Internal MedicinePulmonologyCommunity‑Acquired Pneumonia (CAP)

Community‑Acquired Pneumonia (CAP)

Must-Not-Miss / Red Flags

  • Respiratory failure: SpO₂ <90% on room air or pO₂ <60 mmHg
  • Septic shock: requiring vasopressors or lactate >4 mmol/L
  • Multilobar involvement: high risk for ARDS
Patient Explanation
“You have an infection in your lungs that is causing your cough and fever. We’ll treat you with antibiotics and help you breathe easier.”
Board Fact
“The most common pathogen in CAP is Streptococcus pneumoniae, accounting for 30‑40% of cases.”
ICD-10
J18.9

Definition & Core Concept

Community‑acquired pneumonia (CAP) is an acute infection of the lung parenchyma acquired outside the hospital setting, causing cough, fever, pleuritic chest pain, and alveolar consolidation on imaging.

Epidemiology & Risk Factors

  • Annual incidence: 5‑11 per 1,000 adults
  • Leading cause of infectious death in developed countries
  • Risk factors: age >65, COPD, heart failure, DM, immunocompromised state

Pathophysiology (Rule of 3)

  1. Microaspiration or inhalation of pathogen → colonization of alveoli
  2. Alveolar macrophages overwhelmed → inflammatory response → neutrophil recruitment
  3. Alveolar exudate and consolidation → impaired gas exchange → hypoxemia

Clinical Presentation

  • Productive cough with purulent sputum, fever, pleuritic chest pain, dyspnea
  • Examination: crackles, bronchial breath sounds, egophony, increased tactile fremitus
  • Atypical presentation in elderly: confusion, falls, absence of fever

Diagnostic Workup

Chest X‑ray: lobar consolidation, infiltrate. Labs: CBC, BMP, blood cultures x2. Sputum culture: if severe CAP or risk of MRSA/Pseudomonas. Legionella and pneumococcal urinary antigen: for severe CAP. Procalcitonin: can guide antibiotic duration.

Management Protocol

  1. Risk stratification: CURB‑65 or PORT score; treat as outpatient (score 0‑1), inpatient (score 2), or ICU (score ≥3)
  2. Empiric antibiotics: Ceftriaxone 1‑2 g IV + Azithromycin 500 mg IV/PO for ward patients; add Vancomycin for ICU/severe CAP
  3. Oxygen therapy: target SpO₂ ≥92%
  4. IV fluids for dehydration
  5. Duration: 5‑7 days for most; longer for Legionella or complicated cases

Complications & Prognosis

  • Parapneumonic effusion/empyema
  • Lung abscess
  • ARDS and respiratory failure
  • Sepsis and multi‑organ failure

ICU Criteria

ICU admission if: mechanical ventilation required, septic shock, or ≥3 CURB‑65 criteria.

Clinical Vignette

A 60‑year‑old man with diabetes presents with 3 days of fever, cough with rust‑colored sputum, and right‑sided pleuritic chest pain. CXR shows right lower lobe consolidation. CURB‑65 score = 2 (confusion, BUN >19).

Pearls & Pitfalls

  • “Walking pneumonia” (Mycoplasma pneumoniae) presents with dry cough, headache, and minimal chest findings – macrolides are first‑line.
  • The pneumococcal vaccine reduces severity of CAP but does not prevent all cases.

Discharge & Follow-Up

Repeat CXR in 6‑8 weeks to confirm resolution and rule out underlying malignancy. Administer pneumococcal vaccine before discharge if not previously immunized.

Literature & Guidelines

ATS/IDSA 2024 Guidelines on CAP in Adults. PMID: 38593201.

Personal Clinical Notes