Chapter 9. Ultrasound Evaluation of Peritoneal Fluid

Bedside ultrasound is an optimal first-line imaging modality for the evaluation of peritoneal free fluid. The "focused assessment with sonography in trauma" (FAST) examination is one of the common applications used in evaluating the patient for free peritoneal fluid from hemorrhage. Ultrasound has been shown to be sensitive and specific for significant hemoperitoneum. Ultrasound has long been accepted as a life-saving tool in the hypotensive trauma patient, but its benefits have expanded beyond this. Bedside ultrasound is now readily used to search for free fluid in the non-traumatic patient and in other disease processes. It can be used to evaluate for free peritoneal fluid in patients with end-stage liver disease, renal disease, or congestive heart failure.

In addition, the evaluation for peritoneal fluid is invaluable in other patients who may present with abdominal or flank pain. For example, intraperitoneal free fluid in a pregnant patient with abdominal or pelvic pain usually indicates a ruptured ectopic pregnancy that will require operative intervention. The rapid diagnosis of free fluid at the bedside can expedite patient care and definitive treatment.

Bedside ultrasound evaluation for peritoneal free fluid should be performed in:

• The acute trauma patient with blunt or penetrating torso injuries
• The pregnant or pediatric trauma patient
• The subacute trauma patient with a delayed presentation who complains of worsening abdominal pain
• The elderly patient with undifferentiated abdominal or flank pain
• The pregnant patient with abdominal or pelvic pain
• The patient with undifferentiated shock or dyspnea

• The abdomen and pelvis are best imaged using a wide footprint curvilinear probe with an operating frequency between 3.5 and 5.0 MHz. Lower frequencies may help with more obese patients while higher frequencies can be used in thin or pediatric patients.
• The phased-array or microconvex transducer in the 2.0–5.0 MHz range can also be used to evaluate the abdomen and pelvis for free fluid. The small footprint allows imaging between the ribs, which is particularly helpful in young thin patients.

Total Gain and Time-Gain Compensation

The total gain can be increased to brighten the signal returning from the structure of interest and improve the image on the machine monitor. Often times, the near field has adequate gain, and therefore does not need to be adjusted. In order to compensate for signal loss in the obese patient, time-gain compensation (TGC, far gain) may be increased to enhance the attenuated signal reflecting from deeper tissues in the patient. Decreasing the far gain is often required to adjust for the posterior acoustic enhancement artifact that appears behind the bladder in the pelvic window. The TGC controls should be equalized prior to beginning an exam in order to avoid a false-positive "stripe" that can result when the TGC is set too low.

Depth

The sonographer should start with a depth of 15 cm or more and scan through the entire far field before ...