Pneumonia

• Pneumonia is an infection of the pulmonary parenchyma, distal to the terminal bronchiole, caused by various organisms such as bacteria, viruses, and fungi
• Community-acquired pneumonia (CAP) is the most common infectious cause of death with a high mortality rate (~10 - 20%)
• Defense mechanisms of the lung include:

  • Upper respiratory tract: particles trapped in the nose and nasopharynx, sneeze and cough reflexes
  • Tracheobronchial tree: lined by cilia down to terminal bronchioles, ciliary motion sweeps mucous and trapped particles upwards to the oropharynx
  • Alveoli: alveolar macrophages
• Transmission of infectious agents to the lung can occur through aspiration of organisms that colonize the oropharynx, inhalation of infective aerosols, and haematogenous spread
• Pneumonia can be classified into community-acquired pneumonia (CAP) and nosocomial pneumonia, which includes hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP)
• Etiology of CAP includes typical bacteria like Streptococcus pneumoniae, Influenza, Staphylococcus aureus, and atypical bacteria like Legionella spp, Mycoplasma pneumonia, and Chlamydia pneumonia
• Etiology of CAP also includes respiratory viruses such as Influenza A and B, SARS-CoV-2, Rhinoviruses, and Fungal infections like Histoplasma capsulatum
• Classic diagnosis of community-acquired pneumonia involves a triad of evidence: symptoms and signs of acute infection, localized lung signs of consolidation, and abnormal chest X-ray
• Severity of pneumonia can be determined using scoring systems like the Pneumonia Severity Index (PSI) and CURB-65, with different levels of care based on severity
• Patients with a working diagnosis of CAP should have the severity of illness defined using scoring systems like PSI and CURB-65 to determine the most appropriate site of care
• The American Thoracic Society (ATS) and the Infectious Diseases Society of America (IDSA) suggest minor criteria to identify patients with severe CAP who have progressed to sepsis before the development of organ failure
• Special investigations for pneumonia include blood cultures, sputum Gram stain and culture, respiratory virus screen, multiplex PCR testing, urine streptococcal and Legionella antigen, chest X-ray, and potentially a CT scan of the lung
• Chest X-ray may be normal in 10%-15% of true cases of pneumonia
• A CT of the lung is more reliable than a CXR for pneumonia diagnosis, but it's more expensive, has higher radiation exposure, and is not readily available
• Lung sonar is reliable for pneumonia diagnosis, can be done at the bedside, but specific expertise is necessary
• Biochemistry markers for pneumonia:

  • CRP: Synthesized in the liver, elevates with infection or tissue inflammation
  • PCT is usually significantly elevated (>=0.25 microgram/L) in typical bacterial pneumonia, but not significantly elevated in viral, Mycoplasma, and Legionella pneumonia
  • Leucocytosis in peripheral blood
• Diagnosis of Community-Acquired Pneumonia (CAP) is usually easy in young people without cardio-pulmonary disease
• Difficulties in diagnosing CAP:

  • Elderly may present without fever or signs of infection, only with confusion or decompensated cardiac failure
  • Patients with underlying structural lung disease may have a clinical picture dominated by their lung disease rather than CAP
  • Immuno-compromised patients may not always present with lobar infiltrates, but may have bilateral diffuse disease
• Differential diagnosis for pneumonia includes pulmonary embolism, heart failure with pulmonary edema, lung cancer, atelectasis, pulmonary hemorrhage, drug reactions, and rarer lung diseases like COP and ARDS
• Principles of pneumonia treatment:

  • Therapy should not be delayed as it may affect prognosis
  • Initial therapy is empiric, targeting the most likely pathogen without waiting for a specific etiological diagnosis
  • Empiric regimens target S. pneumoniae and atypical pathogens
• Treatment of CAP as outpatient:

  • For most patients aged <65 years who are otherwise healthy and have not recently used antibiotics: oral amoxicillin plus a macrolide or doxycycline
  • For patients with comorbidities, smokers, or recent antibiotic use: oral amoxicillin-clavulanate plus macrolide or doxycycline
  • For patients unable to use beta-lactam or with structural lung disease: respiratory fluoroquinolone
• Treatment of CAP as inpatient:

  • Initial regimen selection depends on the risk of infection with Pseudomonas and/or MRSA
  • Without suspicion for pseudomonas or MRSA: beta-lactam plus macrolide or respiratory fluoroquinolone
• Adjunctive glucocorticoids in CAP treatment are used to reduce the inflammatory response, especially beneficial for patients with respiratory failure or significant hypoxemia
• Duration of pneumonia therapy depends on the patient's clinical response, typically 5-7 days for mild infection and 7-10 days for severe infection or chronic co-morbidities
• Complications of CAP include sepsis, ARDS, respiratory failure, lung abscess, and pleural effusion which may develop into empyema
• Lung abscess is a purulent infection contained within the lung parenchyma, often arising as a complication of aspiration, with risk factors like alcohol abuse, drug overdose, epilepsy, stroke, and poor dental hygiene