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Chronic obstructive pulmonary disease (COPD) is defined as a disease state characterized by poorly reversible airflow limitation that is usually both progressive and associated with an abnormal inflammatory response of the lung (1). Cigarette smoking is the most important risk factor for the development of COPD. However, only a minority of smokers develop COPD and the reason is still unknown. The pathological hallmarks of COPD are inflammation of the peripheral airways and destruction of lung parenchyma or emphysema. The functional consequence of these abnormalities is expiratory airflow limitation. Since the major determinants of expiratory flow are a driving pressure that promotes flow (elastic recoil of the lung) and an opposing resistance that inhibits flow (airway obstruction), the reduction in flow occurring in COPD is more correctly defined as airflow limitation rather than airflow obstruction, since both …