Pleural Diseases



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.





Congestive Heart Failure


Elevation of pressure in the left atrium and pulmonary veins is the hallmark of congestive heart failure (CHF). Elevation of pulmonary venous pressure increases the amount of interstitial fluid in the lung. In severe cases, flooding of the alveoli causes pulmonary edema, but in less severe cases, interstitial lung water increases and decompresses into the pleural space. Because systemic venous pressure also is elevated, there is limited capability to remove pleural fluid through the intercostal veins. Pleural fluid must be predominantly removed by the lymphatic vessels. Pleural effusions result when the capacity of pleural lymphatic drainage is overcome.6


CHF is the most common cause of clinical pleural effusions. The effusions can be massive, filling the entire hemithorax and compressing the lung. More commonly, they are small and bilateral. The effusions are rarely drained because outcome is heavily influenced by successful management of the underlying CHF, which also clears the effusions.7



Nephrotic Syndrome


In nephrotic syndrome (also known as nephrosis), the kidneys leak more than 3 g of protein per day into the urine. Because patients become protein depleted, there is insufficient oncotic pressure within the blood to hold appropriate amounts of fluid within the blood vessels. These patients become edematous, and fluid leaks into the lung interstitium and pleural space. Pleural effusions are common but usually are small.


Patients with nephrosis are at increased risk of deep venous thrombosis and pulmonary emboli. In nephrosis, protein S, which keeps blood from clotting, becomes deficient from leaking into the urine. The presence of large or asymmetric pleural effusions should raise the possibility of the presence of pulmonary emboli. Pleural effusions associated with pulmonary emboli usually are exudates and contain large numbers of red blood cells.








Exudative Effusions


An exudative pleural effusion is caused by inflammation in the lung or pleura. This type of pleural effusion has more protein and inflammatory cells present than a transudative effusion. Because therapy for pleural effusion depends on the cause, thoracentesis often is performed to determine the specific biochemical and cellular characteristics of the pleural effusion. Box 25-1 lists the common causes of exudative pleural effusion. These account for approximately 70% of all pleural effusions.



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.






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.”



Chylothorax


The thoracic duct is a lymphatic channel that runs from the abdomen through the mediastinum to enter the left subclavian vein. Disruption of the thoracic duct anywhere along its course can cause leakage of chyle into the mediastinum, which may rupture into the pleural space and cause a chylothorax. The most common causes of rupture are malignancy (50%), surgery (20%), and trauma (5%).13 The thoracic duct courses through the right side of the mediastinum in the lower thoracic cavity before crossing to the left side of the mediastinum at T4 to T6. Rupture below this level causes right-sided pleural effusion, whereas rupture above this level causes left-sided pleural effusion.


In a patient who has eaten recently, the effusions are milky white as a result of the presence of chylomicrons (microscopic fat particles) absorbed by abdominal lymphatic vessels. In a fasting patient, these effusions usually are yellow. The effusions may be bloody. A pleural fluid triglyceride concentration greater than 110 mg/dl confirms the diagnosis.14 Computed tomography (CT) should be performed to evaluate the cause of the chylothorax.



Jun 12, 2016 | Posted by in RESPIRATORY | Comments Off on Pleural Diseases
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