Definition of Trapped Lung
Trapped lung is defined as a condition in which the lung is unable to fully expand to fill the hemithorax, rendering the parietal and visceral pleura either partly or completely unopposed. 1 This condition occurs when the lung's expansion is restricted by a fibrous "peel" over the visceral pleura.
Causes of Trapped Lung
Trapped lung can develop through several mechanisms:
- Pleural thickening causing encasement of the lung 1
- Proximal endobronchial obstruction causing distal lung collapse 1
- Chronic atelectasis 1
- Fibrinous or granulomatous pleuritis 2
- Inadequately treated parapneumonic effusion 2
- Complications from cardiac surgery or chest trauma 2
- Long-standing pneumothorax 3
Terminology Distinction
Some authors make a distinction between two related conditions:
- Lung entrapment: An active pleural process (such as malignancy) causes a visceral pleural peel to form, preventing lung expansion 1, 4
- Trapped lung: The fibrous peel has arisen as a consequence of remote inflammation in the pleural space that is no longer active 1, 2, 4
However, in clinical practice, the term "trapped lung" is often used to cover both clinical entities 1.
Diagnostic Features
Trapped lung is characterized by:
- Inability of the lung to expand and fill the thoracic cavity 2, 4
- Restricting fibrous visceral pleural peel 2, 3
- Chronic, stable pleural effusion without evidence of active pleural disease 4
- Initial negative pleural fluid pressure (<10 cm H₂O) at thoracentesis 1, 2
- Increased pleural space elastance 2
- 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 in the absence of endobronchial obstruction 1
Diagnostic Methods
Several methods have been proposed to predict or diagnose trapped lung:
- Pleural manometry during thoracentesis 1, 4
- M-mode ultrasonography 1
- Patient symptoms during aspiration 1
- Radiological imaging showing abnormal visceral pleural thickening 4
- Thoracoscopy for direct visualization 1
However, none of these methods have been proven prospectively, and further evidence is required before they can be adopted into routine clinical practice 1.
Clinical Significance
Trapped lung has important clinical implications:
- It prevents complete drainage of pleural effusions
- It limits the effectiveness of pleurodesis procedures
- It can cause significant dyspnea and reduced quality of life 5
- In malignant pleural effusions, it complicates management and may require specific approaches 6
Management Considerations
The management of trapped lung depends on the underlying cause, symptoms, and patient's overall condition:
- For benign trapped lung: Surgical decortication may be the only definitive therapy 2, 5
- For malignant trapped lung: Indwelling pleural catheters (IPCs) are often recommended 1, 6
- For asymptomatic patients: Observation may be warranted 2
Understanding the definition and mechanisms of trapped lung is essential for proper diagnosis and management of this challenging condition.