Exercise: Pleural Effusion

EXERCISE 4-13. PLEURAL EFFUSION

4-21. Which of the following radiographic signs generally does not suggest the presence of pleural effusion?

A.         Meniscus-shaped opacity in a posterior cost-ophrenic angle on the lateral projection

B.         Biconvex lens-shaped opacity projecting in the midthorax on the lateral projection

C.         Fluid levels that have the same lengths on the PA and lateral views in a hemithorax

D.         Homogenous increased density in a hemithorax with preservation of the vascular shadows in the lungs

E.         Separation of the gastric air bubble from the infe-rior lung margin by more than 2 cm

Radiologic Findings

The frontal chest radiograph (Figure 4-62 A) shows opacity at the lower hemithorax bilaterally, which has a concave bor-der curving upward laterally adjacent to the chest wall. The overall lung volume is low in both the right and left lungs. There is separation of the gastric bubble from the inferior margin of the lung by several centimeters. On the lateral ex-amination (Figure 4-62 B), the opacities obscure the poste-rior heart margin and have a margin curving slightly upward to the posterior chest wall. Neither hemidiaphragm can be followed posteriorly to the chest wall. The findings are those of bilateral pleural effusions (C is the correct answer to Question 4-21).

Discussion

The visceral pleura is the outer lining of the lung, and the parietal pleura is the lining of the chest cavity. Normally, these surfaces are smooth and are separated by a minimal amount of pleural fluid. This provides a nearly friction-free environment for movement of the lung within the thorax. The pleural space, therefore, is a potential space that, in the normal individual, contains no more than 3 to 5 mL of pleu-ral fluid. Fluid may accumulate within the pleural space as a result of conditions that (1) increase pulmonary capillary pressure, (2) alter thoracic vascular or lymphatic pathways, alter  pleural  capillary  or  lymphatic  permeability, or affect diaphragmatic peritoneal and pleural surfaces. Pleural effusions are usually approached clinically according to whether the effusion develops because of alter-ations of the Starling equation, which controls fluid flow and maintenance in body compartments, or whether the pleura is affected primarily by a disease process. Processes resulting from alterations of the Starling equation include congestive heart failure, hypoproteinemia, fluid overload, liver failure, and nephrosis. These effusions are usuallytransudates (clear or pale yellow, odorless fluid without ele-vation of the ratios of pleural fluid to serum protein and lac-tate dehydrogenase [LDH]). Processes that alter pleural capillary or lymphatic permeability include infection, in-flammation, pulmonary embolism, and neoplasms. These effusions are usually exudates (clear, pale yellow or turbid, bloody, brownish fluid; pleural fluid protein: serum protein greater than 0.5; and pleural fluid LDH:serum LDH greater than 0.6). Enlarged lymph nodes or masses within the hila or mediastinum may obstruct lymphatic fluid flow and cause pleural exudates. Abdominal conditions that may produce pleural effusions include pancreatitis, subphrenic abscesses, liver abscesses, ovarian tumors, peritonitis, and ascites.

The most common radiographic sign of pleural effusion is pleural meniscus. The volume of fluid necessary to pro-duce a pleural meniscus within a costophrenic angle varies from individual to individual. Approximately 100 mL of pleural fluid will cause appreciable blunting of the posterior costophrenic angle on the lateral view (Figure 4-63 A), and 200 mL will cause blunting of the lateral costophrenic angle on the PA projection in an upright patient (Figure 4-63 B). A lateral decubitus chest radiograph, with the side containing the pleural effusion placed down (dependent), will demon-strate even smaller amounts of free-flowing pleural effusions (Figure 4-63 C). Each millimeter of thickness of pleural fluid in the lateral decubitus projection corresponds to approxi-mately 20 mL of pleural fluid. Large pleural effusions may usually be aspirated without guidance other than the chest radiograph. Small effusions are more difficult to aspirate and, if thoracentesis is planned, additional imaging guidance with ultrasonography or CT may be used. The effusion may simply be marked and aspirated by the clinical physician, or the effusion may be aspirated by a radiologist. If thoracente-sis is attempted and fails for a large pleural effusion, it may be loculated and further imaging guidance is usually helpful.

When pleural adhesions develop, fluid in the pleural space becomes loculated (Figure 4-64 A–C) and may be trapped in nondependent areas of the thorax. The appearance of pleuralfluid may change and, rather than taking a meniscus shape, may assume the shape of a convex margin away from the chest wall. If air is introduced in the pleural space by penetra-tion of the chest wall, or if fluid is trapped in the fissures, it will assume a biconvex lens shape (Figure 4-65). If a bron-chopleural fistula develops, the patient will have a hydrop-neumothorax that may be recognized by air-fluid levels of different lengths on the PA and lateral chest radiographs (Figure 4-66). When cavities develop in the lung, the fluid levels are usually of the same length (Figure 4-67).