Which view in the Focused Assessment with Sonography for Trauma (FAST) exam will demonstrate free fluid in the peritoneal cavity first in a supine adolescent or adult patient?

The Pelvic View in FAST Exam Will Demonstrate Free Fluid First in Supine Patients

The pelvic view (suprapubic view) is the most likely to demonstrate free fluid first in a supine adolescent or adult patient during the FAST exam, as this space is the most dependent peritoneal space in the supine position. 1

Anatomical Basis for Fluid Collection in FAST Exam

When a patient is positioned supine, gravity directs intraperitoneal fluid to collect in the most dependent areas:

1. Pelvic View (Suprapubic)

   • Also known as the suprapubic, retrovesical, rectovesical view (in males), or retrouterine, rectouterine, and pouch of Douglas view (in females)
   • This is explicitly identified as "the most dependent peritoneal space in the supine position" 1
   • A full bladder provides an ideal acoustic window, though adequate views can often be obtained with a partially filled bladder

2. Right Upper Quadrant (RUQ)

   • Second most common location for fluid accumulation
   • Includes Morison's pouch (hepatorenal space) and the caudal liver edge area
   • Recent research suggests the caudal liver edge and superior paracolic gutter (RUQ3) is the most sensitive sub-quadrant within the RUQ view 2

3. Left Upper Quadrant (LUQ)

   • Less sensitive than RUQ due to the smaller acoustic window provided by the spleen compared to the liver

4. Pericardial View

   • Evaluates for hemopericardium rather than intraperitoneal fluid

Technical Considerations for Optimal Visualization

For the pelvic view:

• Place the probe in the transverse plane immediately cephalad to the pubic bone
• Rock the probe from inferior to the dome of the bladder systematically
• The probe may be rotated 90 degrees into the sagittal plane for additional visualization
• Decrease gain settings to account for posterior acoustic enhancement from the fluid-filled bladder 1

Important Clinical Considerations

• A minimum volume of approximately 500 mL of free fluid is typically required before detection by FAST exam 1
• Patient positioning can affect fluid detection:
  • Trendelenburg position may increase sensitivity for detecting fluid in the right upper quadrant
  • Sitting position may increase sensitivity for detecting fluid in the pelvis 1
  • Right-sided roll has been shown to improve sensitivity of the FAST exam from 40.0% to 47.7% 3

Potential Pitfalls

• Empty bladder: While large volumes of fluid may still be visible with an empty bladder, it's not possible to reliably rule out smaller amounts of free fluid 1
• Non-traumatic fluid collections (e.g., ascites, pleural/pericardial effusions) may be mistakenly attributed to trauma 1
• Blood clots form rapidly in the peritoneum and have sonographic qualities similar to soft tissue, potentially being overlooked 1
• Posterior acoustic enhancement caused by the bladder can result in pelvic free fluid being overlooked if gain settings aren't properly adjusted 1

The pelvic view remains the most sensitive area for detecting free fluid in supine patients due to the gravitational effects on fluid distribution within the peritoneal cavity.