Pneumonia

Detailed Overview

When microorganisms evade upper airway defense mechanisms and reach the alveoli, they trigger a massive inflammatory cascade. The resulting influx of neutrophils and exudate fills the alveoli, compromising gas exchange and leading to hypoxemia. Pneumonia is categorized by acquisition setting: Community-Acquired (CAP), Hospital-Acquired (HAP), and Ventilator-Associated (VAP), which dictate the likely pathogens and empiric antibiotic choices. Despite modern antibiotics, it remains a leading cause of infectious death globally.

Epidemiology & Demographics

CAP affects >5 million adults annually in the US. The mortality rate for outpatient CAP is <1%, but jumps to 10-12% for hospitalized patients and >30% for ICU admissions. Highest risk at age <2 or >65.

Etiological Mechanism

CAP: Streptococcus pneumoniae (most common typical), Mycoplasma pneumoniae, Haemophilus influenzae, Respiratory Syncytial Virus, Influenza, SARS-CoV-2. HAP/VAP: Pseudomonas aeruginosa, MRSA, Klebsiella pneumoniae.

Primary Causes

Aspiration of oropharyngeal flora, inhalation of aerosolized infectious droplets, or hematogenous spread from an extrapulmonary source.

• Advanced Age (>65 years): Declining immune function and diminished gag reflex increase risk of aspiration and infection.
• Chronic Lung Disease: COPD or asthma impairs mucociliary clearance, allowing pathogens to linger.
• Immunosuppression: HIV, chemotherapy, or chronic steroids predispose to both typical and opportunistic pathogens.
• Smoking: Directly damages airway cilia and alveolar macrophages.

Pathogens reach the lower respiratory tract via microaspiration. Alveolar macrophages fail to clear the inoculum, initiating an intense immune response. Cytokines (TNF-alpha, IL-8) recruit neutrophils to the alveoli. The classical stages of lobar pneumonia include: 1) Congestion (days 1-2): prominent vascular engorgement and intra-alveolar fluid. 2) Red hepatization (days 3-4): massive exudation with RBCs, neutrophils, and fibrin filling alveoli (lung looks liver-like). 3) Gray hepatization (days 5-7): RBCs disintegrate, but fibrinosuppurative exudate remains. 4) Resolution (day 8+): enzymatic digestion of the exudate by macrophages. The consolidated alveoli create right-to-left physiologic shunting (V/Q mismatch), leading to hypoxemia.

Characteristic Clinical Presentation

• Productive Cough: Cough producing purulent (yellow/green) or rust-colored (blood-tinged, classic for S. pneumoniae) sputum.
• Fever and Chills: Sudden onset high fever (often >39°C/102.2°F) accompanied by shaking chills (rigors).
• Pleuritic Chest Pain: Sharp pain on inspiration due to inflammation of the adjacent pleura.
• Dyspnea: Shortness of breath and increased work of breathing.

Physical Examination Signs

• Tachypnea (RR > 24 breaths/min) and tachycardia.
• Crackles (rales) and bronchial breath sounds over the area of consolidation.
• Egophony (E-to-A transition) and increased tactile fremitus.
• Dullness to percussion over the affected lobe.

• Parapneumonic Effusion / Empyema: Accumulation of sterile fluid or frank pus in the pleural cavity.
• Lung Abscess: Necrosis of lung tissue leading to a cavitary lesion, often secondary to anaerobic aspiration.
• Sepsis and Septic Shock: Systemic inflammatory response causing severe hypotension and multiorgan failure.

Diagnostic Criteria & Guidelines

Clinical signs of acute infection (fever, cough) combined with the presence of a new pulmonary infiltrate on chest imaging.

Differential Diagnosis

• Acute Bronchitis
• Pulmonary Embolism
• Congestive Heart Failure (Pulmonary Edema)
• Exacerbation of COPD

Laboratory Tests & Biomarkers

• Complete Blood Count: Leukocytosis (WBC > 15,000/microL) with a 'left shift' (increased bands) in bacterial pneumonia.
• Procalcitonin: Elevated (>0.25 mcg/L) strongly suggests bacterial rather than viral etiology.
• Sputum Gram Stain & Culture: Shows >25 PMNs and <10 squamous epithelial cells per LPF, with a predominant organism.
• Urine Antigen Tests: Positive for Streptococcus pneumoniae or Legionella pneumophila serogroup 1.

Imaging Modalities & Findings

• Chest X-ray: Lobar consolidation with air bronchograms (typical of S. pneumoniae) or diffuse bilateral interstitial infiltrates (atypical/viral).
• CT Chest: More sensitive for detecting early infiltrates, cavitation (abscess), or complications like empyema; not routinely required for simple CAP.

• Outpatient Mild CAP
  Stable vitals, CURB-65 score of 0-1, can take oral antibiotics.
• Inpatient Ward CAP
  CURB-65 score 2; requires IV antibiotics and oxygen supplementation.
• Severe CAP / ICU
  CURB-65 score ≥3, requiring mechanical ventilation or vasopressors for septic shock.

Second-Line & Adjunctive Therapy

For penicillin-allergic patients or refractory CAP: Respiratory fluoroquinolone (e.g., Levofloxacin 750mg PO/IV daily) for 5-7 days. For HAP/VAP with risk of Pseudomonas/MRSA: Cefepime 2g IV q8h PLUS Vancomycin (trough target 15-20 mcg/mL).

Surgical & Procedural Management

Generally not indicated, unless there is a complication such as a loculated empyema requiring VATS decortication or a large lung abscess failing medical therapy.

Recommended Lifestyle Changes

• Smoking cessation to allow recovery of respiratory epithelium.
• Adequate oral hydration to help loosen and mobilize respiratory secretions.
• Rest and limitation of physical exertion during the acute phase.

Patient Counseling & Advice

Complete the entire course of prescribed antibiotics even if feeling better in 48 hours. Explain that while the fever should break within 3 days, fatigue and a mild cough can persist for up to a month.

Follow-Up & Monitoring Schedule

Re-evaluate clinically in 48-72 hours if no improvement. A follow-up Chest X-ray in 6-8 weeks is recommended for patients >50 years or smokers to ensure resolution and rule out an underlying lung malignancy that was masked by the infection.

Preventive Strategies

Pneumococcal vaccines (PCV15 followed by PPSV23, or PCV20 alone) for adults >65 or those with comorbidities. Annual influenza and updated COVID-19 vaccinations.

Most young, healthy individuals recover completely in 1-2 weeks. Mortality for admitted patients is significant (10%), mostly driven by age, comorbidities, and progression to sepsis.