Diagnosis of pleural effusions


Upright posterior-anterior chest radiography is the most commonly used diagnostic modality for pleural effusions. Accumulation of pleural fluid occurs first under the lung at the base of the hemithorax. Pleural effusions greater than 300 ml will usually obliterate the costophrenic angle. Larger effusions will cause opacification of the hemithorax. Lateral decubitus views confirm the presence of free pleural fluid and are more sensitive than upright films for the detection of effusions. Decubitus views are sensitive in detecting small effusions; as little as 5 ml of fluid may be detected.

Ultrasonography is superior to radiography for detecting pleural effusions. As little as 2 ml of pleural fluid can be detected using ultrasonography ( Grymjnski^efa/.., 1976). Ultrasound guidance is also useful and can be used to assist aspiration drainage of small or loculated effusions.

CT is also sensitive in detecting small pleural effusions. It is most useful for defining underlying parenchymal disease or pulmonary pathology. Distinguishing malignant from benign disease and evaluation of mediastinal adenopathy are better done using CT, while ultrasonography is superior for localizing free and loculated fluid.

Portable supine radiographs may be the only possible view in the critically ill patient unable to tolerate upright or decubitus views. Small effusions may be undetectable, or appear as a generalized haziness. Bilateral effusions may not be detected at all. Ultrasound and CT play an important role in these patients.


Although radiographic studies and a careful physical examination will usually confirm the presence of a pleural effusion, determining the etiology of the effusion often requires thoracentesis. Pleural fluid analysis categorizes effusions as either transudates or exudates. Transudates are effusions resulting from an imbalance in the Starling force. Systemic disease states, such as congestive heart failure, cirrhosis, and nephrotic syndrome, alter forces governing fluid exchange across pleural surfaces. Exudative effusions, in contrast, occur as the result of pleural membrane disease such as neoplastic and inflammatory processes. Abnormal protein clearance by pleural lymphatics and increased permeability of protein and fluid into the pleural space result in exudative effusions. Pleural fluid with a protein concentration of 3 g/dl and a specific gravity of 1.016 or more is usually considered to be an exudate. Additional tests of pleural fluid are of diagnostic value. Analysis of pleural fluid lactate dehydrogenase and protein content will distinguish exudative from transudative effusions ( Tabled). Lactate dehydrogenase levels above 200

IU, a pleural fluid to serum lactate dehydrogenase ratio above 0.6, and a pleural fluid to serum protein ratio above 0.5 distinguish exudative effusions ( Ljght eLa/

1972). Additional tests of pleural fluid are of diagnostic value for exudative effusions. Amylase, glucose, cell counts, and pH values do not help to distinguish between transudates and exudates.

Table 2 Distinguishing values exudative plural effusions

Low pleural fluid glucose (below 60 mg/dl) or a pleural fluid to serum glucose ratio below 0.5 is typical of infectious effusions, particularly empyema. Tuberculous effusions may have a low glucose value. Rheumatoid effusions also have a characteristic low glucose level. Systemic lupus erythematosus, esophageal rupture, and 15 per cent of malignant effusions have low glucose levels. Low glucose levels are the result of increased glucose metabolism by bacteria and leukocytes which exceeds the rate of glucose transport into the pleural space.

Pleural fluid amylase is elevated above normal serum levels in effusions associated with pancreatitis and pseudocysts. Increased pleural amylase occurs in esophageal rupture and malignancy.

The presence of bacteria in the pleural fluid usually lowers the pleural pH. Thirty per cent of exudative effusions have a pH below 7.3, while transudates typically remain above 7.4. Accumulation of lactic acid by anaerobic glycolysis contributes to the lower pleural pH. Tube thoracostomy drainage should be considered for those effusions which present with gross purulence of effusions with a pH below 7.2. Effusions with pH above 7.2 will often respond to antibiotic therapy alone ( Light.elal

1973). Pleural pH is useful for differentiating tuberculosis from malignant effusions: a pH above 7.4 suggests malignancy, while a pH below 7.3 suggests tuberculosis.

Pleural fluid white cell counts of more than 103/mm3 are found in pleural effusions from 85 per cent of patients with pulmonary infection, 73 per cent with tuberculosis, 42 per cent with neoplasm, 27 per cent with cirrhosis, and 10 per cent with congestive heart failure ( Paddock1..9.84). Predominance of neutrophils favors a diagnosis of bacterial infection, whereas predominance of lymphocytes favors tuberculosis or neoplasm. Red blood cells in the pleural fluid most frequently result from neoplasm. Other causes include congestive heart failure, infection, pulmonary infarction, trauma, and cirrhosis.

Gram stain and culture of pleural fluid are more reliable than sputum cultures for determining the etiology of the underlying pneumonia. Aerobic, anaerobic, and tuberculosis cultures should be prepared. However, tuberculosis cultures are positive in only 30 per cent of cases, even when pleural biopsy confirms the diagnosis of tuberculosis. Specific cultures for fungus and Entamoeba histolytica may also reveal the cause of effusions due to these organisms.


Hemothorax with or without associated pneumothorax is a frequent result of chest trauma and may cause hemorrhagic shock due to massive blood loss into the pleural space. The shock state may be exacerbated by mediastinal shift and decreased.

The most important aspect of managing hemothorax is adequate chest tube drainage of the pleural space. With adequate chest tube drainage and re-expansion of the lung, bleeding will be controlled in 95 per cent of cases (Griffith et a/. 1976). Only 5 per cent of chest trauma cases with hemothorax will require thoracotomy to control hemorrhage. If bleeding is more than 300 ml/h, surgery is indicated.

Chapter References

Griffith, G.L., et a/. (1978). Acute traumatic hemothorax. Anna/s of Thoracic Surgery, 26, 204-10.

Gryminski, J., Krabowka, P., and Lypaowicz, G. (1976). The diagnosis of pleural effusion by ultrasonic and radiologic techniques. Chest, 70, 33-9. Light, R.W., et a/. (1972). Pleural effusions: the diagnostic separation of transudates and exudates. Anna/s of Interna/ Medicine, 77, 507-12.

Light, R.W., et al. (1973). Diagnostic significance of pleural fluid pH and PCO2. Chest, 64, 591-8.

Paddock, F.K. (1984). The diagnostic significance of serous fluids in disease. New England Journal of Medicine, 233, 1010-17.

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    Does a pleural effusion decrease cardiac output?
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