- Clinical diagnosis of COPD (for at least 6 months). The following definition of COPD from the American Thoracic Society (ATS) will be used: COPD is a disease state characterized by the presence of airflow obstruction due to chronic bronchitis or emphysema; the airflow obstruction is generally progressive, may be accompanied by airway hyper-reactivity, and may be partially reversible [American Thoracic Society, 1995].