Diagnosis and Management of Trapped Lung

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

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Diagnosis of Trapped Lung

The diagnosis of trapped lung requires the exclusion of other causes of unexpandable lung after thoracentesis. The causes of unexpandable lung include airway obstruction resulting in atelectasis, severe parenchymal disease, and visceral pleural restriction. All of these conditions may coexist. If pleural pressure is measured, a sharp drop in pressure is usually evident. However, the sharp decrease may not be seen until a significant amount of fluid has been withdrawn; this usually indicates that something other than a purely mechanical cause is responsible for pleural fluid formation. If the fluid is an exudate, active pleural disease is usually present. If the fluid is a transudate, either systemic or local hydrostatic conditions cause pleural fluid formation. Negative initial pleural pressure and an immediate sharp pressure drop favor mechanical restriction as the sole cause of persistence of pleural effusion.^9-11 The pleural fluid usually has low levels of protein and lactate dehydrogenase under these circumstances. A CT scan obtained with negative pleural pressure and preferably air contrast may demonstrate a visceral pleural membrane, severe parenchymal disease, or an endobronchial lesion. Subpleural atelectasis may be seen and does not represent relaxation atelectasis in the presence of negative pleural pressure. Negative pressure in the pleural space is assured if a functioning thoracostomy tube is in place with suction. If the examination is to follow a thoracentesis, fluid may be exchanged with air and the negative pressure can be established just prior to removal of the drainage catheter. However, if pleural pressure measurement is not available, a small-bore thoracostomy tube with drainage system should be placed.

If the lung does not expand after drainage of an effusion with low lactate dehydrogenase and protein levels, a visceral pleural membrane is present, and endobronchial disease and severe parenchymal disease are absent, a presumptive diagnosis of trapped lung can be established. Initially negative pleural pressure (usually < Ð5 cm H₂O) supports the diagnosis. However, trapped lung can only be diagnosed with certainty with successful decortication, which is not indicated in many patients.

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