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Pleural effusion occurs in more than 1.5 million patients each year in the United States.1 The prevalence of ultrasound-detected pleural effusion in critically ill patients has been reported to be as high as 62%;2 mortality is increased for those with an effusion.3 Among the causes of pleural effusions, heart failure, pleural infection, and malignancy are most common.4 Nonmalignant pleural effusions (NMPEs) arise from a vast assortment of conditions (Fig. 76-1); some etiologies have been associated with worse survival than effusions due to malignancy.5,6

Figure 76-1

A non-exhaustive list of causes of pleural effusions and their characterization by Light’s criteria.


The pleural space is defined as the area between the visceral pleura, which lines the lung, including the fissures, and the parietal pleura, which lines the chest wall, diaphragm, and mediastinum. The visceral and parietal pleural membranes consist of a single layer of mesothelial cells, a layer of connective tissue, a superficial elastic layer, a loose connective tissue layer, and a deep fibroelastic layer.7

The normal pleural cavity is estimated to contain 0.26 mL of fluid per kilogram of body weight, with most of the fluid both produced and absorbed on the parietal surface.8,9 Fluid production and resorption are dependent on the balance of hydrostatic and oncotic pressure differences between the systemic and pulmonary circulations and the pleural space. The lymphatic vessels in the parietal pleura are capable of increasing resorption by a factor of 20 in response to an increase in pleural liquid formation before being overloaded. Therefore, both a significant increase in fluid formation and a reduction in fluid resorption are required to produce a clinically significant pleural effusion.8

Pleural effusions typically form by one or more of the following mechanisms: (1) injury to the pleura that changes membrane permeability and leads to exudative effusions, (2) increased intravascular hydrostatic forces and/or decreased oncotic forces that lead to transudative effusions, and (3) extravasation of fluid from lymphatic or vascular structures or from translocation of fluid across the diaphragm.


There is a paucity of large-scale epidemiologic data describing the incidence or etiology of NMPEs. When speaking broadly about pleural effusions, the two most common causes are generally accepted to be related to congestive heart failure and parapneumonic effusion.1 The third most common cause is believed to be a malignant pleural effusion, which is discussed elsewhere in this book. Rounding out the top five causes of pleural effusion are pulmonary embolism and viral illness.10

Table 76-1 depicts the most common causes of pleural effusions in the United States and their estimated annual incidence. Other causes of NMPE include chronic liver disease, renal disease, asbestos ...

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