hospital-acquired pneumonia
Summary
Hospital-acquired pneumonia (HAP) is pneumonia occurring ≥48 hours after hospital admission, not incubating at admission. It is commonly caused by multidrug-resistant organisms including Pseudomonas aeruginosa, MRSA, and Enterobacteriaceae. HAP is a leading cause of nosocomial infection-related mortality and requires broad-spectrum empiric antibiotic coverage.
Detail
Pathophysiology: HAP develops due to microaspiration of oropharyngeal or gastric contents colonized by hospital-associated pathogens, often facilitated by compromised host defenses, mechanical ventilation (leading to ventilator-associated pneumonia, VAP, a HAP subtype), supine positioning, and disruption of normal mucociliary clearance. Biofilm formation on endotracheal tubes contributes to persistent bacterial seeding in VAP.
Microbiology: Key pathogens include Pseudomonas aeruginosa, Staphylococcus aureus (including MRSA), Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Enterobacter species. Risk factors for multidrug-resistant (MDR) organisms include prior IV antibiotic use within 90 days, septic shock at time of VAP, ARDS preceding VAP, ≥5 days of hospitalization before pneumonia onset, and acute renal replacement therapy.
Clinical Presentation: New or progressive infiltrate on chest imaging plus at least two of: fever, leukocytosis/leukopenia, purulent sputum, and worsening oxygenation. Diagnosis can be challenging as clinical criteria overlap with other conditions (heart failure, ARDS, pulmonary embolism).
Management: Empiric therapy should cover MRSA and Pseudomonas based on local antibiogram data and patient risk factors. Typical regimens combine an anti-pseudomonal beta-lactam (e.g., piperacillin-tazobactam, cefepime, meropenem) with vancomycin or linezolid (MRSA coverage), sometimes adding a second anti-pseudomonal agent (aminoglycoside or fluoroquinolone) if risk factors for resistance are high. De-escalation based on culture data is essential for antimicrobial stewardship.
Prevention: VAP bundles include head-of-bed elevation (30-45 degrees), daily sedation vacations, oral care with chlorhexidine, subglottic secretion drainage, and early mobilization.
Board relevance: Understand distinction from community-acquired pneumonia (different pathogen profile), differentiate HAP vs VAP definitions, and recognize risk factors prompting broader antibiotic coverage. Complications include respiratory failure, sepsis, and increased mortality (especially with MDR organisms).
Sources
- Harrison's Principles of Internal Medicine
- IDSA/ATS Guidelines for HAP and VAP (2016)
- First Aid for the USMLE Step 1
- UpToDate: Hospital-acquired and ventilator-associated pneumonia in adults
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