Focused assessment with sonography for trauma

Lore & Background

Before the evolution of ultrasound and its rapid availability in hospitals, surgeons and emergency physicians used Diagnostic Peritoneal Lavage (DPL), an invasive procedure to diagnose hemoperitoneum. In the 1990s, the FAST exam using ultrasound became widely popular after advancement in Europe that showed specificity around 98% while also allowing for faster and cheaper diagnosis of trauma patients, particularly unstable patients who may not be able to make it to CT. The four classic areas examined for free fluid are the perihepatic space (including Morison's pouch), perisplenic space, pericardium, and the pelvis. With this technique, it is possible to identify moderate to large amounts of intraperitoneal or pericardial free fluid, which in the setting of trauma will usually be due to bleeding. The FAST exam is poor at detecting smaller amounts of free fluid, with sensitivity around 85% with 150 mL of fluid.

Reader's Guide

The FAST exam is most useful in trauma patients who are hemodynamically unstable to guide surgical interventions. A positive FAST is defined as the appearance of a dark (anechoic) strip in dependent areas of the peritoneum or abdominal cavity, indicating free fluid or blood. In the right upper quadrant, this typically appears in Morison's pouch. A positive result suggests hemoperitoneum; often CT scan will be performed if the patient is stable, or if unstable, the patient will be taken for a laparotomy. However, this may be a false positive in many menstruating women, as free fluid may be seen behind the uterus normally and would require clinical correlation. In those with a negative FAST result, a search for extra-abdominal sources of bleeding may still need to be performed, as FAST cannot reliably rule out all bleeding or life-threatening injury, including posterior sources or small amounts/early bleeding. The E-FAST adds lung views to detect pneumothorax using signs such as the stratosphere sign (indicating pneumothorax) or seashore sign (normal). Compared with supine chest radiography, bedside sonography has superior sensitivity (49–99% versus 27–75%) and similar specificity (95–100%) for pneumothorax, and can be performed in under a minute. FAST is less invasive than DPL, involves no radiation, and is cheaper than CT, but cannot accurately rule out life-threatening injuries and is of limited value where CT is readily available.

Did You Know?

• The FAST exam has a specificity around 98% for detecting free fluid in trauma patients.
• The E-FAST can detect a pneumothorax by the absence of normal 'lung-sliding' and 'comet-tail' artifact on ultrasound.
• The stratosphere sign (or barcode sign) on M-mode ultrasound indicates a pneumothorax.
• FAST is poor at detecting smaller amounts of free fluid, with sensitivity around 85% with 150 mL of fluid.