HONG KONG RESPIRATORY MEDICINE

A website for HK Thoracic Society, ACCP HK & Macau Chapter

2002 Sep - What is trapped lung and how to manage it?

Dr Maureen Wong, Department of Medicine and Geriatrics, Caritus Medical Centre

Definition and Causes 
The condition occurs when the lung is covered preventing its expansion to the chest wall, leaving a persistent fluid-filled pleural space. There are a number of conditions that prevent the lung from expanding, be it malignant or benign. 

In the latter, it is purely due to the mechanical restriction of the viscer alpleura as a result of fibrinous or granul omatous pleuritis. The fluid andhas protein and LDH values in the transudative range and histopathology shows organized fibrous tissue covering the visceral pleura.

History and Causes
Traditionally, it is recognized as a complication of pneumothorax therapy for tuberculosis. Approximately 5% of patients would develop this complication, either due to fibrosis of the lung or scarring of the visceral pleura.

With the abandonment of therapeutic pneumothorax, the most common cause of trapped lung today is probably a remote complicated parapneumonic effusion or empyema that was insufficiently drained. Other causes include uremic pleuritis, hemothorax, rheumatoid pleuritis, and tuberculous pleuritis.

Persistent pleural effusions after CABG and cardiac surgeries have also been recently reported.

In malignant conditions, local tumor burden, including lesions in the lung or airway, the ability of the mediastinum to shift to the side of the lesion, the ability of the diaphragm to move upward, and the inward movement of the rib cage will have bearing on the approximation of the pleura.

Presentation
In most cases, the presentation of trapped lung is an asymptomatic chronic unilateral effusion in a patient with pleurisy in the past or a remote history of an infectious or inflammatory process known to involve the pleural space. CT may show thickened visceral pleura and partially or completely collapsed lung. In the asymptomatic patient, trapped lung presents a diagnostic dilemma rather than a condition requiring treatment.

Diagnosis in Non-Malignant Conditions
In benign conditions of trapped lung, the diagnosis is often challenging.
The sole cause for the persistence of pleural fluid in trapped lung should
be due to hydrostatic balance. Thoracocenteis and pleural fluid analysis
including cytology are mandatory. Thoracoscopic inspection and biopsy
may also be required. A sterile, lymphocyte -predominant, low protein,
low LDH effusion without malignant cells is characterized but not
diagnostic. Initial pressures may be negative, usually less than -5 cm
H2O, and drop precipitously during fluid withdrawal. Reaccumulation
shortly occurs and the patient usually does not experience much relief
from the thoracocentesis. If a diagnosis is not established, a CT scan
may be obtained to exclude severe parenchymal disease. Replacement of
the fluid with air may show up the fibrinous membrane covering the
visceral pleura. Fibreoptic bronchoscopy should be considered to
exclude endobronchial obstruction. Thorascopy typically reveals a thin,
resilient membrane preventing the lung from expanding. The
predominant histologic finding is mature fibrosis, with few
inflammatory cells in the case of non-specific pleuritis and
granulomatosis pleuritis. If chronicity and stability over time have been
demonstrated, a diagnosis of benign trapped lung can be made with
reasonable confidence. Complete resolution of the effusion after
decortication confirms the diagnosis. However, surgery is not indicated
in the asymptomatic patient with trapped lung.


Figure I shows the apparatus used to measure pleural pressures and
aspirate pleural fluid. A negative initial pressure and elastance (defined
as the decline in pleural fluid pressure in cm H2O after removal of 500
ml of effusion) of more than 25 cm H20/L is in support of the diagnosis.

Prevention and Treatment in Non-Malignant Conditions
Most conditions leading to eventual development of trapped lung are
prevented with appropriate management in the acute inflammatory
phase.

An asymptomatic paient with small trapped lung effusion does not
appear to be at risk for secondary infection or other complications
and therefore, will not benefit from thoracotomy and decortications.

Symptomatic patient need a thorough surgical assessment. In general,
if the lung is badly scarred, re-expansion after decortications is
unlikely and surgery would not be expected to result in symptomatic
relief.

Diagnosis and Management in Malignant Conditions
~ The diagnosis of trapped lung may not be obvious in malignant
condition. Always has high index of suspicion if the hemi-thorax with
the effusion is reduced in size. It indicates that the pleural pressure on
the side with effusion is more negative than the contralateral side. In
many cases, the diagnosis became apparent when the underlying lung
does not expand radiographically after thoracentesis or tube
thoracostomy.

Figure 2

Figure 3
Figure 2 and 3 show the CXR of a patient with known Ca lung and
right pleural effusion. Therapeutic tapping on right side for dysnoea
one month after the initial diagnosis yielded 600 ml of blood stained
fluid. The procedure was stopped because of coughing. Post tapping
CXR showed pneumothorax and trapped lung. The diagnosis might
have been suspected earlier.

Patients with trapped lung tends to have a higher elastance .The effusion also has a lower PH «7.3) and a lower glucose level «3.33 mmol/L.

Tube thoracostomy with the instillation of a sclerosing agent is likely to fail because of the markedly negative pleural pressure.

Treatment options include implantation of pleuroperitoneal shunt, placement of permanent flexible indwelling pleural catheter, decortication +/- pleurectomy.