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PNEUMONIA

Pneumonia, an infection of the lung parenchyma, is classified as community-acquired (CAP), hospital-acquired (HAP), ventilator-associated (VAP), or health care–associated (HCAP). Although the HCAP category initially referred to CAP caused by a multidrug-resistant (MDR) pathogen, it now describes pts with at least two or three risk factors for infection with MDR pathogens (Table 134-1).

TABLE 134-1Risk Factors for Pathogens Resistant to Usual Therapy for Community-Acquired Pneumoniaa

PATHOPHYSIOLOGY

  • Microorganisms gain access to the lower respiratory tract via microaspiration from the oropharynx (the most common route), hematogenous spread, or contiguous extension from an infected pleural or mediastinal space.

  • Many CAP pathogens are components of the normal alveolar microbiota, which is similar to the oropharyngeal microbiota. This observation suggests that alterations in host defenses (e.g., alveolar macrophage activity, surfactant proteins A and D, mucociliary elevator function) allow overgrowth of one or more components of the normal bacterial microbiota. The two most likely sources of an altered alveolar microbiota are viral upper respiratory tract infections for CAP and antibiotic therapy for HAP/VAP.

  • Classic pneumonia (typified by that due to Streptococcus pneumoniae) presents as a lobar pattern and evolves through four phases characterized by changes in the alveoli:

    • Edema: Proteinaceous exudates are present in the alveoli.

    • Red hepatization: Erythrocytes and neutrophils are present in the intra-alveolar exudate.

    • Gray hepatization: Neutrophils and fibrin deposition are abundant.

    • Resolution: Macrophages are the dominant cell type.

  • In VAP, respiratory bronchiolitis can precede a radiologically apparent infiltrate.

COMMUNITY-ACQUIRED PNEUMONIA

Microbiology

Although many bacteria, viruses, fungi, and protozoa can cause CAP, most cases are caused by relatively few pathogens. In >50% of cases, a specific etiology is never determined.

  • Typical bacterial pathogens include S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa.

    • – The incidence of pneumococcal pneumonia is decreasing because of the increasing use of pneumococcal vaccines.

  • Atypical organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., and respiratory viruses (e.g., influenza viruses, adenoviruses, human metapneumovirus, respiratory ...

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