How to diagnose a trapped lung on a chest X-ray (CXR)?

Diagnosing Trapped Lung on Chest X-ray

The most definitive radiographic sign of trapped lung on chest X-ray is the presence of a pleural effusion without contralateral mediastinal shift, suggesting the inability of the lung to expand fully despite the presence of pleural fluid. 1

Key Radiographic Features of Trapped Lung on CXR

Primary Diagnostic Signs

• Absence of mediastinal shift despite presence of a large pleural effusion 1
• Persistent air-fluid level following thoracentesis or chest tube placement
• Pneumothorax ex-vacuo (development of pneumothorax after fluid drainage) 2
• Thickened visceral pleura (may be visible as a white line outlining the collapsed lung)
• Stable, unchanging pleural effusion on serial imaging

Secondary Findings

• Loculated fluid collections that don't change with patient positioning
• Volume loss in the affected hemithorax
• Elevation of the ipsilateral hemidiaphragm
• Fixed position of the mediastinum toward the unaffected side

Diagnostic Algorithm for Trapped Lung on CXR

1. Initial Assessment:

   • Look for pleural effusion without expected mediastinal shift
   • Check for thickened pleural line outlining the collapsed lung
   • Compare with previous imaging to assess chronicity

2. Dynamic Assessment:

   • Review post-drainage films for:
     • Failure of lung to expand after thoracentesis
     • Development of pneumothorax ex-vacuo after fluid removal
     • Persistent air-fluid level

3. Correlation with Clinical Context:

   • History of conditions associated with trapped lung:
     • Previous pleural infection/empyema
     • Hemothorax
     • Cardiac surgery
     • Thoracic radiation
     • Rheumatologic diseases

Limitations of CXR in Diagnosing Trapped Lung

CXR alone has limitations in definitively diagnosing trapped lung. Consider these important caveats:

• Plain radiographs may not clearly demonstrate pleural thickening
• Difficult to differentiate between active pleural processes (lung entrapment) and chronic fibrosis (trapped lung)
• Endobronchial obstruction can mimic radiographic appearance of trapped lung

Complementary Diagnostic Approaches

When CXR findings suggest trapped lung but are inconclusive:

• Pleural manometry during thoracentesis: Initial negative pleural fluid pressure (<10 cm H₂O) is characteristic 1, 3
• CT imaging can better visualize pleural thickening and underlying lung parenchyma
• Ultrasound can identify pleural thickening and assess lung sliding during respiration 1
• M-mode ultrasonography to evaluate lung expansion dynamics

Clinical Pearls

• The diagnosis of trapped lung requires correlation of imaging findings with clinical history and pleural fluid characteristics
• Pleural fluid in trapped lung is typically paucicellular with low LDH, though protein may be in the exudative range 3
• Differentiate trapped lung (chronic, inactive process) from lung entrapment (active inflammatory or malignant process) 4, 5
• Remember that a trapped lung may spontaneously improve in some cases without surgical intervention 2

By systematically evaluating these radiographic features on chest X-ray and correlating with clinical context, trapped lung can be identified, allowing for appropriate management decisions.