Infectious Diseases

Pneumonia

Clinical overview and exam mastery guide for CAP, HAP, and VAP with empiric therapy logic, dosing, MRSA risk decisions, and duration strategy.

Core Diagnosis
Lung parenchymal infection
CAP Typical
S. pneumoniae common
HAP/VAP Risk
Pseudomonas + MRSA
Duration Anchor
CAP 5+ days, HAP/VAP 7 days

Pneumonia Type and Empiric Spectrum Map

CAP S. pneumoniae, H. influenzae Atypicals HAP Pseudomonas risk MRSA consideration VAP Higher MDR risk Culture-guided de-escalation Empiric first, then narrow once microbiology returns Avoid unnecessary broad-spectrum continuation

1. Definition

Pneumonia is infection of lung parenchyma.

Community-Acquired (CAP) Hospital-Acquired (HAP) Ventilator-Associated (VAP)

2. Common Pathogens

CAP

  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Atypicals: Mycoplasma, Chlamydia, Legionella

HAP/VAP

  • Pseudomonas
  • MRSA
  • Gram-negative rods

3A. Community-Acquired Pneumonia (Outpatient, No Comorbidities)

Option MOA Dose Key Risks / Contraindications
Amoxicillin Beta-lactam cell wall synthesis inhibition 1 g orally three times daily GI upset, rash; avoid in severe penicillin allergy
Doxycycline 30S ribosomal inhibition 100 mg orally twice daily Photosensitivity, GI upset; avoid in pregnancy and children under 8 years
Azithromycin (if local resistance low) Macrolide 50S inhibition 500 mg day 1, then 250 mg daily days 2-5 QT prolongation risk; avoid in known prolonged QT

3B. CAP Outpatient (With Comorbidities)

Option 1

  • Amoxicillin-clavulanate + azithromycin
  • Amox-clav dose example: 875/125 mg twice daily

Option 2

  • Respiratory fluoroquinolone (for example, levofloxacin)
  • Levofloxacin dose: 750 mg daily for 5 days
  • MOA: DNA gyrase inhibition
Levofloxacin risks: tendon injury, QT prolongation, dysglycemia. Avoid in pregnancy and myasthenia gravis.

4. Inpatient CAP (Non-ICU)

  • Ceftriaxone + azithromycin is a common regimen
  • Ceftriaxone dose: 1-2 g IV daily
  • Alternative: levofloxacin 750 mg IV daily
  • Ceftriaxone risks: biliary sludge, rash

5. ICU CAP

  • Beta-lactam + macrolide OR beta-lactam + fluoroquinolone
  • Add vancomycin if MRSA risk factors are present
  • Vancomycin dose: 15-20 mg/kg IV every 8-12 hours
  • Target trough (severe infection): 15-20 mcg/mL
  • Risks: nephrotoxicity, red man syndrome

6. Hospital-Acquired Pneumonia (HAP)

  • Must cover Pseudomonas and evaluate MRSA risk
  • Example empiric: piperacillin-tazobactam + vancomycin
  • Piperacillin-tazobactam dose: 4.5 g IV every 6 hours
  • Risks: renal injury, electrolyte imbalance

7. Ventilator-Associated Pneumonia (VAP)

  • Higher MDR risk than many HAP presentations
  • Use anti-pseudomonal beta-lactam + MRSA coverage when indicated
  • De-escalate once culture and susceptibility data are available

8. Duration of Therapy

CAP

  • Minimum 5 days
  • Afebrile for 48-72 hours with clinical stability

HAP/VAP

  • Typical: 7 days
  • Longer course only if complications or poor response

Management Recap Drill

Outpatient CAP: amoxicillin OR doxycycline (or azithro if low resistance).
↓
Comorbid outpatient: amox-clav + azithro OR levofloxacin.
↓
Inpatient CAP: ceftriaxone + azithromycin (or levofloxacin).
↓
HAP/VAP: anti-pseudomonal coverage + MRSA consideration; de-escalate by cultures.

CAP vs HAP/VAP Empiric Treatment Algorithm

CAP pathway Outpatient vs inpatient selection HAP pathway Broad empiric with pseudomonal focus VAP pathway Higher MDR concern Step down by culture results as early as feasible Narrowing therapy improves safety and stewardship

Guideline References (Management)

IDSA / ATS Guidance

https://www.idsociety.org

Guideline Scope

  • CAP outpatient vs inpatient regimens
  • MRSA and pseudomonal risk stratification
  • HAP/VAP empiric therapy and duration recommendations

9. Common Exam Traps

Levofloxacin CAP regimen is often 750 mg daily for 5 days.
Doxycycline is an acceptable first-line CAP option.
Add MRSA coverage only when risk factors are present.
Minimum CAP duration is 5 days with stability criteria.
Always de-escalate when cultures permit.

10. Quick Revision Summary

Must Remember

  • CAP commonly involves S. pneumoniae
  • HAP/VAP carries pseudomonal and MRSA risk
  • Beta-lactam + macrolide is common in inpatient CAP
  • Therapy often lasts 5-7 days depending on syndrome and response

High-Yield Subtopics

  • CAP empiric outpatient logic
  • Comorbidity-driven outpatient escalation
  • HAP/VAP broad-to-narrow sequence
  • MRSA risk-based add-on decisions

Practice Questions

1) Outpatient CAP without comorbidities: name two first-line options.

Answer: Amoxicillin or doxycycline (azithromycin only where macrolide resistance is acceptably low).

2) What is a common high-dose short-course levofloxacin CAP regimen?

Answer: 750 mg once daily for 5 days.

3) Typical inpatient non-ICU CAP combination?

Answer: Ceftriaxone plus azithromycin.

4) Core empiric concept for HAP/VAP?

Answer: Ensure anti-pseudomonal coverage and add MRSA therapy when risk indicates.

5) Minimum CAP duration if clinically stable?

Answer: At least 5 days, with afebrile period and clinical stability.