Trapped Lung

Etiology

Historically, pneumothorax therapy, used to treat tuberculosis, had caused trapped lungs.[2] The most common etiologies include immunologic diseases such as rheumatoid pleurisy, inflammatory pleuritis, improperly drained hemothorax, parapneumonic effusions, and infections like bacterial empyema, tuberculous pleurisy, malignant pleural effusions, asbestos exposure, uremic pleurisy, post-cardiac surgery, chest radiation, and medications.[5][6]

Epidemiology

It is unclear why some patients with pleural inflammation heal without sequela while others develop a restrictive fibrous visceral peel leading to the trapped lung. The exact incidence of trapped lung is poorly documented, but it is expected to be higher than is generally recognized. A study by Dresler et al suggests that among patients who undergo thoracentesis, approximately 10% have non-expandable lungs, 20% of patients who undergo therapeutic thoracentesis, and 30% of those who undergo therapeutic thoracentesis for malignant pleural effusion have non-expandable lung.[7]

Pathophysiology

A restricting peel prevents normal parietal and visceral pleural layer apposition in a trapped lung. The formation of this fibrous restrictive peel requires the presence of a long-standing stable pleuro-pulmonary disease, either from an inflamed lung or a malignant visceral pleural tumor.[3] This fibrotic restrictive visceral pleura prevents the lung from expanding when pleural fluid is removed during thoracocenteses. Negative pleural pressure within the pleural space increases fluid entry into the pleural space. It reduces pleural fluid exit through pleural lymphatics, thus leading to the formation of a chronic pleural effusion. So, a steady state of fluid formation and removal is established, leading to persistent negative pressure pleural effusion of constant volume. This persistence of the pleural effusion is mainly mechanical and can be considered a "pleural effusion ex-vacuo".[2][4]

History and Physical

Symptoms vary depending on the size of the pleural effusion. Patients can be asymptomatic with small effusions and are noticed incidentally on imaging studies like a chest x-ray and computed tomography of the chest or abdomen. Clinical exam shows ipsilateral: decreased breath sounds, stony, dull percussion, and decreased fremitus. When the effusion is significant enough, it can cause respiratory distress and warrant immediate relief through thoracentesis or even a surgical approach. Patients experience discomfort during the thoracentesis owing to further reductions in intrathoracic pressure. Lung entrapment, on the other hand, is an active inflammatory process presenting as chest pain and severe dyspnea. Trapped lung poses a significant risk for inadvertent multiple diagnostic and therapeutic procedures attempted for chronic stable pleural effusion, leading to further complications. Hence, a trapped lung should be considered early in the differential diagnosis of chronic stable pleural effusion when no clinically apparent active disease is found.[2][5]

Evaluation

The pleural fluid analysis in a trapped lung is consistent with a transudate or a protein-discordant exudate and has a paucicellular mononuclear cell predominance. Pleural fluid LDH is low, and protein may be in the exudative range. The elevated total pleural fluid protein may be related to factors other than active pleural inflammation or malignancy and does not exclude the diagnosis. The characteristic diagnostic clue is that the pleural effusion reaccumulates rapidly after thoracocenteses, and the effusion volume due to the trapped lung remains remarkably constant. A high index of suspicion should be maintained in diagnosing and managing trapped lungs, particularly in malignant pleural effusions, to prevent complications from repeated thoracenteses. Repeated thoracenteses can result in various complications like chest pain, the formation of loculations, bleeding, the development of hemothorax, and also recurrent effusions. Trapped lung should be suspected in any patient who presents with a stable chronic pleural effusion, particularly if there is an antecedent history of pneumonia, pneumothorax, thoracic surgery, or hemothorax.[8][9][10]

Pleural Manometry 

The trapped lung is can be diagnosed via pleural manometry during thoracentesis. As it is time-consuming and requires technical expertise, currently, manometry is not routinely used during thoracentesis. With large-volume thoracentesis, pleural elastance changes throughout the procedure. It predicts successful pleurodesis by measuring the absolute closing pressure and overall elastance. When the elastance is high, the probability of the pleural layers being pulled apart increases, interfering with pleurodesis. In the setting of a trapped lung, despite a pleural effusion, the pleural pressure is low and drops significantly with fluid removal. This high pleural elastance (change in pressure/volume) is a hallmark of a trapped lung. These characteristic pleural pressure changes are highly suggestive of a trapped lung. It may aid in the diagnosis of patients with minimal or absent symptoms. However, confirmation with direct visualization via video-assisted thoracoscopy or air contrast computed tomography showing visceral pleural thickening (a pleural rind) is required.

Imaging Studies

Upright and decubitus radiographs and chest computed tomography often show pleural thickening and loculations. Pleural fluid may shift slightly with postural changes but isn't free-flowing. The paradoxical finding that the hemithorax with pleural effusion is reduced in size compared to the contralateral side suggests significantly higher negative pleural pressure on the affected side than on the opposite side. It provides further evidence for the diagnosis of a trapped lung. Post-thoracentesis radiographs may show air in the pleural space with the same size and shape as the prior effusion.[10]

Ultrasound

Point-of-care ultrasound (POCUS)is a rapid, bedside, cost-effective way to diagnose pleural pathologies and must be performed before any pleural intervention. With the patient positioned upright or supine, a low-frequency, 2- to 5-MHz transducer probe is placed in a longitudinal orientation. With the probe marker held cranially from mid-axillary or posterior mid-scapular views, images are viewed in brightness and motion modes. In motion mode, the respiratory movement of an atelectatic lung occurs in a sinusoidal pattern within the surrounding pleural fluid. This represents a “sinusoid sign," which suggests the expandable lung. The absence of this sinusoidal respirophasic lung is known as an “absent sinusoid sign". It suggests an unexpandable lung and coincides with pleural manometry findings.[11] Besides this, thoracic ultrasound is useful for differentiating between malignant and benign diseases, pleural thickening, nodularity, and lung consolidation with or without effusion.[12]

Treatment / Management

There are various treatment options for trapped lungs, and the patient's symptoms determine the choice of treatment modality. Observation is warranted in an asymptomatic patient if the effusion is not worsening.[4] If the patient has dyspnea, symptom alleviation can be achieved through fluid removal via thoracentesis.[13] In patients with a poor prognosis and a shorter expected survival period, repeated thoracentesis or a pleural catheter is advised as the least invasive approach to provide immediate symptom relief. Complications include rapid re-accumulation requiring multiple visits, infection leading to empyema, worsening pneumothorax, hemothorax, and bleeding.[14][15](B2)

Implantable Pleural Catheter

Pleurodesis is considered an effective therapeutic choice for patients with malignant, recurrent pleural effusion. However, pleurodesis is ineffective and not the management of choice in cases of trapped lungs.[16] A chronic indwelling pleural catheter can be considered for symptomatic relief in these patients. This is a small-bore catheter 66 cm long, 15 F silicone rubber catheter with fenestrations along the distal 24 cm, placed within the pleural space. It aids in partial lung expansion and alleviates symptoms, as dyspnea in these patients is predominantly due to pleural space tension relative to the chest wall and diaphragm rather than parenchymal lung pathology or lung collapse.[17][15][18](B2)

Pleuroperitoneal Shunt

The pleuroperitoneal shunt, proposed in 1982 by Weese and Schouten, can provide effective and safe palliation in select patients. With each compression, it can transport up to 1.5 L of pleural fluid into the abdominal cavity. Complications include shunt failure and infection and are not routinely recommended or preferred.[19][20][21](B2)

Surgical Decortication 

Surgical decortication is the only curative treatment modality when a more definitive approach is indicated.[4] It involves removing a fibrous visceral pleural layer covering the lung to allow lung expansion.[22] Pleura can now oppose the chest wall, and this relieves restrictive ventilatory defect, thus having a positive impact on hypoxia and ventilation-perfusion mismatch. It also reduces chest wall pain, discomfort, and dyspnea and prevents recurrent pleural effusions. It is important to exclude other diseases and underlying cardiopulmonary conditions before attempting surgical decortication. Visceral pleurectomy can be performed by either open thoracotomy or the most preferred minimally invasive approach, Video-assisted thoracoscopic surgery (VATS).[23][24] VATS is a minimally invasive surgical technique to diagnose and treat pleuro-parenchymal diseases. A tiny camera (thoracoscope) and surgical instruments are inserted into your chest through 1 or more small incisions in the chest wall during a VATS procedure.

Intrapleural Fibrinolytic Therapy

In select cases, fibrinolytic drugs such as tPA, urokinase, and streptokinase can be introduced into the pleural space to break down fibrin strands and adhesions and promote drainage and expansion of the lung. These can be used as palliative care options in inoperable scenarios.[25] All of these treatment options are, in essence, palliative. Re-expansion of the lung, even after removing this peel, is difficult when the lung has been trapped for a long time.[4](B2)

Differential Diagnosis

It is important to differentiate the trapped lung from lung entrapment to identify active processes requiring specific treatment and avoid unnecessary diagnostic and therapeutic pleural procedures in patients with trapped lungs. Lung entrapment may result from pleural or non-pleural causes, while the trapped lung is caused only by pleural causes. Non-pleural causes that cause lung entrapment include conditions that increase the elastic recoil pressure of the lung. They include endobronchial obstruction due to bronchogenic carcinoma or interstitial diseases like lymphangitic carcinomatosis[26]. The pleural fluid analysis is usually consistent with exudate in lung entrapment, while patients with trapped lungs have a transudate or a protein-discordant exudate with a mononuclear cell predominance. Furthermore, these entities can be differentiated by the particular patterns of pleural elastance. Unlike trapped lung, pleural manometry in lung entrapment shows normal elastance during initial pleural fluid removal. Still, the lung doesn't expand fully as more fluid is removed and the slope of the elastance curve increases. While most cases of lung entrapment resolve with the resolution of the inflammatory process, the resolution is incomplete in a trapped lung. Therefore, a trapped lung and lung entrapment can represent a continuum of the same process. Other differentials include pneumothorax ex-vacuo (particularly right upper lobe collapse), cryptogenic organizing pneumonia, chronic pulmonary consolidations, and bronchiolitis obliterans organizing pneumonia.[27]

Prognosis

The trapped lung is an independent risk factor for short survival compared to patients without non-expandable lungs in mesothelioma patients.[28] Bibby and colleagues found that in a cohort of 229 patients, 33% of patients with Malignant Pleural Mesothelioma have a non-expandable lung. Perhaps most importantly, they have found for the first time that non-expandable lung is an independent predictor of poor survival.[29] It is unclear as to why it has an effect on the survival rate, and further studies are warranted.

Complications

Undiagnosed trapped lung typically presents as chronic stable pleural effusion, and these patients might undergo multiple thoracocenteses without any significant improvement in respiratory symptoms. Repeated thoracenteses can lead to many adverse events related to the procedure, delay in diagnosis and adequate treatment, improper use of resources, worsening of the underlying disease, and above all, significant patient discomfort.[11]