8.2 Infectious Pneumonias & Pleural Effusions
Key Takeaways
- CURB-65 score risk-stratifies CAP: scores 0-1 are managed outpatient, score 2 inpatient medical ward, and scores ≥3 require ICU admission.
- Atypical pneumonias present with indolent systemic symptoms and extrapulmonary signs (e.g., Legionella with hyponatremia and diarrhea; Mycoplasma with cold agglutinins and erythema multiforme).
- Hospital-Acquired Pneumonia (HAP) occurs >48 hours after admission and requires empiric dual coverage against Pseudomonas aeruginosa and MRSA.
- Light's criteria classifies pleural fluid as exudative if pleural/serum protein > 0.5, pleural/serum LDH > 0.6, or pleural LDH > 2/3 ULN serum LDH; transudates stem from hydrostatic/oncotic pressure imbalances.
- Complicated parapneumonic effusions and empyemas (pH < 7.20, glucose < 60 mg/dL, positive Gram stain/culture, or frank pus) mandate tube thoracostomy drainage alongside systemic antibiotics.
8.2 Infectious Pneumonias & Pleural Effusions
Pneumonia and pleural effusions represent major causes of acute pulmonary morbidity. On USMLE Step 2 CK, key competencies include identifying specific etiologic pathogens from clinical vignettes, applying risk-stratification algorithms (CURB-65) for community-acquired pneumonia, differentiating hospital-acquired from aspiration syndromes, and using Light's criteria to guide pleural effusion management.
Community-Acquired Pneumonia (CAP)
Community-Acquired Pneumonia (CAP) is an acute infection of the pulmonary parenchyma acquired outside of healthcare settings. Pathogens are broadly categorized into typical and atypical bacterial species.
Etiologic Organisms & Clinical Signatures
- Streptococcus pneumoniae: The most common cause of CAP overall across all adult age groups. Classic presentation includes acute high fever, rigors, pleuritic chest pain, and productive rust-colored sputum. Radiograph demonstrates classic lobar consolidation.
- Mycoplasma pneumoniae: Frequently affects young adults, college students, and military recruits ("walking pneumonia"). Presents with an insidious onset of headache, dry hacking cough, low-grade fever, and diffuse patchy/interstitial infiltrates. Key USMLE associations: IgM cold agglutinins (causing autoimmune hemolytic anemia), erythema multiforme, and bullous myringitis. Treated with macrolides (azithromycin) or doxycycline.
- Legionella pneumophila: Acquired via inhalation of aerosolized water from contaminated systems (air conditioners, hotel showers, cruise ships). Characterized by high fever with relative bradycardia (Faget sign), gastrointestinal symptoms (watery diarrhea, abdominal pain, nausea), and central nervous system abnormalities (confusion, lethargy). Laboratory testing reveals hyponatremia (SIADH or renal tubular injury) and elevated transaminases. Confirmed by urinary antigen test (detects serogroup 1) or culture on buffered charcoal yeast extract (BCYE) agar. Treated with respiratory fluoroquinolones (levofloxacin, moxifloxacin) or azithromycin.
- Klebsiella pneumoniae: Classic pathogen in individuals with alcohol use disorder, diabetes mellitus, or severe aspiration risk. Characterized by necrotizing lobar pneumonia, upper lobe cavitary lesions, and thick, blood-tinged "currant jelly" sputum.
- Staphylococcus aureus: Secondary bacterial pneumonia following viral influenza infection ("post-flu pneumonia"). Presents with rapid clinical deterioration, necrotizing parenchymal destruction, cavitating infiltrates, and pneumatoceles (fluid/air-filled thin-walled cysts). High prevalence of MRSA requires empiric vancomycin or linezolid.
- Pseudomonas aeruginosa: High risk in patients with structural lung disease (cystic fibrosis, bronchiectasis), severe neutropenia, or recent prolonged broad-spectrum antibiotic therapy.
- Other Notable Atypicals: Chlamydia pneumoniae (pharyngitis followed by subacute bronchitis/pneumonia), Coxiella burnetii (Q fever, livestock exposure, hepatitis, culture-negative endocarditis), Chlamydia psittaci (Psittacosis, exposure to pet birds/parrots, splenomegaly, macular rash).
CAP Risk Stratification: CURB-65 Score
The CURB-65 score is a validated clinical prediction rule used to determine site-of-care disposition (outpatient vs general ward vs ICU) for patients with CAP.
| Criterion | Points | Clinical Definition |
|---|---|---|
| Confusion | 1 | New-onset disorientation to person, place, or time |
| Urea (BUN) | 1 | BUN > 19 mg/dL (> 7 mmol/L) |
| Respiratory Rate | 1 | ≥ 30 breaths/min |
| Blood Pressure | 1 | Systolic BP < 90 mmHg OR Diastolic BP ≤ 60 mmHg |
| Age ≥ 65 | 1 | Age 65 years or older |
Management Disposition & Empiric Antimicrobial Regimens
- Score 0 - 1: Low risk (<1.5% 30-day mortality); Outpatient management.
- Otherwise Healthy: Amoxicillin high-dose (1g TID) OR Doxycycline OR Macrolide (azithromycin/clarithromycin, only if local pneumococcal macrolide resistance is <25%).
- Comorbidities (chronic heart, lung, liver, or renal disease; diabetes; alcoholism; malignancy): Monotherapy with Respiratory Fluoroquinolone (Levofloxacin 750 mg daily or Moxifloxacin 400 mg daily) OR Combination Beta-lactam (Amoxicillin-clavulanate or Cefpodoxime) PLUS Macrolide or Doxycycline.
- Score 2: Moderate risk (8.8% mortality); Inpatient general medical ward.
- Empiric Regimen: IV Beta-lactam (Ceftriaxone 1-2g daily, Cefotaxime, or Ampicillin-sulbactam) PLUS IV/PO Macrolide (Azithromycin) OR Respiratory Fluoroquinolone monotherapy.
- Score ≥ 3: High risk (up to 30% mortality); Intensive Care Unit (ICU) admission.
- Empiric Regimen: IV Beta-lactam (Ceftriaxone 2g daily or Ampicillin-sulbactam) PLUS IV Azithromycin OR IV Beta-lactam PLUS IV Respiratory Fluoroquinolone.
- MRSA Risk: Add IV Vancomycin or IV Linezolid.
- Pseudomonas Risk: Substitute Beta-lactam with Cefepime, Piperacillin-tazobactam, or Meropenem.
Hospital-Acquired (HAP) vs Ventilator-Associated (VAP) Pneumonia
- Hospital-Acquired Pneumonia (HAP): Pneumonia occurring >48 hours after hospital admission that was not incubating at the time of admission.
- Ventilator-Associated Pneumonia (VAP): Pneumonia developing >48 hours after endotracheal intubation.
- Microbiology: High prevalence of multidrug-resistant (MDR) pathogens, predominantly Pseudomonas aeruginosa, MRSA, Acinetobacter baumannii, and enteric gram-negative bacilli.
- Empiric Regimen: Dual antipseudomonal coverage + MRSA coverage:
- Antipseudomonal Beta-lactam: Cefepime, Piperacillin-tazobactam, or Meropenem.
- Second Antipseudomonal Agent: Ciprofloxacin, Levofloxacin, or Amikacin/Gentamicin.
- Anti-MRSA Agent: Vancomycin or Linezolid.
Aspiration Pneumonia vs Aspiration Pneumonitis
Aspiration syndromes result from inhalation of gastric contents or oropharyngeal secretions. Differentiating chemical inflammation from active bacterial infection is critical to prevent unnecessary antibiotic use.
| Feature | Aspiration Pneumonitis | Aspiration Pneumonia |
|---|---|---|
| Pathophysiology | Chemical inflammation caused by aspirated sterile gastric acid | Infectious parenchymal inflammation caused by oral anaerobes + aerobes |
| Risk Factors | Depressed consciousness (anesthesia, drug overdose, seizures) | Dysphagia, stroke, alcohol intoxication, periodontal disease, altered mental status |
| Onset | Sudden, within hours of aspiration event | Insidious, 24 to 72 hours after aspiration event |
| Signs/Symptoms | Sudden hypoxemia, fever, diffuse bilateral crackles, bronchospasm | Fever, cough, productive foul-smelling sputum, localized consolidation |
| Treatment | Supportive care only (antibiotics are NOT indicated initially) | Antibiotics required: Ampicillin-sulbactam (IV) or Amoxicillin-clavulanate (PO); Clindamycin for penicillin allergy |
| Anatomic Site | Dependent segments: Superior segment of RLL (supine) or Posterior segment of RUL | Dependent lung segments (Right lower lobe superior segment most common overall) |
Pleural Effusions: Transudates vs Exudates & Light's Criteria
A pleural effusion is an abnormal collection of fluid in the pleural space. Diagnostic thoracentesis is indicated for new, unexplained effusions >1 cm on lateral decubitus radiograph or bedside ultrasound (except in typical bilateral congestive heart failure, where a trial of diuretic therapy is initiated first).
Light's Criteria Pathophysiology
An effusion is classified as an EXUDATE if AT LEAST ONE of the following three criteria is met:
- Pleural Fluid Protein / Serum Protein ratio > 0.5
- Pleural Fluid LDH / Serum LDH ratio > 0.6
- Pleural Fluid LDH > 2/3 the Upper Limit of Normal for serum LDH
If NONE of these criteria are met, the effusion is classified as a TRANSUDATE.
Pathophysiologic Mechanisms
- Transudative Effusions: Result from systemic factors altering hydrostatic or oncotic pressures across intact pleural capillary membranes.
- Increased Hydrostatic Pressure: Congestive Heart Failure (most common transudate overall).
- Decreased Oncotic Pressure: Cirrhosis (hepatic hydrothorax), Nephrotic Syndrome, severe Hypoalbuminemia/malnutrition.
- Exudative Effusions: Result from local inflammatory processes that increase pleural capillary permeability or cause lymphatic obstruction.
- Increased Capillary Permeability: Infections (parapneumonic effusion, Tuberculosis), Malignancy (lung, breast, lymphoma), Pulmonary Embolism, Autoimmune diseases (Rheumatoid Arthritis, Systemic Lupus Erythematosus), Post-cardiac injury syndrome (Dressler syndrome).
Parapneumonic Effusions & Empyema Management
Parapneumonic effusions accompany bacterial pneumonia and exist along a clinical continuum requiring distinct management strategies.
| Classification | Pleural Fluid pH | Glucose (mg/dL) | LDH (U/L) | Gram Stain / Culture | Clinical Management |
|---|---|---|---|---|---|
| Uncomplicated Parapneumonic | > 7.20 | > 60 | < 1000 | Negative | Systemic Antibiotics alone |
| Complicated Parapneumonic | < 7.20 | < 60 | > 1000 | Negative | Chest Tube Thoracostomy Drainage + Antibiotics |
| Empyema | < 7.20 | < 60 | Variable | Positive or Frank Pus | Chest Tube Thoracostomy Drainage + Antibiotics ± Intrapleural Fibrinolytics / VATS |
Management Nuances
- Chest Tube Drainage Indications: Pleural fluid pH < 7.20, pleural glucose < 60 mg/dL, positive Gram stain or culture, or frank purulent fluid (empyema).
- Loculated or Refractory Empyema: When chest tube drainage is incomplete due to pleural loculations, intrapleural fibrinolytics (tissue plasminogen activator [tPA] combined with recombinant human DNase) or surgical Video-Assisted Thoracoscopic Surgery (VATS) decortication is indicated.
A 58-year-old man with a history of severe alcohol use disorder presents with a 4-day history of fever, chills, and productive cough with foul-smelling sputum. He reportedly lost consciousness following a heavy drinking binge 5 days ago. Temperature is 38.9 C (102.0 F), blood pressure is 118/76 mmHg, pulse is 104/min, and respiratory rate is 24/min. Crackles and dullness to percussion are noted over the right lower lung field. Chest radiograph reveals a consolidation with a central cavitary lesion and fluid level in the superior segment of the right lower lobe. Which of the following antimicrobial regimens is most appropriate for this patient?
A 68-year-old woman with a history of hypertension and osteoarthritis presents with progressive dyspnea and right-sided pleuritic chest pain. Chest radiography demonstrates a moderate right-sided pleural effusion. A diagnostic thoracentesis is performed, yielding straw-colored fluid. Laboratory results are as follows:
Serum:
Pleural Fluid:
Which of the following is the most likely underlying etiology of this patient's effusion?