Chapter 133: Chronic Obstructive Pulmonary Disease

Chapter 133: Chronic Obstructive Pulmonary Disease is a topic covered in the Harrison's Manual of Medicine.

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DEFINITION AND EPIDEMIOLOGY

Chronic obstructive pulmonary disease (COPD) is a syndrome characterized by chronic airflow obstruction. COPD includes emphysema (lung parenchymal destruction), chronic bronchitis (chronic cough and phlegm production), and small airway disease (fibrosis and destruction of small airways) in varying combinations in different pts. 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 133-1): ≥80% is stage I, 50–80% is stage II, 30–50% is stage III, and <30% is stage IV. Pts who do not meet these classic thresholds for airflow obstruction may have emphysema, chronic bronchitis, and respiratory symptoms suggestive of COPD.

TABLE 133-1: GOLD Spirometric Grading Criteria for COPD Severity
GOLD STAGESEVERITYSPIROMETRY
IMildFEV1/FVC <0.7 and FEV1 ≥80% predicted
IIModerateFEV1/FVC <0.7 and FEV1 ≥50% but <80% predicted
IIISevereFEV1/FVC <0.7 and FEV1 ≥30% but <50% predicted
IVVery severeFEV1/FVC <0.7 and FEV1 <30% predicted
Abbreviation: GOLD, Global Initiative for Lung Disease.
Source: From the Global Strategy for Diagnosis, Management and Prevention of COPD 2014. Available from http://www.goldcopd.org; with permission.

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. The impact of electronic cigarettes on the development and progression of COPD is uncertain.

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 may 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.

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