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Pleural effusion

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The pleural cavity is a potential space within the thoracic cavity lined by visceral and parietal pleural membranes

Normally, the pleural apace contains a small physiologic amount of pleural fluid (0.1 mL per kg). An imbalance between normal pleural fluid formation and/or absorption results in accumulation of fluid within the pleural cavity.

The fluid may be either transudative or exudative:

  • a transudate results from abnormal accumulation of pleural fluid due to high capillary and interstitial hydrostatic pressures (e.g. - in heart failure) or abnormally decreased capillary oncotic pressure (e.g. - in nephrotic syndrome).
  • an exudate results from an inflammatory and malignant processes which alter the permeability of the local capillary and pleural membrane permeability or causes lymphatic blockage (1)

It is thought to be a common condition with an estimated 1-1.5 million new cases in the United States and 200 000-250 000 in the United Kingdom each year (3).

A pleural effusion will only be detected:

  • on a chest radiograph when the volume of the effusion exceeds 300 ml
  • clinically when the volume exceeds 500 ml


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