Definition and physiology
- Pleural effusion: an abnormal accumulation of fluid in the pleural space.
- Normally only 10–20 mL of fluid (about 0.1–0.2 mL/kg), similar to plasma but low in protein (< 1.5 g/dL), spread thinly over the visceral and parietal pleura to let the lung slide on the chest wall.
- Fluid is filtered from the systemic capillaries of the parietal pleura and drained by the parietal pleural lymphatics (stomata), which can increase their flow many times. An effusion forms when formation exceeds absorption.
Pathogenesis
- ↑ Hydrostatic pressure (heart failure).
- ↓ Plasma oncotic pressure (hypoalbuminaemia: nephrotic syndrome, cirrhosis, malnutrition).
- ↑ Capillary permeability (inflammation, infection, tumour, PE).
- ↓ Lymphatic drainage (tumour infiltration of the pleura or mediastinal nodes, radiotherapy, yellow nail syndrome).
- Fluid passing from the peritoneum through diaphragmatic defects (ascites in cirrhosis → hepatic hydrothorax; peritoneal dialysis); more negative pleural pressure (atelectasis, trapped lung).
- Direct entry of other fluids: blood (haemothorax), chyle (thoracic duct injury), iatrogenic (misplaced feeding tube or central line).
Image: “Diagram showing a build up of fluid in the lining of the lungs (pleural effusion) CRUK 054” by Cancer Research UK, CC BY-SA 4.0, via Wikimedia Commons.