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PH-RS06

Pleural effusion

How a small amount of pleural fluid is constantly made and drained, why heart failure, low protein, infection and cancer make it accumulate, and how a collection of fluid compresses the lung and changes what you hear and see.

By the end, you can

  • Describe how pleural fluid is formed and drained.
  • Distinguish transudates from exudates by mechanism and give common causes of each.
  • Explain how fluid collects at the lung bases and compresses the lung.
  • Relate breathlessness, stony dullness and reduced breath sounds to the effusion.
  • Explain how an effusion becomes infected (empyema) and why that matters.
  • Recognise the fluid appearances, Light’s criteria and tests of a pleural effusion, and explain how diuretics, drainage and pleurodesis act on its mechanism.

What it is

A pleural effusion is an abnormal collection of fluid in the pleural space. It is a sign of an underlying condition rather than a disease in itself. Transudates form when pressures across normal capillaries are disturbed; exudates form when inflammation, infection or cancer makes the pleura leaky or blocks its drainage.

3 types to tell apart
Transudate
Low in protein: from raised capillary pressure (heart failure) or low blood proteins (cirrhosis, nephrotic syndrome).
Exudate
High in protein: from pneumonia, cancer, tuberculosis, pulmonary embolism or inflammatory disease.
Empyema, haemothorax and chylothorax
Pus, blood or lymph in the pleural space; each has a specific cause and needs drainage.

US spelling: millilitres → milliliters.

Normal function

Pleural fluid is made and drained continuously

The pleural space normally holds only a few millilitres of fluid, a thin film that lets the visceral and parietal slide. Yet fluid passes through it constantly.

Fluid filters out of capillaries in the parietal pleura (supplied by systemic arteries, at relatively high pressure), drawn by the balance of hydrostatic and oncotic pressures. It is removed mainly through lymphatic openings in the parietal pleura over the diaphragm and lower chest wall, which can increase their drainage many times over before fluid builds up.

The lowest points of the pleural space, the , lie where the chest wall meets the , below the .

Point to it on the model
Pleural fluid turnoverSchematic
NormalParietal pleural capillariesSystemic pressure
  • Fluid filters out (from Parietal pleural capillaries to Pleural space)
Pleural spaceA few millilitres
  • Fluid drained (from Pleural space to Parietal lymphatics)
NormalParietal lymphaticsCan drain many times more

Schematic. An effusion forms when filtration exceeds drainage.