Pneumonia (community-acquired, hospital-acquired)

Pneumonia Types - Lungs Under Siege

• Community-Acquired (CAP): Acquired outside a hospital or within 48 hours of admission.
• Hospital-Acquired (HAP): Develops ≥48 hours after hospital admission.
  • Ventilator-Associated (VAP): Arises >48 hours after endotracheal intubation.

• Clinical Syndromes:
  • Typical: Sudden onset, high fever, purulent cough, lobar consolidation. Caused by typical organisms (S. pneumoniae).
  • Atypical ("Walking"): Insidious onset, low-grade fever, non-productive cough, patchy infiltrates. Caused by atypical organisms (Mycoplasma, Chlamydia, Legionella).

⭐ Exam Favorite: Legionella pneumophila, an atypical cause, is classically associated with GI symptoms (diarrhea) and hyponatremia (low sodium).

Etiology - The Usual Suspects

-   **Alcoholism:** *Klebsiella pneumoniae* (currant jelly sputum)
-   **COPD/Smoking:** *H. influenzae*, *Pseudomonas*
-   **Post-Influenza:** *S. aureus* (often MRSA)
-   **Aspiration:** Oral anaerobes (e.g., *Peptostreptococcus*)

⭐ Pseudomonas aeruginosa is a critical pathogen in patients with structural lung diseases like cystic fibrosis or bronchiectasis, often requiring specific antibiotic coverage.

📌 Atypicals: 'My Legion of Clamoring Q-Ts' (Mycoplasma, Legionella, Chlamydophila, Coxiella)

Diagnosis & Severity - Spotting the Shadow

• Clinical & Radiologic Dx:

  • H&P: Fever, cough, dyspnea, crackles on auscultation.
  • Chest X-ray: Key to diagnosis. Look for:
    • Lobar consolidation: Classic for S. pneumoniae.
    • Interstitial infiltrates: Atypical pathogens (e.g., Mycoplasma).
    • Cavities: S. aureus, anaerobes, TB.
  • Labs: ↑ WBC; ↑ Procalcitonin suggests bacterial etiology.

• Severity Scoring (CURB-65 for CAP):

  • Confusion (new onset)
  • Uremia (BUN > 20 mg/dL)
  • Respiratory Rate (≥ 30 breaths/min)
  • Blood Pressure (SBP < 90 or DBP ≤ 60 mmHg)
  • Age (≥ 65 years)
  • Disposition: 0-1 (Outpatient), 2 (Inpatient), ≥3 (ICU).

⭐ Procalcitonin is a specific marker for bacterial infections; levels >0.25 ng/mL strongly support bacterial pneumonia and guide antibiotic stewardship.

Treatment - The Antibiotic Arsenal

• Community-Acquired Pneumonia (CAP): Empiric choice depends on severity.

• Hospital-Acquired (HAP) / Ventilator-Associated (VAP): Cover MRSA & Pseudomonas.

  • MRSA: Vancomycin or Linezolid.
  • Anti-pseudomonal: Piperacillin-tazobactam, Cefepime, or a Carbapenem.

⭐ For CAP, empiric outpatient therapy must cover atypical pathogens. Standard beta-lactams alone are insufficient against Mycoplasma, Chlamydia, and Legionella.

High‑Yield Points - ⚡ Biggest Takeaways

• S. pneumoniae is the most common cause of Community-Acquired Pneumonia (CAP).
• Atypical pathogens like Mycoplasma cause "walking pneumonia," especially in young adults.
• Use the CURB-65 score to determine CAP severity and need for hospitalization.
• Hospital-Acquired Pneumonia (HAP) develops >48 hours post-admission; common culprits are Pseudomonas and MRSA.
• Legionella is linked to contaminated water and classically causes hyponatremia and GI symptoms.
• For severe pneumonia, obtain blood and sputum cultures before starting antibiotics.