Parapneumonic Effusion - Comprehensive Answer (10 Marks)

Definition

Incidence and Significance

• ~4 million cases of bacterial pneumonia occur annually in the US; ~25% require hospitalization
• Up to 40% of hospitalized pneumonia patients develop a parapneumonic effusion
• Patients with PPE are 2.7x more likely to be treatment failures than those without
• Bilateral PPE carries a 7x relative mortality risk; unilateral moderate-to-large PPE carries 3.4x risk
• ~10% of patients ultimately require operative intervention
• Empyema incidence has roughly doubled between 1996-2008 in the US

Pathophysiology - Three Stages

Stage 1: Exudative Stage

• Rapid outpouring of sterile pleural fluid into the pleural space, likely from the pulmonary interstitial spaces
• Fluid characteristics: low WBC, low LDH, normal glucose, normal pH
• With appropriate antibiotics at this stage, the effusion resolves without chest tube drainage

Stage 2: Fibropurulent Stage

• Bacteria invade the pleural space from the contiguous pneumonic process
• Fluid characteristics: large numbers of PMNs, bacteria, and cellular debris
• Fibrin is deposited in a continuous sheet over both visceral and parietal pleura
• Progressive loculation forms - making drainage increasingly difficult
• Pleural fluid pH falls, glucose falls, LDH rises progressively

Stage 3: Organization Stage

• Fibroblasts grow into the exudate from both pleural surfaces
• An inelastic pleural peel forms over the visceral pleura, trapping and encasing the lung
• The lung becomes virtually functionless
• Untreated: fluid may drain through the chest wall (empyema necessitans) or rupture into the lung causing a bronchopleural fistula

Bacteriology

• Pre-antibiotic era: Streptococcus pneumoniae, S. hemolyticus
• 1955-1965: Staphylococcus aureus predominated
• 1970s: Anaerobes most common
• Current era: Mixture of aerobic and anaerobic organisms

• Aerobic organisms isolated more frequently than anaerobes (~53% aerobic only)
• Most common gram-positive aerobes: S. aureus and S. pneumoniae (~70% of gram-positive)
• Most common anaerobes: Bacteroides spp and Peptostreptococcus
• Community-acquired PPE: Streptococcus intermedius-anginosus-constellatus (milleri group) now the most common organism, followed by S. pneumoniae and S. aureus
• Hospital-acquired PPE: S. aureus dominates, with a high proportion being MRSA

Pleural Fluid Analysis - Decision Criteria

• pH <7.00: Definite indication for tube thoracostomy
• pH 7.00-7.20: Borderline; monitor closely or drain
• pH >7.20: Conservative management possible
• Positive Gram's stain or culture with pH <7.0 or glucose <40 mg/dL = definite indication for drainage

Radiological Features

• CXR: blunting of costophrenic angle (minimum ~175-200 mL needed)
• Ultrasound is preferred for: confirming fluid presence, guiding thoracentesis, identifying loculations, distinguishing empyema from lung abscess
• CT scan: demonstrates loculations, underlying lung pathology, thickened pleura ("split pleura sign" in empyema - enhancement of visceral and parietal pleura), and differentiates empyema from lung abscess
• Mediastinal shift toward the effusion side suggests underlying lung collapse/obstruction

Management

Step-wise Approach:

• Indicated when: frank pus, positive Gram's stain or culture, pH <7.00, glucose <40 mg/dL
• Small-bore tubes (9-12 F) are as effective as large-bore tubes in most cases
• Small tubes should be irrigated every 6 hours with saline when used for PPE

• The MIST-2 trial demonstrated that the combination of tPA + DNase administered intrapleurally significantly improved drainage of complicated PPE compared to either agent alone or placebo
• DNase alone: reduces fluid viscosity
• tPA alone: breaks down fibrin loculations
• Combined: synergistic effect with significantly better outcomes

• Indicated for loculated effusions not responding to tube drainage + fibrinolytics
• Permits drainage, lysis of adhesions, and decortication
• VATS has become widespread since the 1990s for management of loculated PPE

• For organized empyema with thick pleural peel trapping the lung
• Removes the inelastic visceral pleural peel to allow lung re-expansion

Historical Graham Principles (still valid today):

1. Drain the pleural fluid - but avoid open pneumothorax in the acute exudative phase
2. Avoid chronic empyema through rapid sterilization and obliteration of the infected cavity
3. Attend carefully to the patient's nutrition

Complications

• Bronchopleural fistula
• Empyema necessitans (spontaneous drainage through chest wall)
• Pleural thickening and trapped lung (fibrothorax)
• Septicemia and multi-organ failure
• Increased mortality: rising in all age groups; death rates from empyema were sixfold higher in 2000-2004 vs. 1950-1975 in one US study

Key Points for Examination

1. Three stages: Exudative → Fibropurulent → Organization (remember progression of fluid chemistry)
2. pH <7.00 = definite chest drain; pH 7.00-7.20 = borderline; pH >7.20 = conservative
3. Glucose <40 mg/dL and LDH >1000 also indicate need for drainage
4. Frank pus on aspiration = immediate chest drain regardless of other parameters
5. tPA + DNase combination is superior to either agent alone for fibrinolysis (key recent advance cited in Light's 6th edition)
6. Small-bore chest tubes are as effective as large-bore when irrigated regularly
7. MRSA is now a major pathogen especially in hospital-acquired empyema
8. Bacteriology has shifted - Streptococcus milleri group is now the most common community organism