What is the role of fast ultrasound (Focused Assessment with Sonography for Trauma - FAST) in emergency settings?

FAST Ultrasound in Emergency Settings

FAST (Focused Assessment with Sonography for Trauma) is a core emergency ultrasound application that should be performed at the bedside during trauma resuscitation to rapidly detect hemopericardium, hemoperitoneum, and hemothorax in hemodynamically unstable patients, enabling immediate life-saving decisions without delaying definitive treatment. 1

Primary Indications

FAST is indicated for:

• Hemodynamically unstable patients with blunt abdominal trauma to detect free intraperitoneal fluid as a marker of bleeding 1
• Penetrating thoracoabdominal trauma when uncertainty exists about abdominal cavity penetration 2
• Any trauma patient with unexplained hemodynamic instability to identify occult bleeding sources 1
• Mass casualty situations for rapid triage of multiple victims 1

What FAST Detects

The examination evaluates four anatomic regions:

• Pericardium for cardiac tamponade detection 1
• Right upper quadrant (hepatorenal space/Morrison's pouch) for hemoperitoneum 1
• Left upper quadrant (splenorenal space) for hemoperitoneum 1
• Pelvis (rectovesical/rectouterine pouch) for dependent fluid collection 1

Extended FAST (E-FAST) adds bilateral thoracic views to detect hemothorax and pneumothorax, providing comprehensive evaluation of traumatic bleeding 1

Performance Characteristics and Clinical Impact

FAST demonstrates high specificity (97-100%) but variable sensitivity (83-100%) for detecting intraperitoneal bleeding, with performance heavily dependent on operator experience and amount of free fluid present 1. The American College of Emergency Physicians reports that in hemodynamically unstable blunt trauma patients, FAST achieved 88% sensitivity and 99% specificity, with negative likelihood ratio of 0.12 1.

Critical advantage: FAST-guided management reduces time to operating room, decreases CT utilization, shortens hospitalization, and lowers complications compared to management without FAST 1. In one randomized controlled trial of 262 blunt trauma patients, FAST utilization resulted in faster disposition to surgery and reduced hospital charges 1.

Integration into Trauma Resuscitation

FAST must be performed as an integral component of trauma resuscitation, concurrent with other life-saving interventions 1. The examination should not delay definitive treatment—if immediate laparotomy is indicated, FAST can still evaluate for pericardial tamponade or pneumothorax before operating room transfer 1.

For hemodynamically unstable patients, the diagnostic algorithm is:

1. Perform E-FAST immediately during resuscitation 1, 3
2. Obtain chest X-ray and pelvic X-ray simultaneously 1, 3
3. If E-FAST and chest X-ray rule out extra-pelvic bleeding sources, proceed directly to pelvic angiography 1, 3
4. If E-FAST shows hemoperitoneum with abundant free fluid (≥3 positive sites), proceed to emergency laparotomy 1
5. Never transport unstable patients to CT scanner—imaging must not delay bleeding control 1, 3

For hemodynamically stable patients, proceed directly to CT scan with IV contrast rather than relying on FAST alone, as CT provides superior anatomic detail and identifies solid organ injuries 1

Critical Limitations and Pitfalls

FAST detects free fluid, not the injury itself—it cannot identify hollow viscus injury, mesenteric vascular injury, or diaphragmatic rupture that produce minimal hemorrhage 1. These injuries require CT or surgical exploration for diagnosis 1.

FAST cannot differentiate fluid types—blood, urine, bile, and ascites appear identical sonographically 1. Clinical correlation is essential.

Small amounts of blood (<500 mL) may be missed, particularly in early evaluation before fluid redistributes to dependent areas 1. Serial FAST examinations should be performed in response to clinical changes to detect previously undetectable fluid accumulation 1.

False positives occur with:

• Retroperitoneal hematoma suffusion appearing intraperitoneal 1
• Intraperitoneal bladder rupture 1
• Pre-existing ascites 1

Operator dependence is significant—adequate training requires didactic education, practical hands-on course, and 50 supervised scans for competency 1

Special Applications

FAST is uniquely suited for austere environments including prehospital care, battlefield settings, remote locations, mass casualty incidents, and resource-limited settings where CT is unavailable 1. The examination is portable, battery-powered, requires no radiation, and provides immediate actionable information 1.

In pregnancy with trauma, FAST evaluation for hemoperitoneum takes priority, though transvaginal ultrasound should subsequently evaluate for intrauterine pregnancy or complications 3, 4

Quality Assurance Requirements

All FAST examinations must be documented with stored images as part of the medical record, retrievable for review and medico-legal purposes 1. Reports should reflect recorded findings interpreted and signed by adequately trained individuals 1.

The examination should not be delayed by image archiving—timely care delivery takes precedence, with documentation completed as soon as feasible 1