Section 6.3: Infectious Pulmonary Diseases

Infectious Pulmonary Diseases: Diagnostics, Risk-Stratification, and Treatment

Infectious pulmonary diseases are major sources of morbidity and represent highly tested topics on the USMLE Step 3. Clinicians must master the risk-stratification of community-acquired pneumonia, the management of hospital-acquired and ventilator-associated infections, the differentiation of aspiration syndromes, and the screening and multi-drug treatment of tuberculosis.

Community-Acquired Pneumonia (CAP)

Community-acquired pneumonia is defined as an acute infection of the lung parenchyma in a patient who acquired the infection in the community (outside of a hospital or healthcare facility). The clinical diagnosis is established by the presence of a new pulmonary infiltrate on chest radiography in a patient with compatible clinical features, such as fever, cough, sputum production, dyspnea, and focal crackles or signs of consolidation (e.g., dullness to percussion, increased tactile fremitus) on physical examination.

Risk Stratification (CURB-65): To determine the appropriate level of care, the clinician should calculate the CURB-65 score:

• C: Confusion (new-onset disorientation in person, place, or time)
• U: Urea > 7 mmol/L (Blood Urea Nitrogen [BUN] > 19 mg/dL)
• R: Respiratory rate >= 30 breaths/minute
• B: Blood pressure (systolic < 90 mmHg or diastolic <= 60 mmHg)
• 65: Age >= 65 years

Each criteria is worth 1 point.

• Score 0-1: Low risk; outpatient management is appropriate.
• Score 2: Moderate risk; consider brief inpatient admission or close outpatient observation.
• Score 3-5: High risk; inpatient hospitalization is indicated, with scores of 4 or 5 strongly suggesting the need for intensive care unit (ICU) admission.

Empiric Pharmacotherapy:

• Outpatient (No Comorbidities): High-dose amoxicillin (1 g three times daily) OR doxycycline (100 mg twice daily). Macrolide monotherapy (e.g., azithromycin) is only recommended if local macrolide resistance rates for Streptococcus pneumoniae are documented to be less than 25%.
• Outpatient (With Comorbidities - e.g., chronic heart, lung, liver, or renal disease, diabetes, alcoholism, active malignancy, or asplenia): Combination therapy with a beta-lactam (amoxicillin-clavulanate, cefpodoxime, or cefuroxime) plus a macrolide or doxycycline; OR monotherapy with a respiratory fluoroquinolone (levofloxacin, moxifloxacin).
• Inpatient (Non-ICU Ward): Combination therapy with an intravenous (IV) beta-lactam (ceftriaxone, cefotaxime, or ampicillin-sulbactam) plus an oral/IV macrolide (azithromycin) or doxycycline; OR IV respiratory fluoroquinolone monotherapy.
• Inpatient (ICU - Severe CAP): Combination therapy with an IV beta-lactam plus an IV macrolide OR an IV respiratory fluoroquinolone. If the patient has risk factors for methicillin-resistant Staphylococcus aureus (MRSA) or Pseudomonas aeruginosa (e.g., prior isolation of the pathogen or recent hospitalization with receipt of IV antibiotics within the past 90 days), add vancomycin or linezolid for MRSA coverage, and swap the beta-lactam for an antipseudomonal agent (cefepime, piperacillin-tazobactam, meropenem, or ceftazidime).

Hospital-Acquired (HAP) and Ventilator-Associated Pneumonia (VAP)

Hospital-acquired pneumonia is defined as pneumonia developing 48 hours or more after hospital admission that was not incubating at the time of admission. Ventilator-associated pneumonia is a subtype of HAP that develops 48 hours or more after endotracheal intubation.

Unlike CAP, HAP and VAP are dominated by multidrug-resistant (MDR) pathogens, including Pseudomonas aeruginosa, MRSA, and gram-negative bacilli such as Klebsiella pneumoniae and Acinetobacter species. Diagnostic criteria include a new or progressive pulmonary infiltrate on chest imaging plus clinical signs (fever, purulent sputum, leukocytosis or leukopenia, and worsening hypoxemia). Prior to initiating antibiotics, lower respiratory tract specimens (sputum, endotracheal aspirate, or bronchoalveolar lavage fluid) must be obtained for Gram stain and culture; however, empiric therapy must not be delayed.

Empiric Antibiotic Regimen: All empiric regimens for HAP/VAP must cover MRSA and Pseudomonas.

• Standard Empiric Regimen: IV Vancomycin (or Linezolid) plus an antipseudomonal beta-lactam (cefepime, piperacillin-tazobactam, ceftazidime, imipenem, or meropenem).
• MDR Risk Escalation: If the patient has risk factors for multidrug-resistant Pseudomonas (prior IV antibiotic use within 90 days, septic shock, acute respiratory distress syndrome [ARDS] preceding VAP, acute renal replacement therapy, or hospitalization >= 5 days before VAP), a second antipseudomonal agent from a different class must be added (e.g., ciprofloxacin, levofloxacin, or an aminoglycoside like amikacin or gentamicin).
• De-escalation: Antibiotics should be narrowed based on culture results at 48-72 hours. The standard treatment duration for uncomplicated HAP/VAP is 7 days.

Aspiration Pneumonia vs. Aspiration Pneumonitis

Step 3 requires clinicians to distinguish between two clinically distinct aspiration syndromes:

1. Aspiration Pneumonitis: A chemical lung injury caused by the inhalation of sterile, acidic gastric contents. It typically occurs in patients with an episode of witnessed vomiting and altered consciousness. Symptoms (abrupt dyspnea, wheezing, hypoxemia) and bilateral infiltrates appear within hours. Treatment is supportive (suctioning, oxygen, bronchodilators). Empiric antibiotics are not indicated unless symptoms fail to resolve within 48 hours (indicating secondary bacterial infection) or the patient has a bowel obstruction (associated with colonized gastric fluid).
2. Aspiration Pneumonia: An infectious process caused by the inhalation of colonized oropharyngeal secretions. Risk factors include dysphagia, stroke, seizures, alcohol intoxication, and neuromuscular disorders. It presents subacutely over days with fever, productive cough (often with foul-smelling sputum), and consolidation in dependent lung zones. When a patient aspirates in a recumbent/supine position, the superior segments of the lower lobes or the posterior segments of the upper lobes are affected; when upright, the lower lobes (especially the right lower lobe due to the wider and more vertical right mainstem bronchus) are involved.

• Treatment: First-line therapy is IV ampicillin-sulbactam or oral amoxicillin-clavulanate. Ceftriaxone plus metronidazole is a preferred alternative. Routine anaerobic coverage is no longer recommended for typical aspiration pneumonia unless there is clinical or radiographic evidence of a lung abscess or empyema, as standard agents cover the primary pathogens (streptococci and oral flora).

Tuberculosis Screening and Treatment

Tuberculosis (TB) screening targets latent tuberculosis infection (LTBI) to prevent progression to active, transmissible disease.

Screening Methods:

• Tuberculin Skin Test (TST/PPD): Induration (not erythema) is measured 48-72 hours after intradermal injection.
  • >= 5 mm: Positive in HIV-infected patients, recent contacts of active TB, patients with fibrotic changes on CXR, or organ transplant recipients/immonosuppressed (e.g., taking prednisone >= 15 mg/day).
  • >= 10 mm: Positive in recent immigrants (< 5 years) from high-prevalence areas, injection drug users, residents/employees of high-risk congregate settings (prisons, homeless shelters), children < 4 years, or patients with comorbid conditions predisposing to reactivation (silicosis, diabetes, end-stage renal disease).
  • >= 15 mm: Positive in individuals with no known risk factors.
• Interferon-Gamma Release Assay (IGRA): A blood test measuring T-cell release of interferon-gamma in response to TB-specific antigens. IGRA is the preferred screening method for patients who have received the Bacillus Calmette-Guerin (BCG) vaccine, as it does not cross-react and yields fewer false-positive results.

Active Pulmonary TB: Active TB presents with chronic cough (> 3 weeks), hemoptysis, fever, drenching night sweats, and weight loss. Chest radiography demonstrates upper-lobe cavitary infiltrates in secondary (reactivation) TB or hilar lymphadenopathy and pleural effusion in primary TB. Diagnosis requires three consecutive morning sputum specimens for acid-fast bacilli (AFB) smear, nucleic acid amplification testing (NAAT) for rapid identification and drug resistance screening, and mycobacterial culture (the gold standard). Place the patient in airborne isolation (negative-pressure room) immediately.

Pharmacotherapy:

• Active TB: Treated with the RIPE regimen for 2 months (Rifampin, Isoniazid, Pyrazinamide, Ethambutol), followed by Rifampin and Isoniazid for 4 months (total 6 months).
  • Rifampin: Side effects include orange/red discoloration of body fluids and hepatotoxicity. It is a potent cytochrome P450 inducer (reduces efficacy of oral contraceptives, warfarin, and protease inhibitors).
  • Isoniazid (INH): Side effects include hepatotoxicity and peripheral neuropathy due to pyridoxine (vitamin B6) deficiency. Always co-prescribe pyridoxine (25-50 mg daily). INH can also cause drug-induced lupus.
  • Pyrazinamide: Side effects include hepatotoxicity and hyperuricemia (which can precipitate acute gouty arthritis).
  • Ethambutol: Side effects include dose-dependent optic neuritis (presenting as decreased visual acuity and red-green color blindness). Baseline and monthly visual acuity and color vision testing are required.
• Latent TB: Preferred regimens include 4 months of daily rifampin, 3 months of once-weekly isoniazid plus rifapentine (directly observed therapy), or 3 months of daily isoniazid plus rifampin.