Introduction
This is the accumulation of fluid within the pleural space.
It is important to note that empyema (collection of pus within the pleural cavity) and haemothorax (collection of blood within the pleural cavity) are different conditions.
Pleural Fluid Samples
|
Sample
|
Cause
|
|
Straw coloured / clear
Yellow / white, foul smelling
Blood
|
Transudate / exudate
Empyema / parapneumonic effusion
Trauma, malignancy, pulmonary infarct
|
Clinical features
- Reduced chest expansion on the affected side
- Mediastinal displacement away from the affected side
- Stony dull to percussion
- Reduced or absent breath sounds
- Reduced or absent vocal resonance
- There will be no additional unusual sounds
- These will generally only be present when the effusion is greater than 500ml!
X-ray
- An effusion of less than 500ml is unlikely to cause anything other than blunting of the costophrenic recess. >500ml will cause a clear fluid level
- An effusion of less than 300ml may not be seen on x-ray
- Remember to look for the meniscus – it is likely to be a very long curve, perhaps rising all the way to the axilla.
- If the fluid level appear perfectly horizontal, it is likely to due co-existing pneumothorax
- If the effusion is large enough, the whole of one lung field may appear opaque, and the mediastinum may be shifted to the opposite side
- Fluid below the lung can simulate a raised hemidiaphragm.
Ultrasound is useful for guiding the aspiration, as well as for diagnosis
Further diagnostics
- Diagnostic aspiration
- Percuss the upper border of the effusion, then go 1-2 intercostal spaces below (any lower and you might end up in the abdomen!)
- Use 5-10ml of lideocaine and inject down to the pleura
- Insert a 21G needle with syringe just above the ribs upper boarder (to avoid the neurovascular bundle)
- Take 10-30ml of fluid
- Send sample for:
- Clinical biochem – glucose, protein, pH, amylase, LDH
- Bacteria culture
- Cytology
- Immunology (if indicated by clinical signs)
- If diagnosis is not possible from fluid sampling, it may be possible with a pleural biopsy, which may be CT guided for the best results.
- You should also take bloods for protein (to check for hypoalbuminaemia), glucose (to compare to the pleural fluid sample - see below)and LFTs, U+E's (to check for renal failure)
Diagnosis - Transudates and Exudates
Exudates
- are fluids that have left the circulatory system and gone into lesions or areas of inflammation.
- The composition of an exudate varies, but it can include pretty much anything that is in blood.
- It will nearly always have water and dissolved solutes, and may also have white and red blood cells, as well as platelets. Exudates have a high protiein content
- Definition - Protein content >35g/L (or more correctly, see Light's criteria below)
- are fluids that have left the circulatory system and gone into lesions or areas of inflammation.
Transudates
- are caused by disturbances is oncotic pressure (i.e. increases in venous pressure), and not by inflammation.
- They have a lower protein content, and will contain fewer cells.
- The fluid typically only contains mononuclear cells (such as macrophages and lymphocytes).
- Definition - Protein content <25g/L (or more correctly, see Light's criteria below)
Essentially, a transudate is caused by a high pressure forcing plasma and some blood products out of the blood across a membrane, whilst an exudates is leaking of fluid from one space to another.
Transudates are pretty much always passive, unwanted losses of fluid, whilst exudates can sometimes be deliberate secretions.
In transudates, proteins have moved down their concentration gradient. In exudates, proteins have been moved against their concentration gradient. On inspection, transudates also often appear more ‘clear’ than exudates
Note that it is difficult to tell a transudate from an exudate if the protien is 25-35gd/L – and so in clinical practice we use Light's Criteria to differentiate transudates from exudates.
According to Light's criteria, the fluid is an exudate if:
- [Plerual Protein : serumprotein] ratio >0.5
- [Pleural LDH : serum LDH] ratio >0.6
- Pleural LDH > 200
Other Tests
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