Trauma
What to know
The FAST examination screens for free fluid in four windows: perihepatic (Morison's pouch), perisplenic, pelvic (rectovesical or pouch of Douglas), and subxiphoid pericardial; an extended FAST adds thoracic views. Free fluid appears as an anechoic stripe, though acute hemorrhage may be echogenic and complex.
Solid organ injury appears as lacerations, subcapsular or parenchymal hematomas, and on contrast-enhanced ultrasound as active extravasation or pseudoaneurysm. Renal injuries may show perinephric fluid or fractured parenchyma. Bladder rupture produces pelvic fluid.
Traps
FAST detects free fluid, not organ injury, and misses retroperitoneal hemorrhage and most hollow-viscus injury. A negative FAST does not exclude injury-repeat or escalate to CT in the stable patient. Do not mistake the pericardial view for a pleural one.
Rule of thumb
In an unstable trauma patient, any positive window counts as a positive FAST; start at Morison's pouch, the most dependent space in the supine patient.
Worked example
A hypotensive patient after blunt trauma has an anechoic stripe between the liver and right kidney-positive FAST; the patient goes to the operating room rather than CT.