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PNEUMONIA

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EPIDEMIOLOGY

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Pneumonia is an infection of the alveoli (the gas-exchanging portion of the lung) emanating from different pathogens, notably bacteria and viruses, but also fungi. Community-acquired pneumonia occurs in 4 million people and results in 1 million hospitalizations per year in the United States.1,2 Pneumonia is the eighth leading cause of death, particularly among older adults,3 and is the most common trigger for sepsis. Those who develop hospital- or other healthcare-associated pneumonia (acquired after placement in a care facility) often have infection from resistant organisms (Table 65-1).4,5

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TABLE 65-1Acquisition Environment Classification for Pneumonia
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PATHOPHYSIOLOGY

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Pathogenic lung organisms are usually aspirated, especially in the hospital or healthcare setting (where eating is often not done sitting upright for dubious reasons), although inhalation is another potential route. Staphylococcus aureus and Streptococcus pneumoniae can produce pneumonia from hematogenous seeding. Patients most at risk for pneumonia are those with a predisposition to aspiration, impaired mucociliary clearance, or risk of bacteremia (Table 65-2).

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TABLE 65-2Risk Factors for Pneumonia
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Some forms of pneumonia produce an intense inflammatory response within the alveoli that leads to filling of the air space with exudate and white blood cells. Bacterial pneumonia results in an intense inflammatory response and tends to cause a productive cough. Atypical organisms often trigger a less intense inflammatory response and create a milder or nonproductive cough.

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In about half of patients with community-acquired pneumonia, no specific pathogen is identified. When an organism is identified, the pneumococcus is still the most common, followed by viruses and the atypical agents Mycoplasma, Chlamydia, and Legionella. Most patients with severe community-acquired pneumonia who were otherwise healthy have S. pneumoniae...

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