Section 2.3: Infectious Pulmonary Disorders (Pneumonia & Tuberculosis)

Infectious Pulmonary Disorders (Pneumonia & Tuberculosis)

Why This Topic Matters for the PANCE

Infectious pulmonary conditions are highly tested on the PANCE. Candidates must demonstrate proficiency in diagnosing and treating community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and tuberculosis (TB). High-yield areas include applying the CURB-65 criteria to determine clinical disposition, selecting empiric antibiotic regimens, interpreting tuberculin skin test (TST) induration, and identifying the classic side effects and clinical indications of the antitubercular RIPE regimen.

Definitions and Pathophysiology

• Community-Acquired Pneumonia (CAP): An acute infection of the lung parenchyma acquired outside of the healthcare system, or developing within 48 hours of hospital admission.
• Hospital-Acquired Pneumonia (HAP): Pneumonia developing >= 48 hours after hospital admission that was not incubating at the time of admission.
• Latent TB Infection (LTBI): Infection with Mycobacterium tuberculosis in which the bacilli are contained by host immune defenses within granulomas. The patient is asymptomatic, non-infectious, has a positive TST or IGRA, and shows a normal chest X-ray.
• Active TB Disease: Symptomatic, infectious illness resulting from uncontrolled replication of M. tuberculosis.

Pathophysiology

• Pneumonia: Pathogens reach the alveoli via microaspiration of oropharyngeal secretions or inhalation of aerosolized droplets. In the alveoli, pathogens multiply and trigger an acute inflammatory response. The alveoli fill with exudate (neutrophils, red blood cells, and fibrin), causing consolidation. This impairs gas exchange, creating a right-to-left intrapulmonary shunt (blood perfusing non-ventilated alveoli), resulting in ventilation-perfusion (V/Q) mismatch and hypoxemia.
• Tuberculosis: M. tuberculosis is an acid-fast bacillus transmitted via airborne droplets. Inhaled bacilli are phagocytosed by alveolar macrophages. If macrophages cannot destroy them, the bacilli replicate intracellularly, triggering a cell-mediated (Type IV delayed) hypersensitivity reaction. CD4+ T-helper cells release cytokines (like interferon-gamma) to activate macrophages. These macrophages differentiate into epithelioid cells and fuse to form multinucleated Langhans giant cells, forming a granuloma with central caseous necrosis (a Ghon focus).
  • Ghon Complex: A Ghon focus plus regional hilar lymphadenopathy.
  • Ranke Complex: A calcified Ghon complex, signifying healed primary infection.
  • Reactivation (Post-Primary) TB: Occurs when immune defenses decline. Bacilli replicate and localize to the apical and posterior segments of the upper lobes due to high oxygen tension.

Clinical Presentation

• Typical CAP: Presents with an acute onset of high fever, shaking chills (rigors), productive cough with purulent sputum, and pleuritic chest pain. Physical exam findings of lobar consolidation include increased tactile fremitus (vocal vibration transmits better through solid tissue), dullness to percussion, egophony (spoken "E" sounds like "A"), and bronchial breath sounds.
• Atypical CAP ("Walking Pneumonia"): Presents subacutely with low-grade fever, dry non-productive cough, headache, sore throat, and marked systemic symptoms (myalgias, fatigue) out of proportion to minimal chest exam findings.
• Active TB: Characterized by a chronic cough (> 3 weeks), hemoptysis, drenching night sweats, low-grade daily fever, unexplained weight loss, and fatigue.

Diagnostic Workup

• Pneumonia: Chest X-ray is the diagnostic standard, showing lobar consolidation (typical CAP) or diffuse, patchy interstitial infiltrates (atypical CAP).
• CURB-65 Score: Determines clinical disposition (outpatient, ward, or ICU):
  • C: Confusion (new onset) - 1 point
  • U: Urea > 7 mmol/L (BUN > 19 mg/dL) - 1 point
  • R: Respiratory rate >= 30 breaths/min - 1 point
  • B: Blood pressure (SBP < 90 mmHg or DBP <= 60 mmHg) - 1 point
  • 65: Age >= 65 years - 1 point
  • Disposition: 0-1 points = outpatient; 2 points = inpatient ward; >= 3 points = intensive care unit (ICU) assessment.
• Tuberculosis Screening: The Tuberculin Skin Test (TST/PPD) is read at 48-72 hours. Measure the diameter of induration (not erythema):

• Note: The Interferon-Gamma Release Assay (IGRA) is preferred in patients who have received the BCG vaccine.
• Active TB Diagnosis: Chest X-ray shows upper lobe apical/posterior infiltrates and cavitary lesions. Diagnostic confirmation requires three serial morning sputum samples for acid-fast bacilli (AFB) smear and culture (Lowenstein-Jensen medium is gold standard). Nucleic Acid Amplification Testing (NAAT) provides rapid confirmation and detects resistance.

Clinical Management

Community-Acquired Pneumonia

• Outpatient (no comorbidities): Amoxicillin (1 g TID) OR Doxycycline OR a Macrolide (Azithromycin, if local resistance is < 25%).
• Outpatient (with comorbidities - COPD, diabetes, chronic kidney/heart disease, alcoholism): Combination of a beta-lactam (Amoxicillin-clavulanate) PLUS a macrolide or doxycycline, OR monotherapy with a respiratory fluoroquinolone (Levofloxacin or Moxifloxacin).
• Inpatient (Non-ICU): IV Ceftriaxone plus IV Azithromycin, OR monotherapy with a respiratory fluoroquinolone.
• HAP/VAP: Must cover Pseudomonas and MRSA: Cefepime (or Piperacillin-Tazobactam) PLUS Vancomycin.

Tuberculosis Management

• Latent TB (LTBI): Rifampin daily for 4 months, OR weekly Isoniazid plus Rifapentine for 3 months, OR Isoniazid daily for 6 to 9 months.
• Active TB (RIPE Regimen): Initial phase of 2 months of Rifampin, Isoniazid, Pyrazinamide, and Ethambutol, followed by a 4-month continuation phase of Rifampin and Isoniazid.
• RIPE Side Effects & Board Clues:
  • Rifampin: Red-orange discoloration of body fluids (tears, urine, sweat). Potent CYP450 inducer (reduces effectiveness of oral contraceptives and warfarin). Hepatotoxicity.
  • Isoniazid: Peripheral neuropathy due to vitamin B6 depletion. Co-prescribe Pyridoxine (Vitamin B6) to prevent this. Hepatotoxicity and Drug-Induced Lupus (anti-histone antibodies).
  • Pyrazinamide: Hyperuricemia (can precipitate acute gouty arthritis) and hepatotoxicity.
  • Ethambutol: Optic neuritis (loss of visual acuity and red-green color blindness). Require baseline and monthly visual exams.

Classic PANCE Traps and Clinical Pearls

• Klebsiella pneumoniae: Suspect in patients with chronic alcoholism or aspiration risk. It presents with "currant-jelly" sputum and upper lobe cavitary lesions.
• Mycoplasma pneumoniae: The most common atypical pneumonia. Look for young adults in close quarters (dorms, barracks) presenting with a dry cough, bullous myringitis, and elevated cold agglutinins.
• Legionella pneumophila: Suspect in patients exposed to aerosolized water systems (hotels, cruise ships). Classically presents with a high fever, watery diarrhea, and hyponatremia.