A 64-year-old woman comes to the emergency department with a 4-day history of productive cough and breathlessness. Her medical conditions include polymyalgia rheumatica, for which she takes low-dose prednisone. Physical examination shows dullness to percussion and decreased breath sounds over the right lower lobe. The abdomen is soft, nondistended, and nontender. Chest x-ray shows a right lower lobe parenchymal opacity and a moderate right pleural effusion. Thoracentesis is performed, yielding serosanguinous fluid. Pleural fluid and serum findings are as follows:
| Pleural fluid | Serum | |
| Total protein | 4.5 g/dL | 6.5 g/dL |
| Lactate dehydrogenase | 40 U/L | 60 U/L |
Which of the following is contributing most to the pathogenesis of this patient's pleural effusion?
Exudative & transudative pleural effusions | ||
Exudate | Transudate | |
Light criteria | Pleural protein/serum protein >0.5 OR | Exudate criteria not met |
Pathophysiology | Inflammatory increase | Change in hydrostatic or oncotic pressure |
Common causes |
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LDH = lactate dehydrogenase; TB = tuberculosis. | ||
Under normal physiologic conditions, pleural fluid enters the pleural space from parietal pleural microvessels and is removed by lymphatics at a constant rate. Pathologic states that disrupt pleural capillary hydrostatic or oncotic pressure, decrease pleural space pressure, reduce lymphatic drainage, or increase vascular membrane permeability can lead to pleural effusion. Pleural effusions that develop due to pressure changes (eg, heart failure, cirrhosis, nephrotic syndrome) are typically transudative, whereas those that develop due to inflammation and consequent increased vascular membrane permeability (eg, infection, malignancy, rheumatologic disease), are typically exudative.
The Light criteria are used to differentiate transudative and exudative pleural effusions and aid in the differential diagnosis. Transudative effusions have a low fluid-to-serum ratio of total protein and lactate dehydrogenase and low absolute levels of lactate dehydrogenase. In contrast, exudative effusions have a high fluid-to-serum ratio of total protein (>0.5) or lactate dehydrogenase (>0.6) or high absolute levels of lactate dehydrogenase (>2/3 the serum upper limit of normal).
This patient with a pleural fluid-to-serum total protein ratio >0.5 has an exudative effusion likely due to pneumonia (ie, parapneumonic effusion).
(Choice A) Decreased plasma oncotic pressure occurs in conditions that cause hypoalbuminemia (eg, cirrhosis, nephrotic syndrome) and can lead to transudative pleural effusion.
(Choice B) Decreased thoracic lymphatic flow (eg, due to malignant involvement of lymphatic ducts) can cause a chylous effusion (chylothorax). Although this type of effusion is usually exudative by Light criteria, the fluid is typically milky white (rather than serosanguinous) in gross appearance due to high triglyceride (ie, chylomicron) content.
(Choice C) Increased intraabdominal hydrostatic pressure occurs with abdominal ascites that develops due to portal hypertension (eg, cirrhosis). The ascites can be forced into the right-sided pleural cavity through fenestrations in the diaphragm, resulting in a transudative pleural effusion known as hepatic hydrothorax.
(Choice D) Increased intrapleural negative pressure occurs in large-volume atelectasis (lung collapse) and can lead to a transudative pleural effusion.
(Choice F) Increased hydrostatic pressure in the pulmonary venous system occurs in heart failure and leads to transudative pleural effusion.
Educational objective:
Transudative effusions are typically caused by alterations in hydrostatic or oncotic pressure (eg, heart failure, cirrhosis, nephrotic syndrome), whereas exudative effusions typically result from inflammation and consequent increased vascular membrane permeability (eg, infection, malignancy, rheumatologic disease). Exudative effusions are characterized by a high ratio of pleural fluid to serum total protein (>0.5) or lactate dehydrogenase (>0.6), or high absolute levels of lactate dehydrogenase.