Community-Acquired Pneumonia (CAP)

Definition

Community-acquired pneumonia (CAP) is an acute infection of the lung parenchyma occurring in a non-hospitalized individual or developing ≤48 hours of hospital admission, not residing in long-term care and without recent healthcare exposure.

Epidemiology

• One of the leading causes of morbidity and mortality worldwide
• Incidence:
• Adults: 5–11 per 1,000 population/year
• Elderly (>65 yrs): 20–40 per 1,000/year
• Mortality:
• Outpatient: <1%
• Hospitalized: 5–15%
• ICU CAP: 25–50%
• Major cause of sepsis and septic shock

Etiology

A. Typical Bacterial Pathogens

B. Atypical Pathogens

C. Viral Causes

• Influenza A/B
• RSV
• SARS-CoV-2
• Adenovirus
• Human metapneumovirus

Viral CAP often predisposes to secondary bacterial pneumonia

D. Risk-Based Pathogens

Pathogenesis

1. Microaspiration of oropharyngeal flora (most common)
2. Inhalation of aerosols
3. Hematogenous spread (rare)
4. Impaired host defenses:
• Smoking
• Alcohol
• COPD
• Diabetes
• Immunosuppression

Lung Response

• Alveolar macrophage activation
• Cytokine release (IL-1, TNF-α, IL-6)
• Neutrophil influx
• Consolidation and impaired gas exchange

Clinical Features

Typical Symptoms

• Fever
• Cough (productive or dry)
• Dyspnea
• Pleuritic chest pain
• Hemoptysis (occasionally)

Systemic Features

• Malaise
• Myalgia
• Confusion (elderly)
• GI symptoms (Legionella)

Physical Examination

• Tachypnea
• Tachycardia
• Fever or hypothermia
• Bronchial breath sounds
• Crackles
• Dullness to percussion
• Reduced air entry

Diagnosis

1. Clinical Diagnosis

• Compatible symptoms PLUS
• Radiographic evidence of pneumonia

2. Imaging

Chest X-ray (Mandatory)

Findings:

• Lobar consolidation
• Interstitial infiltrates
• Patchy bronchopneumonia
• Pleural effusion

No infiltrate = No pneumonia (except very early disease)

CT Chest

• Reserved for:
• Complications
• Non-resolving pneumonia
• Immunocompromised
• Suspected malignancy

3. Laboratory Investigations

Routine Tests

• CBC (↑ WBC / leukopenia = severe)
• CRP, Procalcitonin
• Renal function
• LFTs
• ABG (if hypoxemia)

4. Microbiological Work-up

Outpatients

• Not routinely required

Hospitalized / Severe CAP

• Sputum Gram stain & culture
• Blood cultures (before antibiotics)
• Urinary antigen:
• Streptococcus pneumoniae
• Legionella
• Viral PCR (influenza, SARS-CoV-2)

Severity Assessment

CURB-65 Score

Interpretation

• 0–1: Outpatient
• 2: Hospital admission
• ≥3: Severe CAP → ICU

PSI (Pneumonia Severity Index)

• More accurate but complex
• Preferred for mortality prediction

ATS/IDSA Severe CAP Criteria (2019)

Major Criteria (Any 1 = ICU)

• Septic shock requiring vasopressors
• Respiratory failure requiring mechanical ventilation

Minor Criteria (≥3 = ICU)

• RR ≥30
• PaO₂/FiO₂ ≤250
• Multilobar infiltrates
• Confusion
• Uremia
• Leukopenia
• Thrombocytopenia
• Hypothermia
• Hypotension requiring fluids

Management

A. General Measures

• Oxygen therapy (target SpO₂ ≥92%)
• IV fluids (avoid overload)
• Antipyretics
• Early mobilization
• DVT prophylaxis (hospitalized)

B. Empiric Antibiotic Therapy (ATS/IDSA 2019)

1. Outpatient – No Comorbidities

• Amoxicillin
• OR Doxycycline
• OR Macrolide (only if pneumococcal resistance <25%)

2. Outpatient – With Comorbidities

(Chronic heart/lung/liver/kidney disease, diabetes, alcoholism)

• β-lactam (Amoxicillin-clavulanate or Cefuroxime)
  PLUS macrolide or doxycycline
  OR
• Respiratory fluoroquinolone (Levofloxacin / Moxifloxacin)

3. Inpatient – Non-Severe CAP

• β-lactam + macrolide
• Ceftriaxone + Azithromycin
  OR
• Respiratory fluoroquinolone alone

4. Severe CAP (ICU)

• β-lactam + macrolide
  OR
• β-lactam + fluoroquinolone

5. MRSA Risk

(Add if prior MRSA, post-influenza, necrotizing pneumonia)

• Vancomycin
• Linezolid (preferred if necrotizing)

6. Pseudomonas Risk

(Add if structural lung disease, recent antibiotics)

• Piperacillin-tazobactam
• Cefepime
• Meropenem

Duration of Therapy

• Minimum 5 days
• Patient must be:
• Afebrile ≥48 hrs
• Clinically stable

Severe CAP: 7–10 days
MRSA / Pseudomonas: 10–14 days

Adjunctive Therapies

Corticosteroids

• NOT routine
• Consider if:
• Refractory septic shock
• Severe CAP with high inflammatory markers (selected cases)

Complications

Pulmonary

• Parapneumonic effusion
• Empyema
• Lung abscess
• ARDS

Systemic

• Sepsis
• Septic shock
• AKI
• Multiorgan failure

Non-Resolving Pneumonia

Defined as:

• No clinical improvement after 48–72 hrs
• Radiological non-resolution after 6–8 weeks

Causes:

• Wrong diagnosis
• Resistant organisms
• Tuberculosis
• Malignancy
• Pulmonary embolism

Prevention

Vaccination

• Pneumococcal vaccine
• PCV13 + PPSV23 (as per age/risk)
• Influenza vaccine (annual)
• COVID-19 vaccination

Risk Factor Control

• Smoking cessation
• Alcohol moderation
• Chronic disease control

 Exam Pearls

• Most common CAP pathogen: Streptococcus pneumoniae
• CURB-65 guides admission, not antibiotic choice
• No radiographic infiltrate = not pneumonia
• Macrolide resistance >25% → avoid monotherapy
• Linezolid preferred over vancomycin in necrotizing MRSA CAP
• Minimum antibiotic duration = 5 days
• ICU admission based on ATS/IDSA criteria, not CURB-65 alone