What is a Pleural Effusion?
Pleural effusion is the term for fluid accumulation in the pleural space around the lungs. It is commonly referred to as fluid around the lungs or water surrounding the lungs. It must be differentiated from pulmonary edema which is commonly known as water or fluid in the lungs. The pleural cavity is a potential space between the parietal pleura that lines the inside of the chest wall and the visceral pleura that lines the outer part of the lung. There is a small amount of fluid in this space (pleural fluid) which serves as a lubricant. It ensures that the two pleural layers do not rub against each other when the lung expands and contracts during breathing.
Normally there is about 15 ml of serous fluid in the pleural space. This pleural fluid is clear and similar to interstitial fluid. The pleura do not have secretory cells that produce and secrete mucus as is seen in other cavities. Instead the pleura are serous membranes which allow small amounts of interstitial fluid (tissue fluid) transude continuously into the pleural space. The presence of small tissue proteins gives the fluid a slight mucoid texture. The pleural fluid is constantly taken up by the lympathic vessels to prevent any buildup in the pleural space. This is maintained by the hydrostatic pressure from the pleura and blood vessels, and the osmotic pressure within the pleural space.
Pathophysiology of Pleural Effusion
Accumulation of fluid around the lungs is similar to edema in any part of the body and a pleural effusion is essentially edema of the pleural cavity. This is explained further under pathophysiology of edema.
As with edema elsewhere in the body, an effusion may occur through one or more of the following mechanisms.
- Increased hydrostatic pressure in the vessels which causes excess fluid to be pushed out into the pleural space.
- Decreased osmotic pressure in the vessels which allows fluid to pass into the pleural space.
- Increased vascular permeability when there is blood vessel damage thereby allowing fluid to leak into the pleural space.
- Raised negative pressure within the pleural space which draws fluid into the pleural space.
- Reduced lymphatic drainage of pleural fluid often due to a blockage of the lymphatic vessel or surrounding lymph nodes.
The pleural effusion may be of two types :
- Transudate when the fluid leaks or is pushed into the pleural space as is seen with increased hydrostatic pressure or decreased osmotic pressure (transudative effusion).
- Exudate when the vascular permeability is increased due to damage or the lymphatic drainage of fluid is impaired (exudative effusion).
The types and causes of pleural effusions are discussed in detail under fluid around the lungs.
Symptoms of Pleural Effusion
A build up of fluid in the pleural space can restrict lung expansion. This may manifest as shortness of breath and the severity may vary depending on the extent of the effusion. Rapid shallow breathing may be clearly detectable.
Other signs and symptoms may be associated with irritation of the pleura and not just a result of the effusion. This includes :
- Pleuritic pain – chest pain which is worse on breathing deeply and coughing
- Pleural frictional rub
- Dry cough
Other symptoms, like a fever, may be seen in infectious causes and unintentional weight loss should raise the concern about conditions like pleural tumors.
Article reviewed by Dr. Greg. Last updated on April 12, 2011