Bedside Procedures Thoracentesis and Paracentesis
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Direct answer
Thoracentesis and paracentesis are bedside needle aspirations of pleural and peritoneal fluid, used for diagnosis and symptom relief. Thoracentesis is done in the triangle of safety, ideally under ultrasound guidance, and a chest X-ray afterwards excludes pneumothorax. Removing more than about 5 litres of ascites needs albumin replacement to prevent circulatory dysfunction.
What you must remember
- Triangle of safety: bounded by the lateral borders of pectoralis major and latissimus dorsi, with a horizontal line at about nipple level and the apex in the axilla.
- Needle placement: insert just above the upper border of the rib below, because the intercostal neurovascular bundle runs in the costal groove beneath each rib.
- Light's criteria: a pleural to serum protein ratio above 0.5, a pleural to serum LDH ratio above 0.6, or a pleural LDH above two-thirds of the upper limit of normal serum LDH define an exudate.
- Tuberculous effusion: a lymphocyte-predominant exudate with raised adenosine deaminase, commonly above about 40 U/L, though pleural biopsy may still be needed.
- Re-expansion pulmonary oedema: a rare complication after rapid drainage of a large effusion, which can occur even without a pneumothorax.
- Ascites puncture site: a lower quadrant site lateral to the rectus sheath, avoiding the inferior epigastric vessels, with the bladder emptied first.
- Albumin replacement: 6 to 8 g per litre removed when more than 5 litres is taken, reducing post-paracentesis circulatory dysfunction.
- SAAG and SBP: a serum-ascites albumin gradient of 1.1 g/dL or more indicates portal hypertension, and a polymorphonuclear count of 250 per cubic millimetre or more indicates spontaneous bacterial peritonitis.
A safe large-volume paracentesis, step by step
A 50-year-old man with alcohol-related cirrhosis has tense ascites causing breathlessness, and the ward team wants about 6 litres removed. The steps below set out how to drain safely and avoid the commonest complication.
- Confirm and prepare: Ask the patient to void and use ultrasound to mark a pocket of fluid away from scars and bowel loops. Routine correction of clotting is not needed for a standard paracentesis.
- Send the diagnostic sample first: Fluid goes for cell count with differential, serum-paired albumin, protein, Gram stain and culture, with cytology if malignancy is suspected. Bedside inoculation into blood culture bottles improves the yield for culture.
- Drain steadily: Insert the catheter after local anaesthesia and drain slowly rather than all at once, since rapid removal lowers intravascular volume and can precipitate hypotension.
- Replace albumin: For the planned 6 litres, give 6 to 8 g of human albumin per litre removed to prevent post-paracentesis circulatory dysfunction, which can cause hyponatraemia and renal impairment.
- Monitor afterwards: Check blood pressure, pulse, sodium and creatinine, and watch for leakage or infection. Fever or abdominal pain after the tap raises the question of spontaneous bacterial peritonitis.
Where the examiner tests bedside judgement
Procedure questions reward the safe site and the right replacement, not only the technique.
- Trap: choosing any upper abdominal site for an ascites tap Correct: a lower quadrant site lateral to the rectus sheath avoids the inferior epigastric vessels, and the bladder is emptied first.
- Trap: placing a thoracentesis needle at the lower border of a rib Correct: the neurovascular bundle lies beneath each rib, so the needle goes just above the upper border.
- Trap: skipping a chest X-ray because the procedure seemed uneventful Correct: pneumothorax is a common complication, and a post-procedure film is standard practice.
Frequently asked questions
Where should a thoracentesis needle be inserted?
Thoracentesis is performed in the triangle of safety, with the needle placed just above the upper border of the rib below. Ultrasound guidance is preferred because it reduces the risk of pneumothorax and injury to adjacent organs.
Which pleural fluid criteria separate exudates from transudates?
Light's criteria separate exudates from transudates: a pleural to serum protein ratio above 0.5, a pleural to serum LDH ratio above 0.6, or a pleural LDH above two-thirds of the upper limit of normal serum LDH. Meeting any one criterion classifies the pleural effusion as an exudate.
Why is albumin given after a large-volume paracentesis?
Albumin is given after large-volume paracentesis to prevent post-paracentesis circulatory dysfunction, which can cause hypotension, hyponatraemia and renal impairment. The usual regimen is 6 to 8 grams of albumin per litre removed when more than 5 litres is drained.
What does a serum-ascites albumin gradient of 1.1 g/dL indicate?
A serum-ascites albumin gradient of 1.1 g/dL or more indicates that the ascites is due to portal hypertension, as in cirrhosis, cardiac failure or Budd-Chiari syndrome. A lower gradient points to peritoneal causes such as peritoneal carcinomatosis or tuberculous peritonitis.
What ascitic fluid count defines spontaneous bacterial peritonitis?
A polymorphonuclear cell count of 250 per cubic millimetre or more defines spontaneous bacterial peritonitis, even when the patient has few symptoms. Ascitic fluid for culture should be inoculated into blood culture bottles at the bedside to improve the yield.