Trapped Lung Definition and Diagnosis
Trapped lung is defined as a condition in which the lung is unable to fully expand to fill the hemithorax due to a fibrous "peel" over the visceral pleura that restricts lung expansion, occurring as a consequence of remote inflammation in the pleural space that is no longer active. 1
Pathophysiology and Mechanism
Trapped lung develops through several mechanisms:
- Formation of a fibrous visceral pleural peel following remote pleural inflammation
- Occurs during the organizational stage of pleural infection when fibroblasts infiltrate the pleural cavity
- Thin intrapleural membranes reorganize to become thick and non-elastic (the "peel")
- These solid fibrous pleural peels prevent lung re-expansion 2, 1
It's important to distinguish trapped lung from lung entrapment:
- Trapped lung: A consequence of remote inflammation where the fibrous peel is the primary problem and active inflammation is no longer present 1, 3
- Lung entrapment: An active pleural process (malignant or inflammatory) that prevents lung expansion 1, 3
Diagnostic Features
Trapped lung can be identified by several characteristic findings:
- Initial negative pleural fluid pressure (<10 cm H₂O) at thoracentesis 1, 4
- Absence of contralateral mediastinal shift on chest radiograph with a large pleural effusion 1
- Cut points of >19 cm H₂O with removal of 500 ml of fluid and >20 cm H₂O with removal of 1 L of fluid are predictive of trapped lung 1
- Post-thoracentesis hydropneumothorax (pneumothorax after fluid drainage) 3
- Development of chest pain preventing complete drainage of pleural effusion 3
- Chronic, stable pleural effusion without evidence of active pleural disease 5, 4
- Increased pleural space elastance during drainage 4
Diagnostic Tools
Several methods can help diagnose trapped lung:
- Pleural manometry during thoracentesis (most useful for identifying unexpandable lung) 1, 3
- M-mode ultrasonography 1
- Thoracoscopy for direct visualization 1
- Ultrasound to confirm the presence of pleural fluid collection 2
- Documentation of chronicity and stability of the effusion 4
Clinical Context
Trapped lung most commonly occurs as a consequence of:
- Inadequately treated parapneumonic effusion or empyema 4
- Cardiac surgery 4
- Chest trauma 4
- Other inflammatory processes involving the pleura 4
- Historically, it was recognized as a complication of therapeutic pneumothorax for tuberculosis treatment 4
Management Considerations
The management approach depends on symptoms and underlying cause:
- Surgical decortication is the definitive therapy for symptomatic benign trapped lung 1, 4, 6
- In asymptomatic patients, observation rather than decortication is warranted 4
- Indwelling pleural catheters (IPCs) are often recommended for malignant trapped lung 1
- Before considering surgical intervention, other causes of dyspnea should be excluded 6
Important Caveats
- Diagnosis requires confirmation of the absence of active inflammatory or malignant pleural process, bronchial obstruction, or severe underlying lung disease 4
- Chest CT scans should not be performed routinely for diagnosis 2
- Ultrasound should be used to guide thoracocentesis or drain placement when intervention is needed 2
- Trapped lung can significantly impact quality of life by causing dyspnea in symptomatic cases 7