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FAST Ultrasound: Trauma Assessment (Mobile)

Focused Assessment with Sonography for Trauma (FAST) is a rapid, portable, repeatable bedside ultrasound used to detect free intraperitoneal and pericardial fluid in patients with blunt or penetrating trauma. In many African emergency settings it has replaced diagnostic peritoneal lavage and triages haemodynamically unstable patients directly to laparotomy without delay for CT. Its principal strength is speed and portability; its principal limitation is that it detects free fluid rather than the specific organ injury, so a negative scan never excludes significant injury.

Indications

  • Blunt abdominal trauma, especially from road-traffic collisions and falls
  • Penetrating thoraco-abdominal trauma to assess for haemoperitoneum and pericardial blood
  • Haemodynamically unstable trauma patient to guide immediate operative decision-making
  • Undifferentiated hypotension in the trauma bay
  • Serial reassessment of a patient managed non-operatively or one who deteriorates
  • Extended FAST (eFAST) for suspected pneumothorax or haemothorax
  • Resource-limited emergency departments lacking immediate CT access

Contraindications & Cautions

  • No absolute contraindication; ultrasound is non-ionising and safe in pregnancy and children
  • Do not delay transfer to theatre in an exsanguinating patient for a scan - clinical judgement overrides
  • Reduced reliability with subcutaneous emphysema, morbid obesity, large bowel gas or existing ascites
  • A negative FAST does not exclude retroperitoneal, hollow-viscus, diaphragmatic or solid-organ injury without free fluid - never use it to rule out injury
  • Operator dependence: reliability depends on training and experience

Patient Preparation

  • Perform as part of the primary/secondary survey without interrupting resuscitation
  • Patient supine (Trendelenburg increases sensitivity for small volumes in the pelvis/upper abdomen)
  • Expose chest and abdomen; apply gel; maintain spinal precautions
  • Brief the patient if conscious; formal consent is not required in the emergency setting
  • Ensure the machine is charged/available at the bedside and images can be saved for the record
  • Empty bladder via catheter can degrade the pelvic window - a moderately full bladder aids the acoustic window

Technique & Parameters

  • Use a low-frequency curvilinear/phased-array probe (2-5 MHz); portable/handheld devices are acceptable
  • Four core views: right upper quadrant (hepatorenal/Morison pouch), left upper quadrant (splenorenal recess and subdiaphragmatic space), pelvis (rectovesical/rectouterine pouch in longitudinal and transverse), and subxiphoid pericardial view
  • Morison pouch is the most dependent and most sensitive site in the supine patient
  • eFAST adds bilateral anterior chest (lung sliding, absent sliding + lung point = pneumothorax) and the costophrenic angles for haemothorax
  • Scan systematically, sweeping through each region; total examination typically under 5 minutes
  • Repeat the scan after 10-30 minutes or on deterioration - sensitivity improves as fluid accumulates
  • Can be performed by trained non-radiologist emergency clinicians following structured competency training

Systematic Review

  • RUQ: assess the hepatorenal interface, the inferior liver tip and the subphrenic space for an anechoic stripe
  • LUQ: the splenorenal recess and, importantly, the subdiaphragmatic space where left-sided fluid collects first
  • Pelvis: scan cephalad-to-caudad in two planes for fluid posterior to the bladder/uterus
  • Pericardium: subxiphoid four-chamber to detect anechoic fluid between pericardium and myocardium and signs of tamponade
  • eFAST lungs: confirm bilateral lung sliding and A-lines; seek lung point, barcode/stratosphere sign for pneumothorax and anechoic fluid above the diaphragm for haemothorax
  • Assess for gross solid-organ laceration if visible, though FAST is not designed to characterise it
  • Estimate fluid volume qualitatively (trace vs moderate vs large) and note location

Key Findings & Significance

  • Anechoic free fluid in Morison pouch - most sensitive single sign of haemoperitoneum in blunt trauma
  • Fluid in the splenorenal recess or left subphrenic space indicating left upper quadrant injury
  • Fluid in the pelvic recesses (rectovesical or pouch of Douglas), the most dependent site overall
  • Anechoic pericardial collection with diastolic right-heart collapse indicating tamponade
  • Positive FAST in an unstable patient mandates immediate laparotomy
  • eFAST: absent lung sliding with a lung point confirms pneumothorax; anechoic fluid above the diaphragm indicates haemothorax
  • Sensitivity for haemoperitoneum is roughly 85-95% and specificity is high, but sensitivity falls for small volumes (<200-500 mL)

Differential Considerations

  • Anechoic perihepatic/perisplenic fluid -> haemoperitoneum vs pre-existing ascites vs ruptured ectopic (in the pregnant patient)
  • Pelvic free fluid in a woman -> traumatic haemoperitoneum vs physiological pelvic fluid vs ruptured ovarian cyst/ectopic
  • Pericardial fluid -> traumatic haemopericardium vs pre-existing pericardial effusion
  • Absent lung sliding -> pneumothorax vs mainstem intubation vs bulla vs pleural adhesion (confirm with lung point)
  • Fluid above the diaphragm -> haemothorax vs pleural effusion vs ascites seen through the diaphragm
  • Negative FAST with instability -> retroperitoneal haemorrhage, pelvic fracture bleeding, hollow-viscus or thoracic/extremity source

Pearls & Pitfalls

  • A negative FAST never excludes injury - retroperitoneal, hollow-viscus and diaphragmatic injuries produce little or no free fluid
  • Repeat scanning increases sensitivity; a single early negative scan can miss slowly accumulating blood
  • In the unstable patient a positive abdominal FAST is an indication to operate, not to obtain CT
  • Perinephric fat, a fluid-filled stomach or gallbladder can mimic free fluid - trace anatomy carefully
  • Left-sided fluid collects in the subphrenic space before the splenorenal recess, so scan cephalad on the left
  • Mirror-image and edge-shadowing artefacts around the diaphragm can be mistaken for haemothorax
  • Epicardial fat pad can mimic a small pericardial effusion - it is echogenic and only anterior
  • The stable patient with a positive FAST should proceed to CT for organ-specific characterisation and grading

Structured Report

  • State indication, device used and who performed the scan
  • Report each of the four windows as positive, negative or indeterminate for free fluid
  • Describe location and qualitative volume of any fluid detected
  • State pericardial findings and any signs of tamponade
  • For eFAST, report lung sliding, pneumothorax and haemothorax findings per hemithorax
  • Give an overall FAST result (positive/negative/equivocal) and correlate with haemodynamic status
  • Recommend next step: laparotomy if unstable and positive, CT if stable and positive, and repeat FAST or CT if negative with ongoing concern

References

  • ACS/ATLS (Advanced Trauma Life Support) guidance on FAST in the primary survey
  • WHO Guidelines for Essential Trauma Care
  • ACEP Emergency Ultrasound Guidelines (FAST/eFAST)
  • RadioGraphics: US in Blunt Abdominal Trauma reviews
  • Royal College of Radiologists ultrasound training standards

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.