Chronic obstructive pulmonary disease (COPD) is a disease state characterized by chronic airflow obstruction; thus, pulmonary function testing is central to its diagnosis. The presence of airflow obstruction is determined by a reduced ratio of the forced expiratory volume in 1 s (FEV₁) to the forced vital capacity (FVC). Among individuals with a reduced FEV₁/FVC ratio, the severity of airflow obstruction is determined by the level of reduction in FEV₁ (Table 131-1): ≥80% is stage I, 50-80% is stage II, 30-50% is stage III, and <30% is stage IV. Cigarette smoking is the major environmental risk factor for COPD. The risk of COPD increases with cigarette smoking intensity, which is typically quantified as pack-years. (One pack of cigarettes smoked per day for 1 year equals 1 pack-year.) Individuals with airway hyperresponsiveness and certain occupational exposures (e.g., coal mining, gold mining, and cotton textiles) are likely also at increased risk for COPD. In countries in which biomass combustion with poor ventilation is used for cooking, an increased risk of COPD among women has been reported. COPD is a progressive disorder; however, the rate of loss of lung function often slows markedly if smoking cessation occurs. In normal individuals, FEV₁ reaches a lifetime peak at around age 25 years, enters a plateau phase, and subsequently declines gradually and progressively. Subjects can develop COPD by having reduced maximally attained lung function, shortened plateau phase, or accelerated decline in lung function.

Symptoms often occur only when COPD is advanced; thus, early detection requires spirometric testing. The PaO₂ typically remains near normal until the FEV₁ falls to <50% of the predicted value. Hypercarbia and pulmonary hypertension are most common after FEV₁ has fallen to <25% of predicted. COPD pts with similar FEV₁ values can vary markedly in their respiratory symptoms and functional impairment. COPD often includes periods of increased respiratory symptoms, such as dyspnea, cough, and phlegm production, which are known as exacerbations. Exacerbations are often triggered by bacterial and/or viral respiratory infections. These exacerbations become more common as COPD severity increases, but some individuals are much more susceptible to developing exacerbations than others with similar degrees of airflow obstruction.