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Pneumonia is an infection of the lung parenchyma and remains one of the most common causes of infectious morbidity and mortality globally. Community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and ventilator-associated pneumonia (VAP) differ in microbiology, severity, and management approach.
Microorganisms overcome respiratory defenses (mucociliary clearance, alveolar macrophages) through aspiration of oropharyngeal flora, inhalation of aerosolized particles, or hematogenous spread. Bacterial pneumonia (Streptococcus pneumoniae most common) triggers an acute inflammatory exudate filling alveolar spaces. Atypical organisms (Mycoplasma, Legionella, Chlamydophila) cause a more indolent, interstitial pattern. Viral pneumonia (influenza, SARS-CoV-2) damages respiratory epithelium directly.
Clinical reasoning integrates symptoms (productive cough, fever, pleuritic chest pain), exam (crackles, bronchial breath sounds, dullness to percussion), and chest imaging showing consolidation or infiltrates. CURB-65 or PSI guides inpatient vs. outpatient management. CAP empirical therapy covers typical and atypical organisms — a beta-lactam plus macrolide or respiratory fluoroquinolone. Legionella and pneumococcal urine antigens should be obtained in severe CAP. Key differentials include pulmonary edema, lung cancer, and pulmonary hemorrhage.
Key imaging focus: Lobar vs interstitial patterns, air bronchograms, silhouette sign
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The CURB-65 score assigns one point each for Confusion, BUN >19, Respiratory rate ≥30, Blood pressure <90/60, and age ≥65. Score 0-1: outpatient; score 2: consider hospitalization; score ≥3: hospital admission, ICU evaluation for score ≥4-5.
Outpatient low-risk CAP: amoxicillin or doxycycline. Outpatient with comorbidities: respiratory fluoroquinolone or beta-lactam plus macrolide. Inpatient non-ICU: beta-lactam plus macrolide or respiratory fluoroquinolone. ICU: beta-lactam plus azithromycin or fluoroquinolone, with anti-pseudomonal coverage if risk factors present.
Legionella urinary antigen is recommended in severe CAP (ICU admission), CAP with hyponatremia, travel-associated pneumonia, outbreak situations, or pneumonia unresponsive to beta-lactam therapy. It detects serogroup 1, which causes 80% of Legionella pneumonia.