Pathophysiology

• Most common: microaspiration (droplet infection) of airborne pathogens or oropharyngeal secretions

• Aspiration of gastric acid (aspiration pneumonitis) , food, or liquids

• Hematogenous dissemination (rare)

1. Failure of protective pulmonary mechanisms (e.g., cough reflex, mucociliary clearance , alveolar macrophages )

2. Infiltration of the pulmonary parenchyma by the pathogen → interstitial and alveolar inflammation

3. Impaired alveolar ventilation → ventilation/perfusion (V/Q) mismatch with intrapulmonary shunting (right to left)

4. Hypoxia due to increased alveolar-arterial oxygen gradient

   • Hypoxia is worsened when the affected lung is in the dependent position, as perfusion to the dependent lung is better compared to the nondependent lung.

   • In the case of a large unilateral pulmonary abscess, it may be helpful to position the patient so that the affected lung is in the dependent position in order to prevent the pus from filling the unaffected lung.

• Classic (typical) pneumonia of an entire lobe; primarily caused by pneumococci

• Characterized by inflammatory intra-alveolar exudate, resulting in consolidation

• Can involve the entire lobe or the whole lung

mostly commonly a descending infection that affects the bronchioles and adjacent alveoli

• Primarily caused by pneumococci and/or other streptococci

• Characterized by acute inflammatory infiltrates that fill the bronchioles and the adjacent alveoli (patchy distribution)

• Usually involves the lower lobes or right middle lobe and affects ≥ 1 lobe

• Manifests as typical pneumonia

• Necrotizing bronchopneumonia and pneumatocele are caused by Staphylococcus aureus and are often preceded by an influenza infection.

interstitial inflammation, typically caused by Mycoplasma and viral infections

• Characterized by a diffuse patchy inflammation that mainly involves the alveolar interstitial cells

• Bilateral multifocal opacities are classically found on chest x-ray.

• Manifests as atypical pneumonia

• Often has an indolent course (walking pneumonia)

• Miliary pneumonia: multiple small infiltrations caused by hematogenous dissemination (e.g., of tuberculosis)

• Cryptogenic organizing pneumonia: characterized by inflammation of the bronchioles and surrounding structures