pleural effusion
Meaning of pleural effusion: Fluid around the lung is a finding rather than a diagnosis, and analyzing a sample of it is what points toward the cause.
Definition of pleural effusion
A pleural effusion is an abnormal build-up of fluid in the pleural space, the thin gap between the membrane wrapping the lung and the membrane lining the chest wall. A small amount of fluid normally lubricates that gap. When disease drives far more into it, the lung cannot expand fully, which is felt as breathlessness or chest discomfort.
What is the pleural space?
Two thin membranes are involved. One covers the surface of the lung, the other lines the inside of the chest wall and the top of the diaphragm. Between them lies the pleural cavity, holding only a film of fluid, just enough to let the two surfaces slide over each other with every breath. Fluid is produced and reabsorbed continuously, and an effusion develops whenever production outpaces removal or drainage is blocked. That thin layer also holds the two membranes tightly together, which is part of what keeps the lung expanded, so a large volume of fluid between them lets the lung fall inward and shrink.
What separates a transudate from an exudate?
| Transudate | Exudate | |
|---|---|---|
| Mechanism | Pressure or protein imbalance in the blood vessels, membranes intact | The membranes themselves inflamed, injured or invaded, so they leak |
| Protein in the fluid | Low relative to blood | High relative to blood |
| Usual causes | Heart failure, liver cirrhosis, low blood protein | Pneumonia, other infection, cancer, blood clot in the lung, autoimmune disease |
| Where it directs attention | The heart, liver or kidneys | The pleura and the lung themselves |
The classification is settled by comparing protein and an enzyme called lactate dehydrogenase in the fluid against the same measures in blood drawn at the same time. A malignant pleural effusion, caused by cancer involving the pleura, falls on the exudate side, and tumor cells can sometimes be identified in the fluid under a microscope.
How is it found and sampled?
Fluid settles at the bottom of the chest under gravity and blunts the normally sharp angle where the diaphragm meets the ribs on a chest radiograph. Ultrasound detects smaller volumes and shows whether the fluid is one free-moving pool or divided into pockets by strands of fibrous tissue, which changes how easily it can be drained. Removing fluid through a needle, a procedure called thoracentesis, serves two purposes at once: it relieves the pressure limiting the lung and it supplies a sample for chemistry, cell counts, culture and examination for tumor cells. A pericardial effusion is the equivalent collection around the heart, a different space with a different set of consequences.
Also known as
Used in a sentence
A chest x-ray identified a pleural effusion that was subsequently sampled for laboratory analysis.
Good to know
Analysis of the fluid itself, comparing measures such as its protein content against that of the blood, helps classify a pleural effusion as transudative or exudative and points toward its underlying cause.
Word origin
Where “pleural effusion” comes from
Formed from pleural, from Greek pleura, meaning rib or side, and effusion, from Latin effundere, meaning to pour out.