Pneumonia - Hospital-Acquired

Definition

Hospital-acquired pneumonia (HAP) is a pneumonia occurring 48 hours or more after admission to a hospital, not incubating at the time of admission, often caused by multidrug-resistant organisms and associated with increased morbidity and mortality.

Epidemiology

• Second most common nosocomial infection after urinary tract infections.
• Incidence: 5–10 cases per 1,000 hospital admissions.
• Higher risk in ICU patients, elderly, immunocompromised individuals, and those with chronic lung disease.
• Associated with mechanical ventilation (ventilator-associated pneumonia, VAP).
• Mortality rate: 20–50%, higher in ICU or MDR pathogen infections.

Etiology

• Bacterial pathogens:
• — Gram-negative bacilli: Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Acinetobacter spp.
• — Gram-positive cocci: Staphylococcus aureus (including MRSA).
• — Less common: Legionella spp., anaerobes (aspiration-related).
• Risk factors: prolonged hospitalization, mechanical ventilation, prior antibiotics, immunosuppression, severe underlying disease.
• Polymicrobial infections are common.

Pathophysiology

• Aspiration of colonized oropharyngeal secretions.
• Inhalation of contaminated aerosols or ventilator tubing in mechanically ventilated patients.
• Bacterial adherence to alveolar epithelium with evasion of host defenses.
• Inflammatory response: alveolar filling with exudate, impaired gas exchange.
• Systemic inflammatory response may lead to sepsis, septic shock, and multi-organ dysfunction.