What is pneumonia?

What is Pneumonia?

Pneumonia is an acute infection of the lower respiratory tract that causes inflammation of the air sacs in the lungs, which fill with fluid, leading to respiratory symptoms and potentially significant morbidity and mortality. 1

Definition and Pathophysiology

• Pneumonia is characterized by inflammation of the alveoli and distal airways due to infection by various microorganisms, including bacteria, viruses, and fungi 2, 1
• The infection causes the air sacs to fill with fluid, which impairs gas exchange and leads to respiratory symptoms 3
• Diagnosis combines clinical presentation (fever, cough, auscultatory findings), laboratory data, and radiographic confirmation of pulmonary infiltrates 2

Types of Pneumonia

Based on Acquisition Setting:

• Community-acquired pneumonia (CAP): Infection acquired outside of the hospital setting 2

  • Accounts for approximately 1.4 million emergency department visits, 740,000 hospitalizations, and 41,000 deaths annually in the US 4
  • Diagnosed when symptoms develop in previously healthy individuals in the community 2

• Hospital-acquired pneumonia (HAP): Develops after 48 hours of hospitalization 5

  • Second most common nosocomial infection after bloodstream infections 2
  • Higher risk of drug-resistant pathogens 2

• Ventilator-associated pneumonia (VAP): Occurs in mechanically ventilated patients more than 48 hours after intubation 5

  • Associated with 15-30% mortality rate 5

Based on Radiographic Patterns:

• Lobar pneumonia: Typically associated with community-acquired bacterial infections 6
• Bronchopneumonia: More commonly seen with hospital-acquired infections 6
• Interstitial pneumonia: Often associated with atypical pneumonias caused by viruses or organisms like Mycoplasma pneumoniae 6

Common Causative Pathogens

Community-Acquired Pneumonia:

• Children under 5 years: Predominantly viral agents (e.g., respiratory syncytial virus, parainfluenza virus) 2
• Ages 5-25 years: Mycoplasma pneumoniae is a major cause 2
• Adults and elderly:
  • Streptococcus pneumoniae (50-90% of pyogenic pneumonia cases) 2
  • Haemophilus influenzae, Staphylococcus aureus, and mixed aerobic-anaerobic bacteria are less common 2
  • Atypical pathogens include Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella species 2
  • Up to 40% of identified pathogens in hospitalized patients are viruses 4

Hospital-Acquired Pneumonia:

• Pseudomonas aeruginosa (16.9%), Staphylococcus aureus (12.9%), Klebsiella species (11.6%), and Enterobacter species (9.4%) are most common 2
• During influenza pandemics, secondary bacterial pneumonia is common, with S. aureus being particularly prevalent 2

Clinical Presentation

• Symptoms: Fever, cough (may be productive), dyspnea, chest pain, malaise 2, 4
• Signs: Tachypnea, abnormal breath sounds, crackles, evidence of pulmonary consolidation 2
• In elderly or immunocompromised patients: May present atypically with confusion, worsening of underlying conditions, or falls 2
• Complications: Respiratory failure, pleural effusion, empyema, lung abscess, sepsis, acute respiratory distress syndrome 6, 4

Diagnosis

• Clinical assessment: Presence of fever and/or acute respiratory symptoms 2
• Chest radiography: Essential for confirming diagnosis and identifying complications 2, 6
  • May show lobar consolidation, interstitial patterns, or bronchopneumonia 6
• Laboratory tests: Complete blood count, blood cultures, sputum Gram stain and culture 2
• Specific pathogen identification:
  • Respiratory cultures (sputum, bronchial washings, pleural fluid) 2
  • Blood cultures for bacteremic pneumonia 2
  • Specific tests for atypical pathogens (e.g., nucleic acid probes for Mycoplasma, antigen detection for Legionella) 2
  • Testing for COVID-19 and influenza when these viruses are circulating 4

Treatment Approach

• Antimicrobial therapy: Should be initiated promptly based on likely pathogens and local resistance patterns 2
  • For community-acquired pneumonia in hospitalized patients without risk factors for resistant bacteria: β-lactam/macrolide combination (e.g., ceftriaxone with azithromycin) 4
  • For atypical pneumonia: Macrolides (e.g., azithromycin) or tetracyclines 2
• Duration: Minimum of 3-7 days, depending on clinical response 2, 4
• Supportive care: Oxygen therapy, hydration, and respiratory support as needed 2
• Corticosteroids: May reduce 28-day mortality in severe CAP when administered within 24 hours 4

Special Considerations

• Azithromycin: Indicated for community-acquired pneumonia due to Chlamydophila pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, or Streptococcus pneumoniae in patients appropriate for oral therapy 7

  • Not recommended for patients with moderate to severe illness requiring hospitalization or with risk factors such as cystic fibrosis, suspected bacteremia, or immunodeficiency 7

• Prevention: Pneumococcal and influenza vaccines in appropriate at-risk populations, smoking cessation 2