5.2 : Pleural Effusion I: Introduction

Pleural effusion is an abnormal fluid accumulation in the pleural cavity, a narrow space between the lungs and the chest wall. It is not a disease per se but rather a symptom or indication of an underlying disease. In normal circumstances, this space contains a small amount of fluid (5 to 15 mL), a lubricant facilitating the non-frictional movement of the pleural surfaces.

There are two main types of pleural effusion: transudative and exudative. They are differentiated using Light's criteria, which consider the levels of proteins, lactate dehydrogenase (LDH), and the ratio of pleural fluid to serum levels of these substances. An effusion is considered exudative if it meets one or more of the following criteria:

• A pleural fluid protein/serum protein ratio exceeding 0.5
• A pleural fluid LDH/serum LDH ratio surpassing 0.6
• A pleural fluid LDH level surpasses two-thirds of the higher limit of normal for serum LDH.

If the effusion does not meet any of these criteria, it is considered transudative.

While the type of effusion is defined by its fluid composition, their differing effects and their respective mechanisms of fluid accumulation provide a complete understanding of the underlying conditions.

Transudative pleural effusions are critical, typically linked to systemic factors affecting the blood vessel's hydrostatic or oncotic pressure. Common causes include:

• Congestive Heart Failure (CHF): A common cause of transudative pleural effusion is heart failure. In CHF, the heart cannot pump blood effectively, increasing venous pressure. The escalated pressure may lead to fluid leakage into the pleural space.
• Cirrhosis of the Liver: Liver cirrhosis can lead to a decrease in oncotic pressure due to the liver's decline in the production of proteins, especially albumin. Albumin is essential in maintaining oncotic pressure, and its reduction may lead to fluid accumulation in the pleural space
• Nephrotic Syndrome: This is a kidney disorder characterized by proteinuria, a condition where there is an abnormal amount of protein in the urine. This loss of proteins, including albumin, can contribute to decreased oncotic pressure and transudative pleural effusion.
• Hypoalbuminemia: reduced levels of albumin in the blood can reduce oncotic pressure, promoting fluid movement into the pleural space.

Next, exudative pleural effusions are characterized by fluid accumulation with a higher protein content in the pleural space. They often result from inflammation and increased permeability of the pleural membranes. This process crucially maintains the balance of fluid in the pleural space between the pleural membranes, which consist of a visceral and a parietal layer. When these membranes become inflamed or their permeability increases, it can lead to the buildup of exudative fluid.

Common causes of exudative pleural effusions include:

• Pneumonia: Bacterial, viral, or fungal lung infections can lead to an inflammatory response in the pleura, resulting in the accumulation of exudative fluid.
• Malignancy: Cancer involving the pleura (primary pleural tumors or metastasis from other sites) can cause exudative pleural effusions. Lung, breast, and mesothelioma are malignancies associated with pleural effusions.
• Pulmonary Embolism: Blood clots that migrate to the lungs (pulmonary embolism) can cause irritation and inflammation of the pleura, leading to an exudative effusion.
• Connective Tissue Disorders: Conditions such as rheumatoid arthritis and systemic lupus erythematosus can cause inflammation in the pleura, resulting in exudative effusions.
• Pulmonary Infarction: Infarction of lung tissue, often related to pulmonary embolism or vasculitis, can lead to exudative pleural effusions.