Community-Acquired Pneumonia (CAP) in Adults

Community-Acquired Pneumonia (CAP) in Adults

Introduction

• Definition: Acute infection of the lung parenchyma acquired outside of hospitals.
• Significance: Most common infectious cause of death in the U.S.
• Incidence: Approximately 5.2–6.8 cases per 1,000 adults per year.
• Hospitalization: Increases with age and presence of comorbidities.

Etiology & Pathogens

• Typical pathogens:
  • S. pneumoniae
  • H. influenzae
  • M. catarrhalis
• Atypical pathogens:
  • M. pneumoniae
  • C. pneumoniae
  • Legionella spp.
• Viral pathogens:
  • Influenza
  • RSV
  • SARS-CoV-2
• Aspiration pathogens:
  • Anaerobes
  • Enteric gram-negatives

Pathogenesis of Community-Acquired Pneumonia (CAP)

• Traditional Model:

  • CAP is caused by inhalation or aspiration of a respiratory pathogen into sterile alveoli.
  • The local inflammatory response to the pathogen results in pulmonary signs and symptoms like cough, sputum production, dyspnea, crackles, and hypoxemia.
  • Release of cytokines into the bloodstream leads to systemic signs and symptoms of pneumonia, including fever, fatigue, tachycardia, and leukocytosis.

• Evolving Understanding:

  • With the discovery of the lung microbiome, the traditional model has evolved.
  • When a respiratory pathogen arrives in the alveolar space, it likely has to compete with resident microbes to replicate.
  • Resident microbes may also modulate the host immune response to the infecting pathogen.
  • CAP might also arise from uncontrolled replication of microbes that normally reside in the alveoli.

• Steps Describing CAP Pathogenesis:

  • Step 1: Arrival of pathogens in alveolar space

  • Step 2: Uncontrolled multiplication of pathogens

  • Step 3: Local production of cytokines primarily by alveolar macrophages

  • Step 4: Recruitment of neutrophils into the alveolar space and introduction of cytokines into systemic circulation

  • Step 5: Generation of alveolar exudate

Clinical Presentation

• Symptoms:
  • Cough
  • Fever
  • Dyspnea
  • Pleuritic chest pain
  • Fatigue
  • Confusion (especially in elderly)
  • Tachypnea
  • Gastrointestinal (GI) symptoms
• Atypical presentation in elderly:
  • Confusion is more common

Diagnosis

• Symptoms + Chest X-ray showing infiltrates.
• Pulse oximetry or ABG for hypoxia assessment.
• Labs: CBC, procalcitonin (selective), cultures (if severe).

Severity Assessment Tools

• CURB-65:
  • Criteria: Confusion, Urea >7 mmol/L, Respiratory Rate ≥30/min, Blood Pressure <90/60 mmHg, Age ≥65 years
  • Score ≥2: Hospitalization advised
• PSI (PORT):
  • Detailed severity assessment tool

Empiric Treatment Strategy

• Outpatients (no comorbidity):
  • Amoxicillin or doxycycline
• Outpatients (with comorbidity):
  • Amoxicillin-Clavulanate + macrolide or fluoroquinolone
• Inpatients (non-ICU):
  • β-lactam + macrolide or fluoroquinolone alone
• ICU:
  • β-lactam + azithromycin or fluoroquinolone

Drug-Resistant Pathogens

• Risk factors:
  • Recent antibiotics
  • Structural lung disease
  • Colonization
• Watch for:
  • MRSA
  • Pseudomonas
  • Recent hospital exposure

Prognosis & Complications

• Mortality:
  • <1% (outpatient)
  • 5–15% (hospital)
  • >25% (ICU)
• Complications:
  • Effusion
  • Empyema
  • Sepsis
  • Lung abscess
  • Renal Failure (RF)

Key Takeaways

• CAP = Infection + infiltrate
• Use CURB-65 or PSI for severity assessment
• Tailor treatment by setting and comorbidities
• Avoid broad-spectrum overuse
• Recognize atypical signs in elderly

Acute Bronchitis vs. Pneumonia in Adults

• Acute bronchitis is typically diagnosed based on the presence of compatible clinical findings (eg, acute onset, persistent cough; recent or
  concurrent upper respiratory infection) and lack of clinical evidence of pneumonia. Chest radiography is typically not needed for diagnosis.

• Chest radiography is required to diagnose pneumonia and is indicated when pneumonia is suspected or when acute bronchitis cannot be
  clinically distinguished from pneumonia. Indications for obtaining chest imaging include abnormal vital signs (pulse >100/min, respiratory
  rate >24 breaths/min, temperature >38°C [100.4°F], or oxygen saturation

• Distinguishing between these two disorders is important because management strategies differ. Acute bronchitis is typically self-limited;
  symptom control and patient education are the cornerstones of care. Pneumonia is associated with significant morbidity and is generally
  treated with antibiotics.

• Acute Bronchitis

  • Inflamed and edematous large airways.
  • Mucus in airway.
  • Often caused by respiratory viruses, uncommonly by lower virulence bacteria (eg, Mycoplasma pneumoniae)
    • Other Features
      • Cough: Acute onset, persistent cough.
      • Respiration: Mild dyspnea and/or wheeze.
      • Body temperature: Normal or low-grade fever (
      • Chest examination: Lacks signs of lung consolidation.
      • Other findings: Recent or concurrent upper respiratory tract infection.

• Pneumonia

  • Pus, mucus, and fluid-filled alveoli, which can lead to consolidation (lobar pneumonia).
  • Inflamed and edematous small and large airways and surrounding parenchyma (bronchopneumonia).
  • More often caused by more virulent bacteria (eg, Streptococcus pneumoniae).
    • Other Features
      • Cough: Acute or subacute onset cough
      • Respiration: Dyspnea
      • Body temperature: Fever (>100.4°F/38.0°C)
      • Chest examination: Signs of lung consolidation (eg, crackles, rhonchi, egophony, tactile fremitus)
      • Other findings: Other abnormal vital signs (eg, tachycardia,
        decreased oxygen saturation)

Pneumonia Imaging

• Mycoplasma pneumoniae pneumonia: Chest radiograph

• Pneumococcal pneumonia: Complications

• Mycoplasma pneumonia: Chest imaging

• Pneumococcal pneumonia: Lateral chest radiograph

• Pneumococcal pneumonia: Chest radiograph