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Pathophysiology
Inspiration of ambient air exposes the lungs to environmental and infectious particulate matter. These respiratory pathogens enter the lower respiratory tract by one of three routes:
- Direct inhalation of infectious droplets
- Aspiration of oropharyngeal contents
- Hematogenous spread from another infection site
Following inhalation, the pathogen colonizes the nasopharynx and then reaches the lung alveoli via microaspiration. When the inoculum size is sufficient and/or host immune defenses are impaired, infection results.
As in for immunocompromised patients, they lack robust defense mechanisms and therefore at a higher risk for immediate infection. Lung infections also suppress the antibacterial activity of the lungs by impairing alveolar macrophage function and mucociliary clearance, thus setting the stage for bacterial pneumonia.
Pneumonia arises when there is inflammation and damage of the lung parenchyma due to replication of the pathogen, production of virulence factors, and the host’s reduced immune response.
Clinical Manifestations
CAP has a wide spectrum of associated clinical features. It is for this reason that CAP is often part of the differential diagnosis of almost all respiratory illnesses. These presentations range from mild pneumonia characterized by fever, shortness of breath, and cough, to severe pneumonia with sepsis and respiratory distress. These symptoms are directly related to the immune response of the patient.
