Pleural Fluid Collection (Pleural Effusion)
Pleural effusion is an abnormal accumulation of fluid in the pleural space between the visceral and parietal pleura, typically exceeding the normal amount of 0.1-0.2 mL/kg body weight, and is classified as either transudative or exudative based on the underlying pathophysiologic mechanism. 1, 2
Definition and Pathophysiology
Pleural fluid collection occurs when there is an imbalance between pleural fluid formation and drainage, resulting in excess fluid accumulation in the pleural space. Under normal circumstances, approximately 0.3 mL/kg of pleural fluid circulates continuously within this space 1.
The pathophysiologic mechanisms leading to pleural effusion include:
- Increased pulmonary capillary pressure
- Increased pleural membrane permeability
- Decreased negative intrapleural pressure
- Decreased oncotic pressure
- Obstructed lymphatic flow 1, 2
Classification
Pleural effusions are classified into two main categories:
1. Transudative Effusions
- Result from systemic conditions that alter the balance of hydrostatic forces
- Normal capillary permeability to proteins
- Common causes: congestive heart failure, cirrhosis, renal failure 1, 2
- Characteristics: clear fluid, protein <25 g/L, LDH <1000 IU/L, glucose >2.2 mmol/L 1
2. Exudative Effusions
- Result from altered pleural surface and/or increased capillary permeability
- Common causes: infection, malignancy, autoimmune disorders 1, 2
- Further classified based on progression:
Clinical Presentation
Patients with pleural effusion may present with:
- Dyspnea (often out of proportion to the size of the effusion in cases like pulmonary embolism) 1
- Pleuritic chest pain
- Cough
- Decreased breath sounds on the affected side
- Dullness to percussion 2
Diagnostic Approach
Imaging
Chest Radiography:
- Initial imaging modality
- Can detect >175 mL on frontal view and >75 mL on lateral view 1
- May show blunting of costophrenic angles, meniscus sign
Thoracic Ultrasound:
- Essential for all patients at initial presentation
- Can detect >20 mL of pleural fluid
- Guides safe diagnostic aspiration
- Identifies septations and loculations 2
CT Scan:
- Reference standard for imaging
- Can detect >10 mL of pleural fluid
- Helps differentiate empyema from lung abscess
- Shows "split pleura" sign in empyema 1
Pleural Fluid Analysis
A diagnostic pleural fluid sample should be obtained with a fine bore (21G) needle and analyzed for:
- Appearance (clear, cloudy, purulent)
- Protein and LDH (to differentiate transudate from exudate)
- pH (particularly important for parapneumonic effusions)
- Glucose
- Gram stain and culture
- Cytology
- Additional tests as indicated (e.g., adenosine deaminase for TB, amylase for pancreatitis) 1, 2
Management
Management depends on the underlying cause and classification:
Transudative Effusions
- Treat the underlying condition (e.g., heart failure, cirrhosis)
- Therapeutic thoracentesis may be performed for symptomatic relief 2
Exudative Effusions
- Simple parapneumonic: Antibiotics alone usually sufficient
- Complicated parapneumonic/Empyema: Requires chest tube drainage
- Malignant effusions: May require drainage for symptom relief and possible pleurodesis 1, 2
Special Considerations
- Fluid collections are dynamic and may reaccumulate after drainage
- In infectious cases, drainage appears to confer survival advantage
- Repeated thoracenteses or continuous pleural drains may be necessary 1
Common Pitfalls
Assuming bilateral effusions are always transudative: Even bilateral effusions can be exudative in nature 2
Inadequate sampling: Insufficient fluid volume can lead to false-negative cytology results 2
Overlooking pulmonary embolism: No specific pleural fluid tests exist to diagnose this condition 2
Failing to consider medication-induced effusions: A thorough medication history is essential 1, 2
Delaying drainage of infected collections: Prompt drainage of empyema and complicated parapneumonic effusions is associated with better outcomes 1