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.

FAST Ultrasound: Trauma Assessment (Mobile)

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References