Pulmonary Disease - Volume 15, Part 1, October 2014

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