Pulmonary Medicine

Trapped Lung Syndrome

What every physician needs to know:

Trapped lung syndrome refers to a condition in which the lung does not fully expand during pleural drainage to oppose the chest wall. This form of non-expandable lung is the sequalea of prior pleural inflammation that results in the creation of a fibrous peel on the visceral pleura. The resulting negative pleural pressure causes a pleural effusion ex-vacuo. As hydrostatic forces are the sole cause of increased pleural fluid formation, pleural fluid analysis will reveal a transudative effusion.


It is important to differentiate trapped lung from entrapped lung (also known as lung entrapment). Like trapped lung, lung entrapment refers to a lung that will not fully expand with pleural drainage. The major difference is that lung entrapment may result from either pleural or non-pleural causes, while trapped lung results from pleural causes only. Patients with active pleural inflammation can have thickening of the visceral pleura, causing non-expandable lung, especially toward the end of pleural drainage. Non-pleural causes of lung entrapment include diseases that increase the elastic recoil pressures of the lung, such as endobronchial obstruction causing atelectasis or interstitial disease, such as lymphangitic carcinomatosis.

Are you sure your patient has trapped lung syndrome? What should you expect to find?

As patients with trapped lung do not have active pleural inflammation, they are usually asymptomatic, and the effusion is often found during a routine physical exam or chest imaging done for another reason. If the effusion is sampled, pleural fluid analysis will reveal a transudate. Pleural manometry (measuring pleural pressure during thoracentesis) is simple to perform and is extremely useful in diagnosing patients with trapped lung.

Under normal conditions, if one were to add fluid to a closed system (the thorax), the pressure would rise; and as the fluid is removed, the pressure would fall until a steady state is reached. In the chest, the pleural pressure at functional residual capacity (FRC) is slightly negative (-3 to -5 cmH20) because the balance of forces of the chest have a tendency to expand, and the lung's elastic recoil results in a tendency for the lung to collapse.

In the setting of trapped lung, despite the presence of a pleural effusion, the pleural pressure is low, and it drops significantly with the removal of fluid. This high pleural elastance (change in pressure/change in volume) is a hallmark of trapped lung.

Beware: there are other diseases that can mimic trapped lung syndrome:

Patients with lung entrapment are usually symptomatic from their effusion, as there is typically some active pleural inflammation, and pleural fluid analysis will demonstrate an exudative effusion. Common causes of lung entrapment include malignant pleural effusions, complicated parapneumonic effusions, and endobronchial obstruction that causes a post-obstructive pneumonia with atelecatsis.

Pleural manometry in patients with lung entrapment can show a normal pleural elastance during the initial removal of fluid; however, as more fluid is removed and the lung does not fully expand, the slope of the elastance curve increases.

It has been shown that the development of a vague type of chest discomfort during thoracentesis is likely due to dropping pleural pressures that should be a sign that terminating thoracentesis should be considered. In this setting, some authors advocate that atmospheric air be allowed to enter the pleural space in order to bring the pleural pressure back to a more physiologic range and to diminish the patient's discomfort. A chest CT scan can then be obtained to demonstrate visceral pleural thickening and non-expandable lung (Figure 1).

Figure 1.

Chest CT showing thickened visceral pleura

How and/or why did the patient develop trapped lung syndrome?

Pleural effusions resolve when the underlying disease that has caused the imbalance in hydrostatic or oncotic (or both) pressures has resolved. If a patient with lung entrapment has resolution of the active pleural inflammation, and the pleura heals in a way that does not result in any thickening of the visceral pleura, the pleural physiology will return to normal. An example of this process is in patients who have completely recovered from an episode of community-acquired pneumonia with a parapneumonic effusion.

However, the same patient's pleura may heal in such a way that a visceral peel develops, and though there is no further pleural inflammation, a pleural effusion persists solely because of the negative pleural pressure created by the visceral peel. In this example, if one were to sample the fluid early in the course of the patient's illness, the fluid would be exudative, and the elastance curve would be consistent with lung entrapment. If the fluid were sampled later in the resolution and healing process, it would be a transudate and the elastance curve would be consistent with trapped lung.

Which individuals are at greatest risk of developing trapped lung syndrome?

It is unclear why some patients form a visceral pleural peel while others heal their pleural space with no residual visceral pleural thickening.

What laboratory studies should you order to help make the diagnosis, and how should you interpret the results?

Patients with a trapped lung have a transudative effusion, whereas those with an entrapped lung have an exudative effusion. It is important to examine the pleural elastance curves and to distinguish between these two syndromes and put the pleural fluid analysis and physiology into the patient's clinical context, as there may be overlap between lung entrapment and trapped lung during the healing phase of the disease.

What imaging studies will be helpful in making or excluding the diagnosis of trapped lung syndrome?

In both causes of non-expandable lung, post-drainage imaging may reveal a pneumothorax. The cause of pneumothorax in these patients may be likened to pouring milk out of a bottle: As the milk pours out, a vacuum is created in the bottle and air enters. Similarly, in the setting of non-expandable lung, as fluid is removed, pleural pressure drops and eventually air has to enter the pleural cavity. Though the air may enter from the atmosphere (i.e., between the catheter and skin), it is likely that local deformation forces develop and create small tears in the visceral pleura.

Unlike patients with spontaneous or traumatic pneumothorax, which are often located apically on an upright chest x-ray, patients with non-expandable lung may have basilar pneumothoraces, as it is often the dependent part of the lung that fails to expand fully. If a chest CT is obtained after drainage, visceral pleural thickening can be readily identified.

What non-invasive pulmonary diagnostic studies will be helpful in making or excluding the diagnosis of trapped lung syndrome?

Pleural manometry is easily performed at the bedside during thoracentesis, but it is necessary to use the syringe-pump method, as opposed to a vacuum bottle, if one is interested in measuring pleural pressure. Although pleural manometry may add five minutes to the procedure, the information gained about the underlying pleural physiology is well worth the time and effort.

The simplest way to measure pleural pressure is by using the drainage tubing as a U-shaped water manometer. With the tubing connected to the catheter in the patient's chest and the collection bag, the syringe is removed, and the difference in height from catheter insertion site to where drainage ceases is the pressure in the pleural space at the level of the catheter's entry into the chest. For example, if fluid stops draining with the tubing 6cm above the catheter insertion site, the pleural pressure is +6 cmH20. If the fluid stops flowing with the tubing 16cm below the skin insertion site, the pleural pressure is -16 cmH20. If pressures are measured at regular intervals (e.g., every 240 mL, or four pumps of a 60 mL syringe), one can easily plot out the pleural elastance curve (change in pressure on the Y axis vs. change in volume on the X axis).

The pressure curve of lung entrapment starts out with a normal and relatively flat pressure-volume curve with little change in pressure for any change in volume. As the terminal part of the lung fails to expand with continued drainage, the pressure drops additionally with any given change in volume, and the curve steepens. The pressure curve of trapped lung typically starts out at zero or slightly negative and drops precipitously during the initial withdrawal of pleural fluid.

What diagnostic procedures will be helpful in making or excluding the diagnosis of trapped lung syndrome?

As pleural manometry is easy to perform and does not add additional risk or cost to the procedure, it should be performed during every thoracentesis. The air-contrast chest CT scan and sometimes chest ultrasonography may show visceral pleural thickening.

What pathology/cytology/genetic studies will be helpful in making or excluding the diagnosis of trapped lung syndrome?

Not applicable.

If you decide the patient has trapped lung syndrome, how should the patient be managed?

Dyspnea patients with pleural effusions see effects on the pressure-volume curve from the chest wall's being expanded to a disadvantageous position and not from hypoxemia or lung collapse. As such, patients with lung entrapment (such as from malignant pleural effusions) can be relieved of their dyspnea by implantation of a tunneled pleural catheter. Despite the lung's not expanding, their dyspnea improves because the chest wall comes down to a more compliant point on its pressure-volume curve.

As the large majority of patients with trapped lung are asymptomatic, treatment aimed at their pleural effusion is not necessary. However, some patients may be dyspneic from a restrictive ventilatory defect and may benefit from decortication. It is important to make sure the patient's dyspnea is due to the non-expandable lung and not another underlying cardiopulmonary condition before sending the patient for decortication.

What is the prognosis for patients managed in the recommended ways?

Patients with trapped lung do well. Because they are typically not symptomatic and do not require treatment, the most important consideration is to avoid unnecessary interventions that cause iatrogenic injury, such as inappropriate hospitalization and chest tube placement after thoracentesis when the X-ray reveals a basilar pneumothorax. It is not likely that the lung will re-expand with chest tube drainage, and unless the patient becomes symptomatic, it may be better to let the pleural fluid re-accumulate.

What other considerations exist for patients with trapped lung syndrome?

Not applicable.

What’s the evidence?

Heidecker, J, Huggins, JT, Sahn, SA, Doelken, P. "Pathophysiology of pneumothorax following ultrasound-guided thoracentesis". Chest. vol. 130. 2006. pp. 1173-1184.

Light, RW, Jenkinson, SG, Minh, VD, George, RB. "Observations on pleural fluid pressures as fluid is withdrawn during thoracentesis". Am Rev Respir Dis. vol. 121. 1980. pp. 799-804.

Doelken, P, Huggins, JT, Pastis, NJ, Sahn, SA. "Pleural manometry: technique and clinical implications". Chest. vol. 126. 2004. pp. 1764-1769.

Feller-Kopman, D, Walkey, A, Berkowitz, D, Ernst, A. "The relationship of pleural pressure to symptom development during therapeutic thoracentesis". Chest. vol. 129. 2006. pp. 1556-1560.

Huggins, JT, Sahn, SA, Heidecker, J, Ravenel, JG, Doelken, P. "Characteristics of trapped lung: pleural fluid analysis, manometry, and air-contrast chest CT.". Chest. vol. 131. 2007. pp. 206-213.

Feller-Kopman, D, Parker, MJ, Schwartzstein, R. "Assessment of pleural pressure in the evaluation of pleural effusion". Chest. vol. 135. 2009. pp. 201-209.

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