resulting from the extent of disease and primary
malignancy. 1 If thoracentesis is initially nondiagnos-tic, repeat thoracentesis can improve diagnostic
yield, but subsequent sampling has diminishing
utility. In one series, the diagnosis of malignancy
was made in 65% of patients by fluid cytology analysis from the initial sample and 27% from the second sample, but only 5% from the third sample.30
However, it is also important for the clinician to
assess how the patient responds to large volume
drainage as the effect on the patient’s symptoms
and their physiologic responses to fluid removal
will influence management options. Up to 50%
of patients will not have significant symptom
palliation because they may be symptom-limited
by other comorbid conditions, generalized de-conditioning, or incomplete lung reexpansion. A
trapped lung is generally described as a lung that
cannot expand completely after removal of pleural
fluid; it may result from pleural-based malignancies
or metastases, loculations and adhesions, or bronchial obstruction. Trapped lung is associated with
high elastance (PEL) affecting pleural pressure-volume relationships (Figure 1). While clinically
often considered together, some authors differentiate the category of incomplete lung expansion into
2 subgroups. In this context, the term trapped lung
is used specifically to describe a mature, fibrous
membrane that prevents lung reexpansion and is
caused by a prior inflammatory pleural condition.31
Entrapped lung describes incomplete lung expan-
sion resulting from an active disease process,
such as ongoing infection, rheumatologic pleurisy,
or malignancy. Differences in pleural manometry
can be seen in the 2 subgroups. Pleural manom-
etry can be helpful to monitor for the generation
of high negative intrapleural pressures during fluid
removal; negative pressures in excess of –19 cm
H2O are suggestive of trapped lung.32 Patients may
develop symptoms of chest pain or discomfort dur-
ing fluid drainage in these situations.
While most malignant pleural effusions are protein-rich exudates, approximately 2% to 5% may be
transudates.33,34 Pleural fluid may frequently appear
hemorrhagic, but comparison of the hematocrit in
the pleural fluid to the serum with a ratio greater
than 0.5 is used to distinguish a true hemothorax
from bloody appearing pleural fluid.35 The cell
count may be lymphocyte-predominant, but other
cell types, such as eosinophils, do not exclude
malignancy.36 Fluid may have a low glucose concentration and pH as well. While there is conflict-
Figure 1. Pressure/volume curves in normal lung compared to entrapped and trapped lung physiology. There
is minimal pleural pressure change in normal lung as fluid
is removed. Entrapped lung initially has a pleural space
elastance (PEL) that mirrors normal lung but then increases significantly. Trapped lung has a high PEL throughout
the course of volume removal. (Adapted from Huggins
JT, Doelken P, Sahn SA. The unexpandable lung. F1000
Med Rep 2010;2:77.)
PleuralPressu
re
(cmH2 O
)
20
0
–20
–40
012
3
Pleural Fluid Volume Removed (L)
Trapped lung
Entrapped lung
Normal lung