Information ⬇
- Microorganisms gain access to the lower respiratory tract via microaspiration from the oropharynx (the most common route), inhalation of contaminated droplets, hematogenous spread, or contiguous extension from an infected pleural or mediastinal space.
- Before disease manifests, the size of the organism burden must overcome the ability of macrophages and other components of innate immunity (e.g., surfactant proteins A and D) to clear bacteria.
- 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 intraalveolar 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.
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