Ultrasound technology is readily available and can provide guidance for invasive abdominal procedures in multiple planes without ionizing radiation. Further, real-time guidance of the needle tip position allows the operator to avoid inadvertent puncture of vital structures during the performance of invasive and soft tissue procedures of the abdominal cavity. Recently, this technology has a demonstrated benefit in abdominal procedures, such as paracentesis, liver and kidney biopsy, percutaneous gallbladder drainage, and abscess drainage.
Transducer selection for abdominal procedures most commonly includes a curved or phased array. A curved array allows the user to visualize a larger field, but the actual transducer size is larger than a phased array. Curved array is the transducer of choice for most abdominal procedures since a larger field can be visualized during the exam. A sector array is smaller and is most commonly used for abscesses that are close to the diaphragm and allows for an intercostal approach.
Ultrasound use in paracentesis improves both the ease and efficiency of the procedure and reduces unnecessary abdominal punctures in patients with scant fluid. It is particularly helpful in obese patients to assess the depth to the peritoneum and in patients with loculated effusions to define the largest locule for drainage. Ultrasound use can identify intra-abdominal pathology that may increase the risk of bowel perforation during the procedure.1
Diagnostic paracentesis is indicated as a part of the initial evaluation of patients with new onset ascites and in patients with a known history of ascites secondary to liver cirrhosis, who develop clinical deterioration, including the signs and symptoms of fever, abdominal pain, rapid worsening of renal function, worsened hepatic encephalopathy, leukocytosis, acidosis, gastrointestinal bleeding or sepsis, and to rule out underlying spontaneous bacterial peritonitis (SBP).2 Paracentesis can also be used for the evaluation of intra-abdominal fluid in trauma patients. Therapeutic paracentesis is performed in patients with tense or diuretic-resistant ascites to alleviate difficulty with breathing or abdominal pain.
Paracentesis is contraindicated in patients with disseminated intravascular coagulation, a platelet count <50 × 109 per liter, an international normalized ratio (INR) >2, and a local skin infection, visible scar, hematoma, or cutaneous vein at the site of needle entry.3 Patients with underlying renal insufficiency who have an increased tendency for bleeding should be carefully evaluated prior to the procedure.
A 3.5–5 MHz broadband curved array is preferred for the evaluation of ascites and for assistance with ultrasound-guided paracentesis.1,4 Prepackaged paracentesis procedure kits are commercially available and contain all necessary equipment, including an aspiration catheter, catheter bag, blood collection tubing, 19-gauge introducer needle with 5-French catheter, lidocaine for skin anesthesia, skin preparation solution, and dressing pack with sterile draping (Figure 25-1). For real-time ultrasound-guided procedures, a sterile sleeve is used to cover the ultrasound probe.
The optimal site for paracentesis is where the depth of ascitic fluid is maximal and the abdominal wall is the thinnest.5 It is common practice to perform a paracentesis in the left lower quadrant (LLQ). Paracentesis in the right lower quadrant is generally avoided because of an increased risk of bowel perforation in patients with a distended cecum. The best location for paracentesis is 5 cm superior and medial to the anterior superior iliac spine. One prospective study of 52 patients with ascites secondary to cirrhotic liver disease demonstrated that the LLQ was the optimal location for paracentesis because it was easier and safer than an infraumbilical, midline approach because of a lower incidence of “dry tap” and bleeding.5 Preprocedure scanning with ultrasound is recommended to avoid complicating factors like cutaneous veins, the inferior epigastric artery, and areas with previous scars and operations (Figure 25-2).
After obtaining informed consent, the patient is asked to empty the bladder prior to the procedure to decrease the risk of urinary bladder perforation (Figure 25-3). The patient is positioned supine. Prior to starting the procedure, the four abdominal quadrants are scanned to evaluate the extent of the ascites. The liver and spleen should also be evaluated. Scanning the entire abdomen allows the identification of complicating factors for intra-abdominal paracentesis, like intraperitoneal septation (Figure 25-4). Noncomplicated ascites usually has a characteristic anechoic contrast on ultrasound (Figure 25-5). An abnormal density, loculation, or septation may be identified on ultrasound and presents visually as an echogenic mass.6
Ultrasound examination of the left lower quadrant in a patient with ascites demonstrates a collection of ascites adjacent to a full urinary bladder. A full bladder poses an increased risk of bladder perforation; therefore, the patient should be asked to empty bladder prior to the procedure.
After screening the abdominal cavity with ultrasound, the optimal site is identified, cleaned, and draped. The ultrasound probe is covered with a sterile sheath and the abdomen is rescanned to confirm the site of needle entry. Using a 25–30 gauge needle, the skin is infiltrated with local anesthetic using 1% or 2% lidocaine. A combination of lidocaine and sodium bicarbonate may be used to reduce the burning associated with the local injection of lidocaine. Next, a 22-gauge 1.5 cm needle can be used to infiltrate the local anesthetic to the subcutaneous tissues and anterior abdominal wall. A 19-gauge needle with an echogenic tip is then introduced into the peritoneal cavity. To avoid precipitating a peritoneal fluid leak, a “Z-line technique,” which introduces the needle at an angle and avoids creating a direct tract, is recommended. A 5-French catheter can be used for drainage of the peritoneal fluid.
A free-hand technique, in which paracentesis is guided by real-time ultrasound, is recommended to avoid direct injury to the bowel for a small pocket of peritoneal fluid (Figure 25-6). In this approach, the needle is introduced into the peritoneal cavity and fluid collected under direct vision (Figure 25-7). A sample of fluid is submitted to the clinical laboratory for appropriate diagnostic studies.
For a therapeutic tap of a large volume of ascites, a catheter-over-the-needle assembly can be used. In this approach, a needle is introduced into the peritoneal cavity, and once the needle passes through the ascitic collection, a catheter is introduced over the needle. Follow-up imaging with ultrasound can document the proper and safe position of the tip of the catheter. Then, drainage tubing connected to a vacuum device can be attached to the catheter. On occasion, the bowel will obstruct the catheter; when this occurs, the drainage tubing can be clamped with hemostats and the tubing separated from the catheter. This allows the suction to be temporarily removed from the catheter. The catheter can then be pulled back a few millimeters until ascites again begins to escape from the catheter. The drainage tubing is reattached and the hemostats removed to continue draining the ascites (Video 25-1).
A number of diagnostic tests can be performed on the ascitic fluid to assist with clinical interpretation.7–9 These include:
Gram stain and culture: A gram stain and culture is recommended to rule out infection of the peritoneal fluid and may identify secondary peritonitis.
Cell count and differential: This test allows for the diagnosis of inflammation or SBP. The sensitivity and specificity depends on the polymorphonuclear (PMN) count. A PMN count >500 cells/mm3 has a sensitivity of 70–100% and a specificity of 86–100%.
Glucose: Ascitic fluid glucose is low in secondary peritonitis.
Albumin: Albumin levels allow for a determination of the etiology of the ascitic fluid. The serum-ascites albumin gradient (SAAG) is the absolute difference between the serum albumin and the ascitic albumin level. A SAAG >1.1 g/dL identifies ascites due to portal hypertension. A SAAG <1.1 g/dL indicates an exudative ascites that can be due to infection or malignancy.10
Total protein: An ascitic fluid protein level >2.5 g/dL supports a diagnosis of an exudative peritoneal effusion, while a protein level <2.5 g/dL supports a diagnosis of a transudative effusion. The differential diagnosis associated with a transudate includes cirrhosis, congestive heart failure, portal vein thrombosis, and myxedema, and that associated with an exudative effusion includes peritoneal carcinomatosis, tuberculosis, infection, connective tissue disease, bowel obstruction, and infarction.
Amylase: A high amylase level in ascitic fluid >1000 indicates leakage of pancreatic enzymes into the peritoneal cavity.
Triglycerides: Chylous ascites with triglycerides level >200 mg/dL is highly suggestive for intra-abdominal malignancy or infection (filariasis, tuberculosis).
Bilirubin: High ascites bilirubin >6 mg/dL indicates choleperitoneum (bile in the peritoneal cavity).
Cytology: Malignant cells may be present in the peritoneal fluid and indicate carcinomatosis.7–9
The incidence of bleeding from diagnostic paracentesis is estimated at approximately 0.2%. This was found to increase cost-related procedure by about $19,066 and increase length of hospital stay.11 Hemorrhage related to large volume therapeutic abdominal paracentesis >4 L is estimated at approximately 2%.12 An inferior epigastric artery pseudoaneurysm has been reported in two cases after large volume paracentesis. These were treated by percutaneous embolization.13,13a This complication can be avoided by scanning of the abdomen with ultrasound to define the optimal entry site that is lateral to the rectus abdominis muscles and avoids large intra-abdominal veins. It was found that ultrasound had decrease the risk of bleeding after paracentesis by 19%.11
Other complications may include intestinal or urinary bladder perforation, secondary peritonitis, and reaccumulation of peritoneal fluid. Large volume paracentesis, >10 L, is usually avoided because it may lead to a reduction in central venous pressure and associated hypotension. In addition, activation of the rennin-angiotensin-aldosterone system occurs, which is extremely sensitive in patients with liver cirrhosis. This phenomenon is worse in patients who have liver cirrhosis with ascites but without peripheral edema, and may precipitate hyponatremia, renal insufficiency, and hepatorenal syndrome if the patient does not receive simultaneous intravenous albumin.13,13a,14 This is due to the absence of edema, which can functionally re-equilibrate with plasma.
Drainage of an intra-abdominal abscess by using puncture and aspiration was described in the 1930s.15
Real-time sonography has the advantages of being portable, readily available at the bedside, and a success rate of approximately 90% for intra-abdominal abscesses.
Ultrasound is able to differentiate between a solid mass and cyst in approximately 95% of cases.16 Using a gray scale, the cyst will appear bright white, while the adjacent organs appear gray. This is referred to as the “light bulb” sign.17 Ultrasound provides real-time imaging during catheter placement. Color-Doppler flow can be used to identify crossing blood vessels, which allow proper planning of an optimum approach.18 When compared with computerized tomography (CT), ultrasound is more accurate in localizing and guiding the drainage of small, deep abscesses19 (Figure 25-8).
The drainage of abnormal fluid collections is recommended for fluid that is suspicious for infection. Percutaneous fluid drainage is a safe and expeditious option for critically ill patients, who are too unstable for an operative intervention or need a temporizing measure to improve their condition and reduce morbidity. Percutaneous drainage provides a rapid, inexpensive, and immediate treatment in elderly, unstable patients. While this procedure is usually performed in the radiology department with the assistance of CT or fluoroscopy, the role of bedside ultrasound is important for those patients, who are too sick for transportation to the radiology suite and whose transportation conveys additional risks for potential complications.
Percutaneous abscess drainage is contraindicated if the catheter path is through a vital organ (lack of safe route). If possible, consider correction of coagulopathy prior to the procedure.1,17 Ultrasound may be limited in differentiating between an abscess and other abdominal fluid collections, such as blood. Bowel gas may reduce image quality. Surgical dressing or drains may limit ultrasound evaluation.
The site of the abscess must be evaluated by ultrasound prior to the procedure by using a 5–7 MHz transducer; for very superficial abscesses a 10 MHz transducer may provide better resolution.16 One needs to evaluate a safe path for catheter placement, the size of the abscess, the content of the abscess (light bulb sign), and the proximity to the skin surface.17 The key to a successful procedure is a careful, well-planned needle path and an appropriate angle of entry prior to starting.
The catheter chosen for the procedure depends on the indication, with smaller catheters (6–8 French) used for aspiration of fluid collections, while larger caliber catheters (10–14 French) may be needed for more viscous fluid collections.
The procedure site is sterilized and draped. Lidocaine 1–2% is used to anesthetize the skin. A sterile sheath is used to cover the ultrasound probe. The catheter is introduced into the abscess under direct ultrasound visualization.
Different techniques are available for abscess drainage. The trocar technique is the preferred method for a large abscess with thick fluid; however, this method is associated with more severe complications due the large size of the introducer needle. In the Seldinger technique, a needle, guide wire, and dilators are sequentially introduced prior to the introduction of the catheter.
For a superficial abscess, a 22-gauge needle can be used; for deeper collections, a larger bore needle is recommended. For deeper abscesses using a Seldinger technique, the skin is prepped and draped and the ultrasound probe is covered with a sterile sleeve. The probe is held in parallel with the needle tract and the skin is anesthetized. The 18-gauge introducer needle is advanced along the parallel ultrasound plane (beam), carefully observing and avoiding surrounding anatomical structures. After placing the needle tip into the collection, a syringe is attached to the needle and a sample of aspirate is removed for cultures. The syringe is removed and a 0.035 guide wire is placed through the needle lumen into the collection. Direct visualization of the wire can be achieved by carefully tugging on the wire while observing for movement within the collection. The tract is dilated to the desired size of the catheter, most commonly a 10-French, 30 cm, multipurpose “pigtail” catheter. Starting with a 6-French dilator, sequential dilation can occur with 8- and 10-French dilators. The drainage catheter is advanced over the wire and into the abscess (Figure 25-9). The metal stiffener (inner portion of the catheter mechanism) should only be advanced a few millimeters passing through the border of the collection; the catheter is slightly longer than the stiffener. After passing through the wall of the collection, hold the stiffener while advancing the catheter over the stiffener into the collection. Remove the stiffener from the catheter, remove the wire, and pull the string, which causes the distal end of the catheter to coil (pigtail). The catheter is secured using either suture or an adhesive device. The catheter is attached to the drainage bag and a dressing placed over the catheter entry site. If it is difficult to visualize catheter placement, push a combination of 3 cc of sterile saline with 1 cc of air through the catheter. Perform this technique while scanning; you should see air bubbles floating within the collection.