Background and Indications for Examination
Ascites is the accumulation of free fluid in the peritoneal cavity and is typically caused by portal hypertension due to hepatic failure. Other common causes of ascites include renal failure, congestive heart failure, infection, or malignancy. Ascites may be extremely uncomfortable when large amounts of fluid are present, and may be life threatening if it becomes infected or compromises respiratory efforts or venous return. Physical examination is not sensitive for ascites and cannot reliably determine the optimal location for drainage. Bedside ultrasound can reliably detect ascites and aid in the removal of fluid.
Both critical care and emergency medicine physicians have long performed bedside paracentesis. The traditional blind approach to remove intraperitoneal fluid has been employed until the recent introduction of ultrasound-guided techniques. Ultrasound-guided paracentesis is now the standard of care amongst both critical care and emergency medicine physicians. When compared to the older blind approach, ultrasound-guided paracentesis may increase the success of the procedure and decrease the risk of vascular injury or bowel perforation.
Paracentesis can be diagnostic or therapeutic and sometimes both, depending on the patient’s presentation. Diagnostic paracentesis may be required to rule out an infection or malignancy. Ascitic fluid is always at risk for bacterial translocation which can lead to peritonitis. Therapeutic paracentesis can also be performed to relieve abdominal pain or shortness of breath. When massive, ascitic fluid can inhibit excursion of the diaphragm and cause respiratory distress, or compress the inferior vena cava (IVC) impairing right heart filling which lead to hemodynamic instability. In these patients, removal of fluid can relieve cardiopulmonary distress and provide significant symptomatic relief.
- Assessment of new onset ascites and/or ascites of unclear etiology
- Concern for spontaneous bacterial peritonitis in a patient with known ascites (may present with abdominal pain, fever, or altered mental status)
Probe Selection and Technical Considerations for Paracentesis
The initial probe for detection of fluid should be a large footprint curvilinear probe in the 3.5–5.0 MHz range. This probe is best for identifying large pockets of fluid in the upper and lower quadrants, explained fully in Chap. 9.
Following the identification of the optimal fluid pocket for aspiration, a higher-frequency linear probe, in the 8–12 MHz range, should be used for actual procedural guidance. This will provide better resolution, improving identification of the inferior epigastric vessels and visualization of the needle entering the fluid pocket.
The standard equipment required for a sterile procedure, including appropriate local anesthesia (1%–2% lidocaine with or without epinephrine), is needed for a paracentesis. In addition, if available, a packaged “paracentesis kit” or similar supplies are required which include:
- 18–20 gauge needle for fluid withdrawal
- For a diagnostic paracentesis only, a 60-cc syringe can be used to withdraw fluid directly
- For a large-volume paracentesis done for therapeutic purposes:
- Vacutainer bottles
- Tubing connected to a luer lock to drain fluid into vacutainer bottles
- A luer lock to connect tubing to the paracentesis needle
- An 18-gauge needle to insert into vacutainer bottles
- Vacutainer bottles
Blind versus Ultrasound-Guided Paracentesis
The traditional approach to paracentesis relies on the physical examination alone. Ascites accumulates in a gravity-dependent manner within the peritoneum. The classic findings suggestive of ascites include a distended abdomen, presence of a fluid wave, and shifting dullness. Limitations of blind paracentesis are as follows:
- It cannot assess for the adherence of bowel or omentum to the peritoneal lining.
- It cannot determine the distance from the skin surface to the peritoneum or from the peritoneum to underlying bowel or omentum.
- It cannot determine the optimal location for drainage.
- It may not identify the inferior epigastric vessels.
Ultrasound-guided paracentesis helps to eliminate the limitations associated with the blind technique. This method reliably detects fluid and determines the optimal site for needle access and drainage. The focused abdominal ultrasound should include views of both the upper and lower abdominal quadrants, including the pelvis. The inclusion of multiple sites helps to identify the location with the largest pocket of fluid. Ultrasound is very sensitive for fluid when the volume exceeds 500 cc. Bedside ultrasound allows the clinician to directly visualize ascitic fluid deep to the intended insertion site. In addition, it helps to locate adjacent bowel, omentum, and blood vessels that must be avoided by the sonographer in order to prevent perforation, infection, or bleeding. Direct ultrasound visualization is ideal in patients with small pockets of ascitic fluid or when vulnerable structures are near the intended insertion site.
In situations where a patient presents with a large amount of fluid and resultant tense ascites, ultrasound can help to evaluate compartment syndrome by imaging the IVC. These patients will have a baseline compression and pronounced respiratory collapse of their IVC, which indicates the need for more rapid intervention and fluid drainage. In patients with significant cardiorespiratory distress, a bedside echo may also detect impaired right-sided heart filling.
Ultrasound-guided paracentesis can be performed as either a static or dynamic procedure. Static guidance uses ultrasound to identify the largest fluid pocket and the optimal site of needle entry by marking the spot on the patient’s skin, and noting the angle of approach needed prior to doing the procedure. Dynamic guidance allows the clinician to anesthetize accurately down to the peritoneum, ensures adequate needle entry, and avoids vascular or bowel injury in real time. It may also be used to evaluate why the catheter may have stopped draining during the procedure, and to assess the adequacy of drainage. The single-user approach where the clinician holds the probe in one hand and the needle in the other provides optimal procedural guidance. A two-person approach can also be utilized where an assistant may hold the probe or can help in visualizing the needle entry into the peritoneum on the ultrasound monitor.
Performing Ultrasound-Guided Paracentesis
The most commonly chosen position for paracentesis is a lateral decubitus position. This position shifts peritoneal fluid to the dependent side and increases the distance between the peritoneum and underlying bowel or omentum. If the patient cannot tolerate the decubitus position, the procedure can also be performed with the patient supine and their head slightly elevated in order to drain fluid into the dependent lower quadrants.