In preparation for scanning, the patient should be in a comfortable supine, semi-reclined position. This position will allow the ascites to accumulate in the lower abdomen. When deciding where to perform the paracentesis, the right or left lower quadrants are generally favored over the more traditional infraumbilical approach. One study compared the left lower quadrant to the infraumbilical area and found the depth of the fluid was generally greater in the lateral abdomen (6). In any given patient, however, the most accumulated fluid may be in any of these three areas, and ultrasound provides the advantage of locating the largest and most accessible fluid.

Scanning for a paracentesis site needs to be systematic, beginning in one location and progressing through the others (Fig. 13.4). With the lateral quadrants, place the low frequency transducer just above the anterior superior iliac crest, and work in a cephalad direction along the pericolic gutter. As the transducer is moved up the abdomen, fan the probe in a medial direction toward the midline and in
a lateral direction toward the edge of the peritoneum. The transducer may need to be shifted in a medial or lateral direction, depending on the size of the patient’s abdomen. When a potential site for paracentesis is identified, rotate the probe 90 degrees to ensure that the fluid is present in two planes. When scanning the infraumbilical area, place the probe a couple of centimeters below the umbilicus in the midline, and move in a caudal direction. Again, check each area of fluid in two perpendicular planes. The minimal depth of fluid needed to safely attempt paracentesis is generally considered about 3 cm (Fig. 13.5). For those experienced with the use of ultrasound guidance, a depth of 2 cm may be acceptable. The decision to proceed with paracentesis is determined by the clinical situation.

After an insertion site is chosen and the skin is prepared for asepsis, the clinician can choose between a static and a dynamic technique. If the fluid collection is large, a static approach is satisfactory, as long as the patient remains in the same position. In general, a dynamic technique is safer and recommended. If a dynamic technique is chosen, the linear probe is covered with a sterile sleeve and placed on the sterile field or on a sterile procedure tray. The two techniques used for any needle guidance procedure using ultrasound are the in-plane or long axis approach and the out-of-plane or short axis approach. Whichever the clinician is more comfortable with is acceptable. The short axis approach is easier, although it carries a risk of inserting the needle too deep, since only the section of the needle directly under the transducer is seen. When there is an abundance of fluid, this method should not be a problem. When there is little fluid to access, particularly when sensitive structures are nearby, the long axis approach is preferred. This approach, though more difficult, provides real-time visualization of the entire length of the needle as it enters the peritoneum. Either technique can be performed by one clinician using the sterile transducer; the needle is inserted with the dominant hand while the other hand holds the transducer.

Whichever approach is used, one must be sure to reexamine the chosen site in two perpendicular planes to assure an adequate volume of fluid. Bowel will tend to move over time due to peristalsis. It also moves when the patient adjusts his/her upper body position or coughs. If the fluid is no longer present at the premarked site, scan the adjacent sterilized area, and if need be, repeat the entire search and aseptic preparation. After the proposed site and path of the needle catheter is determined, local anesthesia should be injected down to and including the parietal peritoneum. Using the short axis approach the needle catheter is inserted next to the middle of the long edge of the linear probe as the probe is held perpendicular to the skin. The transducer should be moved forward as the needle advances, ideally scanning the tip of the needle. If need be, the clinician can halt the needle and shift the transducer away from and then back toward the needle to clarify where the tip of the needle is located. When the needle is about to enter the peritoneum,
the inner wall of the abdomen will be seen tenting inward. At this point, short controlled jabs with the needle may help to pierce the peritoneal fascia. Once in the peritoneal cavity, the needle is seen as a hyperechoic point within the anechoic fluid. If the long axis approach is used, the entire needle is followed as it tracks through the abdominal wall and into the peritoneum ( VIDEO 13.3). With either approach, make sure that the needle has advanced about a centimeter into the ascites before collecting fluid and advancing the catheter. At this stage the ultrasound transducer can be set aside and the procedure can be completed in the usual manner.