Pleural Diseases

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Chapter 25

Pleural Diseases

Charlie Strange

A spectrum of pleural diseases affects respiratory function. An understanding of pleural anatomy, physiology, and pathology is essential to delivering effective respiratory care. This chapter focuses on the two major disease processes that occur in the pleural space: pleural effusion and pneumothorax.

Pleural Space

Overview and Definitions

Each lung is covered by a thin membrane called the visceral pleura, which adheres closely to the subjacent alveoli of the lung. The visceral pleura dips into the fissures of the lung, allowing the surgeon easy access between the lung lobes and allowing pleural fluid to travel freely between the lobes while remaining in the pleural space.

The ribs and connective tissue of the chest wall are covered on the inner surface by a similar membrane called the parietal pleura. The parietal pleura can be thought of as a sac that covers not only the rib surface (costal pleura) but also the diaphragm (diaphragmatic pleura) and the mediastinum (mediastinal pleura).

The blood vessels and airways that enter the lung connect to the mediastinum at the lung hilum. At this juncture, the visceral pleura meets the mediastinal parietal pleura to form a single, continuous pleural membrane (Figure 25-1).

Because the lung usually is completely inflated, it might be thought that the pleural membranes always touch. However, freeze-fracturing has shown that there is a space between the visceral and parietal pleura that averages 10 to 20 µm in width and is filled with pleural fluid. This thin film of fluid allows the lung to slide over the ribs and allows for a gliding movement that takes little energy and produces little friction.

The average person has approximately 8 ml of pleural fluid per hemithorax.¹ The pleural fluid is estimated to have a total protein concentration similar to that of interstitial fluid elsewhere in the body: between 1.3 g/dl and 1.4.²

In humans, the pleural spaces surrounding each lung are completely independent, being separated by the mediastinum. This is not the case in all other mammals. The slaughter of the American buffalo could occur with a single spear or rifle shot because the pleural spaces of the buffalo lung are connected. Consequently, air in the pleural space collapses both lungs. An analogous situation can occur in any patient who has undergone median sternotomy, during which both pleural spaces were entered. Common operations resulting in this condition are lung volume reduction surgery and bilateral lung transplantation.

The pleural space is under negative pressure except during forced expiration. The intact thoracic rib cage provides elastic recoil pressure outward, whereas the intrinsic recoil pressure of the lung is inward toward the lung hilum. The diaphragm further decreases the intrapleural pressure below the atmospheric pressure to allow inspiration to occur. In an upright person, the pressure is more negative at the lung apex than at the lung base because of the weight of the lung and the effects of gravity. The net effect of the negatively pressurized pleural space is that fluid moves into the pleural space from adjacent sites when a communication is present. A patient with ascitic fluid and a diaphragmatic defect preferentially pulls fluid into the chest.

Pleural Effusions

Any abnormal amount of pleural fluid in the pleural space is called pleural effusion. The many causes of pleural effusion are categorized according to etiologic factor and the content of the fluid.³

Pleural fluid enters the pleural space across both the visceral and the parietal pleurae, particularly when the interstitial pressure within either the lung or the chest wall is increased. The main route for pleural fluid removal is small holes within the parietal pleura called stomata, which are large enough to allow a red blood cell to enter and be cleared from the pleural space. The parietal pleural stomata connect with intercostal lymphatic vessels under the ribs that drain posteriorly into the mediastinum. In the mediastinum, these lymphatic vessels enter lymph nodes before draining into the thoracic duct, a large lymphatic channel within the chest, which empties into the left subclavian vein. Abnormalities of increased pleural fluid production or blockade of drainage can cause pleural fluid to accumulate.

Transudative Effusions

Any pleural effusion that forms when the integrity of the pleural space is undamaged is called a transudative pleural effusion. A pleural fluid total protein concentration less than 50% of the serum total protein level and lactate dehydrogenase values in the pleural fluid less than 60% of the serum value indicate the presence of a transudative pleural effusion. In the absence of serum values, an absolute pleural fluid lactate dehydrogenase level less than two-thirds normal for serum suggests the presence of a transudate. These numbers were derived from large patient series in which pleural fluid and serum protein concentrations were measured while the cause of the effusion was being determined and corrected.⁴

The classification system listed in Box 25-1 is not perfect, and refinements continue to be proposed. For practical purposes, these numbers help narrow the possible causes of pleural fluid formation. Transudative pleural effusions form when hydrostatic and oncotic pressures are abnormal (Figure 25-2).⁵ The list of diseases that cause transudative pleural effusions is short. These diseases remain relatively easy to diagnose.

Box 25-1 Causes of Pleural Effusion

Transudative pleural effusion

Lymphatic obstruction

Peritoneal dialysis

Atelectasis

Central venous catheter in pleural space

Urinothorax

Trapped lung

Exudative pleural effusion

Gastrointestinal disease

Pancreatic disease

Intraabdominal abscess

Splenic infarction

Esophageal perforation

Abdominal surgery

Endoscopic variceal sclerotherapy

Collagen vascular disease

Rheumatoid pleurisy

Systemic lupus erythematosus

Drug-induced lupus

Immunoblastic lymphadenopathy

Sjögren syndrome

Familial Mediterranean fever

Churg-Strauss syndrome

Wegener granulomatosis

Drug-induced pleural disease

Procarbazine, bleomycin, mitomycin

Methotrexate

Practolol

Miscellaneous diseases and conditions

Benign asbestos pleural effusion

Post–cardiac injury syndrome

Fetal pleural effusion

Uremic pleural effusion

Parapneumonic Effusion

Pleural effusions form in pneumonia because inflammation in the lung increases interstitial lung water and pleural fluid production. Most effusions are small and resolve with resolution of bacterial pneumonia.⁹ Complicated parapneumonic pleural effusion develops when the pleural fluid has a high enough protein content to clot. The clotting causes fibrin strands to span the visceral and parietal pleurae. The net result is collection of pleural fluid into different loculi within the pleural cavity. These often cannot be drained by a single chest tube.

Progression to empyema is marked by the presence of bacteria within the pleural space, seen as pus or bacteria on Gram stain. Empyema necessitates drainage. Whether complicated parapneumonic effusions necessitate drainage is controversial, although most physicians perform drainage because some of these effusions can progress to empyema.¹⁰

Parapneumonic effusions are common causes of persistent fever among patients with pneumonia in the intensive care unit (ICU). Sampling by thoracentesis is commonly performed to exclude empyema. Pleural fluid drainage can improve ventilation if the fluid volume is large.

Viral Pleurisy

Viral lung infections can cause pleural inflammation (pleurisy), small pleural effusions, and pain. The effusions may be so small they may be overlooked on a routine chest radiograph; when they can be seen, the effusions often are too small to sample. Pleural pain, which is called pleurodynia and which can be the result of many other pleural processes, often is difficult to manage. A typical patient with pleurodynia has shallow respirations; deeper breaths are limited by pain. The subsequent atelectasis can cause oxygenation difficulty secondary to shunting.

Tuberculous Pleurisy

In many parts of the world, any lymphocyte-predominant exudative effusion is considered tuberculosis until proven otherwise. Tuberculous pleural effusions occur when a caseous granuloma in the lung ruptures through the visceral pleural surface causing an exudative inflammatory effusion. Experiments in which purified protein derivative (PPD) is placed into the pleural space of animals have shown that such effusions result from the body’s immune reaction to tuberculin proteins.

Although these patients require respiratory isolation, only 25% of them have sputum that subsequently grows Mycobacterium tuberculosis. The PPD skin test result is negative in 30% of patients when they come to medical attention but turns positive in 6 to 8 weeks in almost everyone.¹¹

Malignant Disease

Malignant disease is the most common cause of large unilateral pleural effusions among individuals older than 60 years. Common cancers that form malignant pleural effusions include lung cancer and breast cancer, although any cancer can metastasize to the pleural surface. The effusions usually are lymphocyte predominant; malignant cells are found during cytologic examination of the pleural fluid.

Some malignant pleural effusions, such as effusions from lymphoma, respond to therapy for the malignant disease. However, most patients with symptomatic malignant pleural effusions need primary therapy with pleurodesis. In pleurodesis, the visceral and parietal pleural membranes are fused by talc, other chemicals, or surgery to obliterate the pleural space.

Postoperative Causes

Various operations involving the chest or upper abdomen produce pleural fluid.¹² Effusions following cardiac surgery usually are predominant on the left side and tend to be bloody. These effusions are particularly prevalent after a cutdown of the internal mammary artery for coronary artery bypass.

Small transudative pleural effusions are common when there is any atelectasis in the lung. Upper abdominal operations cause inflammation of the diaphragm and effusion that has been termed sympathetic. Lung surgery in which the lung is unable to fill the thoracic cavity leaves a space under negative pressure, which fills with inflammatory pleural fluid. When the lung is unable to fill the space because of small postoperative size or visceral pleural fibrosis, the resulting pleural effusion can never be completely drained because of the “trapped lung.”