Transudative pleural effusion is most commonly due to which mechanism?

• A. Increased hydrostatic pressure
• B. Decreased capillary permeability due to inflammation
• C. Infection of pleural space
• D. Malignant invasion

Answer: (A)

Transudative pleural effusions come from systemic factors that shift fluid balance, not from local pleural disease. The main mechanism is increased hydrostatic pressure in the capillaries, which drives more fluid out of vessels into the pleural space and can overwhelm the pleural lymphatics’ ability to clear it. This is classically seen in congestive heart failure, where elevated venous pressures raise filtration into the pleural space. Because transudates are low in protein and LDH, they reflect simple filtration rather than inflammation or directly abnormal pleural tissue.

In contrast, decreased capillary permeability due to inflammation, infection of the pleural space, or malignant invasion all promote exudative effusions. These exudates have higher protein and LDH and result from local pleural inflammation, infection, or tumor-related changes in vascular permeability or lymphatic drainage.