according to Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines and had definite airflow limitation with a post-bronchodilator forced expiratory volume in 1 second (FEV1) / forced vital capacity (FVC)<0.7.  Exclusion Criteria:  age <40 years; subjects with airway diseases other than COPD; acute exacerbation of COPD in the past 3 months; active tuberculosis; cardiovascular or cerebrovascular events in the past 3 months; cognitive dysfunction such as vascular dementia or Alzheimer's disease; refusal to participate in this study.