The first and most important step in paracentesis is confirming there is ascites to begin with!
Physical examination findings can be misleading, and inserting a needle blindly into the abdomen can cause complications unnecessarily. Note the black free fluid in the image, with echogenic bowel loops floating within.
Several approaches are commonly used. Each one starts with an assessment for peritoneal fluid, localization of area suitable for paracentesis (no nearby vessels, large enough pocket, etc.). Next, measure the distance from skin to peritoneum to establish a sense of how far the needle lust penetrate before expecting to yield ascites. This is followed by either:
- Real time (dynamic) ultrasound guidance. Probe held in non-dominant hand (or by assistant); dominant hand guides needle with real-time guidance using short- or long-axis technique
- Static guidance: Mark the location for paracentesis (use a pen, pressure from a pen cap or fingernail, or even using a nearby mole/skin blemish as a landmark!), and then proceed with the tap using that mark as a guide. It is critical that the mapping is performed immediately before the paracentesis, and the patient remains in the same position. If the patient moves or is re-positioned, the patient must be scanned again because the pocket of fluid would have shifted due to the highly mobile floating bowel loops.
As the needle is inserted into the abdomen using either technique, it is wise to hold slight negative pressure on the plunger of the syringe. This way, as soon as fluid or blood is encountered, the operator will note both a pressure change and a flash of fluid into the syringe.
One of the most common pitfalls in gallbladder sonography is confusion with the structure which abuts it in the right upper quadrant – the duodenum. This loop of bowel can easily be mistaken for the gallbladder especially if it contains a mixture of fluid and solid materials. So how can we tell them apart?
