An unexpandable lung may result from visceral pleural restriction, an endobronchial lesion, or chronic atelectasis. The most common causes of visceral pleural restriction are malignancy and infection; others include inﬂammatory pleurisy, such as rheumatoid disease, and coronary artery bypass graft (CABG) surgery.
There are two distinct phases of the unexpandable lung caused by visceral pleural restriction: (1) the early phase, called lung entrapment, and (2) the late phase, termed trapped lung. Lung entrapment caused by malignancy is associated with two pathophysiologic mechanisms responsible for pleural ﬂuid formation: (1) malignant involvement of the pleura, promoting capillary leak and impaired pleural lymphatic drainage, and (2) visceral pleural restriction from the tumor burden, resulting in hydrostatic imbalance. A trapped lung from remote infection results in pleural ﬂuid formation only from the unexpandable lung and hydrostatic forces.
Patients with a trapped lung may present either with exertional dyspnea if the extent of unexpandable lung is large or with a small, persistent effusion discovered on routine chest radiography if the visceral pleural restriction is small. When a portion of the pleura is restricted and the adjacent lung cannot occupy the resultant space, ﬂuid ﬁlls the space in vacuo along a pressure gradient.
A therapeutic thoracentesis will cause a rapid and signiﬁcant decrease in pleural pressure that can be documented by manometry, resulting in anterior chest pain without relief of dyspnea. The diagnosis of trapped lung can be further substantiated by allowing air entry through an open stopcock into the pleural space with rapid cessation of the chest pain. A chest computed tomography (CT) scan will conﬁrm a “visceral pleural peel,” which is typically smaller than 3 mm in thickness and not likely to be detected when not outlined by air. The character of the pleural ﬂuid will be determined by the stage of the unexpandable lung. With a parapneumonic effusion causing lung entrapment, the ﬂuid will be exudative by protein and lactate dehydrogenase criteria with neutrophil predominance. When the inﬂammatory or infectious process has resolved, the sole cause of the effusion will be an imbalance in hydrostatic pressures and thus a transudate. However, because lung entrapment and trapped lung represent a continuum of the same disease process, the timing of thoracentesis is critical in revealing whether the ﬂuid is exudative (early) or transudative (later). Although most patients with inﬂammatory lung entrapment have resolution, others develop a pleural peel and trapped lung.
Therefore, the classic pleural effusion from trapped lung is a serous transudate with a low number of mononuclear cells.
In an asymptomatic patient with a small, trapped lung, reassurance is all that is necessary. With a large, symptomatic trapped lung and restrictive physiology, the underlying lung should be examined by CT scan. If the underlying lung is normal, decortication can be recommended in the appropriate circumstance.