Diagnosis and Management of Trapped Lung

By Peter Doelken, MD, FCCP; and Steven A. Sahn, MD, FCCP

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Pathophysiology and Technical Considerations

The pleural fluid in trapped lung exists in hydrostatic equilibrium with oncotic gradients intact.⁶ The persistence of a pleural space under these conditions requires a mismatch of the shape of the thoracic contents and the chest wall or a size mismatch resulting in deformation forces exceeding the hydrostatic and oncotic forces that favor fluid removal from the pleural space. Similar conditions are found in the normal pleural space, along the lobar margins, where increased deformation forces result in a thicker pleural liquid layer, a steeper pressure gradient, and more negative pressures.^7,8

When pleural liquid pressure is measured, the initial mean pressure has been shown to be negative in trapped lung and to drop sharply upon fluid withdrawal. However, in malignant effusion, negative initial mean pressures and rapid decreases may also be encountered.^9-12 This is likely related to coexistent malignant lung entrapment or other malignancy-related causes of unexpandable lung. Therefore, the demonstration of negative initial pleural liquid pressure or increased elastance of the pleural space is not sufficient to establish the diagnosis of trapped lung. In contrast, positive initial mean pleural pressure virtually excludes trapped lung as the sole cause of a pleural effusion. On occasion, a patient with trapped lung may experience pain after only a small amount of fluid has been withdrawn; this is usually accompanied by excessively negative pressure, and the pain may be alleviated by opening the thoracentesis system to the atmosphere. Similarly, patients with lung entrapment associated with malignancy or other causes may experience sudden onset of pain coincident with a sharp decrease in pleural pressure. In these patients, considerable amounts of fluid may already have been withdrawn prior to the onset of pain. Again, alleviating the negative pressure usually results in immediate relief. Thoracostomy with small-bore catheters and a standard drainage system (controlled suction and gravity drainage) has the advantage of not resulting in excessively negative pressure. Small-bore tube thoracostomy may be the method of choice for demonstration of lung entrapment when pleural space manometry is not available during thoracentesis. However, thoracostomy tubes in conjunction with one-way valve drainage systems do not allow the free exchange of air and fluid and should not be used if lung entrapment is suspected.

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