Ascites and Shifting Dullness

Expansion

Free intraperitoneal fluid obeys gravity, while gas-filled bowel floats on top of it. Supine, the fluid therefore collects in the flanks and pelvis and the bowel rises to the midline.

• Dull in the flanks, resonant centrally
• Rolling the patient towards you shifts the fluid, and the previously dull flank becomes resonant after 30 seconds or so, shifting dullness
• A fluid thrill requires larger volumes and a hand placed on the abdomen to damp transmission through the wall

For paracentesis, the standard site is the left lower quadrant, roughly 15 cm lateral and superior to the pubic symphysis, or lateral to the rectus sheath at the junction of the outer and middle thirds of a line from umbilicus to anterior superior iliac spine. The reasons are anatomical: it avoids the inferior epigastric vessels (which run behind rectus), the caecum on the right, the bladder centrally, and it enters where the abdominal wall is thin.