Chapter 15 Acute Exacerbation of Chronic Obstructive Pulmonary Disease
Definitions
Epidemiology
COPD is a morbid, expensive, and lethal medical condition, affecting an estimated 16 million adults and accounting for 1.4 million emergency room visits in 1998. COPD was directly responsible for 1.9% of hospitalizations in 1998 and was a contributing cause of an additional 7% of hospitalizations during that year. Deaths related to COPD have been increasing steadily in the United States over the past 20 years, especially among women. COPD is now the fourth leading cause of death in the United States.
Pathogenesis
Tobacco smoking accounts for almost all cases of COPD. Cigarette smoke increases the number of neutrophils and macrophages in the airways. These cells release proteases (enzymes that degrade proteins), which destroy lung parenchyma in individuals who are unable to mount an adequate anti-protease response. As a result, mucus production is increased and airways become narrowed, lose their support structure, and develop increased collapsibility that leads to airway obstruction.
During an acute exacerbation of COPD, inflammation, bronchospasm, and mucus hypersecretion lead to airway narrowing, worsening ventilation–perfusion (V/Q) mismatching, and hypoxemia. The work of breathing increases to overcome the high airway resistance, but this leads to increased oxygen consumption by the respiratory muscles, which further lowers the oxygen content in venous blood.
Precipitants
COPD exacerbations become more frequent with increasing disease severity. An important trigger of these exacerbations is viral upper respiratory tract infections (especially rhinovirus), which cause airway inflammation and oxidative stress (Box 15-1). Although bacteria are often cultured from patients with an acute COPD exacerbation, it is unclear whether these organisms are pathogens or colonizers, because bacteria can also be cultured from many patients with stable COPD. The most common pathogenic bacteria are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. COPD exacerbations are more common in the winter (upper respiratory tract infections are more common during these months, and cold temperatures to cause a small reduction in lung function). Exacerbations have also been linked to increasing environmental pollution, which is thought to increase susceptibility to viral respiratory infections.
