Recognizing a Pleural Effusion

Normal anatomy

• The parietal pleura lines the inside of the thoracic cage and the visceral pleura adheres to the surface of the lung parenchyma including its interface with the mediastinum and diaphragm.

• The enfolds of the visceral pleura form the interlobar fissures —the major ( oblique ) and minor ( horizontal ) on the right, only the major on the left. The space between the visceral and parietal pleura, i.e., the pleural space, is a potential space normally containing only about 2 to 5 mL of pleural fluid (see Fig. 2.1 ).

Case Quiz 7 Question

This 56-year-old male presented to the emergency department with shortness of breath after a motor vehicle collision. An upright chest x-ray was obtained. There is a serious injury that required immediate intervention suggested by a fundamental finding on this image. The answer is at the end of this chapter.

Normal physiology

• Normally, several hundred milliliters of pleural fluid are produced and reabsorbed each day. Fluid is produced primarily at the parietal pleura from the pulmonary capillary bed and is resorbed at both the visceral pleura and by lymphatic drainage through the parietal pleura.

Conventional radiography is frequently the first step in detecting a pleural effusion. Other modalities used include CT and ultrasound (US). CT and US are both sensitive in detecting small amounts of fluid. CT is best in evaluating the disease underlying the effusion or in complete opacification of the hemithorax by effusion. Ultrasound can be especially helpful in guiding an intervention to remove the pleural fluid. The fundamental appearances of pleural effusion are similar regardless of the modality.

Fluid accumulates in the pleural space when the rate at which the fluid forms exceeds the rate by which it is cleared.

• The rate of formation may be increased by:

  • Increasing hydrostatic pressure , as in left-heart failure.

  • Decreasing colloid osmotic pressure , as in hypoproteinemia.

  • Increasing capillary permeability , as can occur in toxic disruption of the capillary membrane in pneumonia or hypersensitivity reactions.

• The rate of resorption can decrease by:

  • Decreased absorption of fluid by lymphatics , either from lymphangitic carcinomatosis or from increased venous pressure, which decreases the rate of fluid transport via the thoracic duct.

  • Decreased pressure in the pleural space , as in atelectasis of the lung due to bronchial obstruction.

Pleural effusions can also form when there is transport of peritoneal fluid from the abdominal cavity through openings in the diaphragm or via lymphatics from a subdiaphragmatic process.

Pleural effusions are divided into transudates or exudates depending primarily on their protein content and their LDH (lactate dehydrogenase) concentrations. Transudates typically have lower protein concentrations, lower specific gravity, and lower LDH concentrations than exudates.

Transudates are usually bilateral and tend to form when there is increased capillary hydrostatic pressure or decreased osmotic pressure. Causes include:

• Congestive heart failure , primarily left-heart failure, is the most common cause of a transudative pleural effusion.

• Hypoalbuminemia

• Cirrhosis

• Nephrotic syndrome

Exudates are usually unilateral and tend to be the result of inflammation, connective tissue disease, or malignancy.

• The most common cause of an exudative pleural effusion is malignancy.

• An empyema is an exudate containing pus.

• In a hemothorax, the fluid has a hematocrit that is greater than 50% of the blood hematocrit.

• A chylothorax contains increased triglycerides or cholesterol.