What is trapped lung and how does it cause hydrops ex‑vacuo (passive pleural effusion)?

Trapped Lung and Hydrops Ex-Vacuo

Definition and Pathophysiology

Trapped lung is a mechanical condition where the lung cannot fully expand to fill the hemithorax due to a restricting fibrous visceral pleural peel that arose from remote, now-inactive pleural inflammation. 1, 2

The key pathophysiologic features include:

• The fibrous peel prevents the visceral and parietal pleura from opposing each other, creating a persistent pleural space that fills with fluid purely due to hydrostatic equilibrium 1, 2
• The fluid accumulation occurs passively—this is the "hydrops ex-vacuo" phenomenon, where fluid fills the negative pressure space created by the unexpandable lung 2, 3
• The pleural effusion persists not because of active inflammation or malignancy, but solely because the mechanical restriction prevents lung expansion and pleural apposition 2, 4

Distinguishing Trapped Lung from Lung Entrapment

The European Respiratory Society differentiates two related but distinct entities 1:

• Trapped lung: Fibrous peel from remote, inactive inflammation (e.g., old parapneumonic effusion, prior cardiac surgery, chest trauma) 1, 2
• Lung entrapment: Active pleural process (malignancy, ongoing infection) causing visceral pleural peel formation 1, 4

Both create unexpandable lung, but trapped lung represents a chronic, stable condition without active disease, while lung entrapment involves ongoing pathology 1, 4

Common Etiologies

Trapped lung most commonly results from 2, 3:

• Inadequately treated parapneumonic effusion or empyema (most common modern cause) 2
• Cardiac surgery complications 2
• Chest trauma 2
• Historical: therapeutic pneumothorax for tuberculosis treatment 2

Clinical Presentation

The hallmark presentation is chronic, stable, unilateral pleural effusion in a patient without active pleural inflammation or malignancy 2, 4:

• Dyspnea is the primary symptom, though some patients remain asymptomatic 2, 5
• Absence of contralateral mediastinal shift despite large effusion volume is a key radiographic clue 1
• Patients may develop chest pain during attempted pleural drainage or post-procedure pneumothorax when drainage is attempted 4

Diagnostic Features

The European Respiratory Society notes several diagnostic approaches, though none are prospectively validated 1, 6:

• Initial pleural fluid pressure <10 cm H₂O at thoracentesis strongly suggests trapped lung 1, 7
• Pleural manometry showing abnormal lung expansion during drainage 1, 4
• Imaging demonstrating visceral pleural thickening (the "peel") 4
• Documentation of chronicity and stability without active inflammatory or malignant process 2

Critical Management Principle

Pleurodesis is contraindicated and will fail in trapped lung because the visceral and parietal pleura cannot appose 7. Attempting pleurodesis subjects patients to unnecessary procedures, prolonged hospitalization, and significant pain without therapeutic benefit 7.

Recommended Management

The European Respiratory Society and European Association for Cardio-Thoracic Surgery recommend indwelling pleural catheters (IPCs) as first-line management for trapped lung 6, 7:

• IPCs provide symptomatic improvement in >94% of patients 6, 7
• They allow outpatient management with intermittent drainage, avoiding repeated hospitalizations 8
• IPCs result in shorter hospital stays and reduced morbidity compared to other interventions 6

IPC Outcomes and Complications

Expected benefits include 6, 8:

• High symptomatic relief rates (>94% across multiple studies) 6
• Improved mobility and ease of management in palliative patients 8
• Catheters can remain in situ until death in most patients 1

Common complications include 6, 8:

• Pain (35% of patients, typically lasting <3 days) 8
• Local cellulitis 6
• Catheter blockage or displacement 6
• Rare but serious: empyema, catheter fracture 1, 6

Alternative Options

For highly selected patients 6, 7:

• Video-assisted thoracoscopic decortication may be considered in younger patients with good performance status and benign trapped lung, though evidence is limited 6, 5
• Pleuroperitoneal shunts are no longer recommended due to high complication rates 6

Clinical Pitfall to Avoid

The major pitfall is failing to recognize trapped lung before attempting pleurodesis, which leads to procedure failure, unnecessary patient suffering, and wasted resources 7. Always assess for complete lung expansion before considering pleurodesis by checking for contralateral mediastinal shift on imaging and measuring initial pleural pressure during drainage 1, 7.