The exact role of infections as triggers of acute respiratory failure is difficult to establish because the respiratory tract in COPD patients is routinely colonized by micro-organisms. Moreover, some patients may have viable pathogens in their airways without symptoms of infection, while others may suffer severe infection by opportunistic micro-organisms.

One reason for this apparently erratic behavior is the inoculum effect. Pathogens with low virulence can result in severe infections if a large number arrive in the lower airways. Another important factor facilitating the growth of micro-organisms is a defective local immune system. Bacterial residence time is increased because of the thickening of the bronchial mucosa and reduced mucous clearance. These factors are enhanced by malnutrition, which is common in COPD patients.

In addition to the common respiratory pathogens ( Streptococcus pneumoniae, Hemophilus influenzae, and Mycoplasma pneumoniae), Moraxella catarrhalis and viridans streptococci are increasingly being recognized as precipitants of acute respiratory failure ( Cu^[tis..and Hudson 1994). Nevertheless, the lack of correlation between antibiotic treatment and resolution of acute respiratory failure suggests that viruses can also have an important responsibility in many episodes. Outbreaks of acute respiratory failure coincident with influenza epidemics are clear examples that viral infection is typically the key determinant. However, the difficulties in diagnosing active viral diseases preclude stronger statements.

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