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.