In residency, I spent a great deal of time at the bedside assessing for shifting dullness in patients who may have had ascites. Today, when I ask residents about shifting dullness, I often get blank stares. “Why would I do that when I can use the ultrasound”, they wonder. Ultrasound guidance for bedside procedures has increased dramatically over the last decade. Indeed, for some procedures such as central venous catheter placement, ultrasound guidance has become the standard of care. Its role in paracentesis, however, is less well-defined.
Blind paracentesis is most often performed in the left lower quadrant at a location that mirrors McBurney’s point after a clinical assessment for ascites. A study of bedside clinical acumen, however, reveals that the physical exam has a 58% chance of recognizing ascites.1 Also, blind paracentesis is often unsuccessful for small volumes of fluid. The success rate when 500 ml of fluid is present is 78%. This rate drops to 44% when 300 ml of fluid are present.2 Another finding complicates the picture further. Several studies have shown that ascitic fluid is consistently found in smallest quantities in the left lower quadrant. Fluid first pools in the perihepatic region or occasionally the perivesicular region but the left pericolic gutter is consistently the last area to fill.3,4 We can compensate for this by positioning patients so that gravity works with us, and we frequently choose the left because the risk to the spleen, which sits higher in the abdomen is thought to be less than risk to an enlarged liver or potentially distended bladder. Also, the sigmoid colon is much more mobile than the cecum, theoretically lowering the risk of bowel perforation. But without ultrasound we lack the ability to diagnose ascites consistently, when we do make an accurate diagnosis we frequently end up with a dry tap and we usually choose the location least likely to contain fluid.
There are no randomized trials to assess the utility of bedside ultrasound for paracentesis. However, a retrospective review of 1297 paracenteses, in which 56% were performed with ultrasound guidance, showed fewer adverse events (1.4 vs 4.7 p=0.01 ), post-paracentesis infections (0.41 vs 2.44 p=0.01) and hematomas (0 vs 0.87 p=0.01) when ultrasound was used.5
To be clear, I’m not advocating for real time ultrasound guidance for our Class C cirrhotics who come to the ED every two weeks to have 8 liters of ascites removed. In fact, I cringe to see residents “checking for fluid” on patients such as these. But in cases of new or subtle ascites, bedside ultrasound is an invaluable tool to confirm the diagnosis, identify small pockets of ascites and guide us safely to the fluid.
1. Cattau EL, Benjamin SB, Knuff TE et al: The accuracy of the physical examination in the diagnosis of suspected ascites. JAMA 1982; 247: 1164-1166
2. Giacobene JW, Siler VE: Evaluation of diagnostic abdomi- nal paracentesis with experimental and clinical studies. Surg Gynecol Obstet 1960; 110: 676-686
3. Yeh HC, Wolf BS: Ultrasonography in ascites. Radiology 1977; 124: 783-790
4. 99. Proto AV, Lane EJ, Marangola JP: A new concept of ascitic fluid distribution. AJR 1976; 126: 974-980
5. Patel PA et al: Evaluation of hospital complication and cost associated with using ultrasound guidance during abdominal paracentesis procedure. J Medical Economics, 2011; October 19.