Point-of-Care Ultrasound: FAST

The focused assessment with sonography for trauma (FAST) is a rapid, bedside, goal directed ultrasound examination done by the treating clinician during the primary or secondary survey of an injured patient, to detect free intraperitoneal fluid (haemoperitoneum) and pericardial effusion (haemopericardium). It is repeated as needed. A 2 to 5 MHz curvilinear probe (or a phased array probe) is used.

The classical FAST examination has four windows:

• The right upper quadrant (hepatorenal space, Morison pouch, with the right subphrenic space and the inferior pole of the liver and paracolic gutter).
• The left upper quadrant (splenorenal space and subphrenic space).
• The suprapubic window (pouch of Douglas in women or rectovesical pouch in men, with the bladder as an acoustic window).
• The subxiphoid or parasternal cardiac window (pericardium).

The extended FAST (eFAST) adds bilateral anterior chest views for pneumothorax (absence of lung sliding) and the pleural spaces for haemothorax. The examination takes 2 to 5 minutes, with no radiation, and is done while resuscitation continues.

Trauma is a leading cause of death in people under 45 years, and haemorrhage is the commonest preventable cause of death. FAST has a sensitivity of 65 to 95% (higher in hypotensive patients) and specificity of 95 to 99% for detecting significant haemoperitoneum in blunt abdominal trauma, and reduces time to operation, use of CT and use of diagnostic peritoneal lavage.

In haemodynamically unstable patients with a positive FAST, direct transfer to the operating room is indicated. In stable patients, a negative FAST does not exclude injury (solid organ injury without free fluid, retroperitoneal, hollow viscus, and diaphragm injuries are missed), and CT is required as the mechanism and clinical picture dictate. eFAST detects pneumothorax with a sensitivity of 80 to 95% (higher than supine chest radiograph at 40 to 50%) and specificity of 95 to 100%.

Indications. Blunt abdominal or chest trauma with hypotension or suspected internal bleeding. Penetrating torso trauma (precordial, thoracoabdominal; detects pericardial effusion). Unexplained shock in trauma. Multiple trauma with altered consciousness. Pregnant trauma patients (the first line imaging modality). Paediatric trauma (with caution; lower sensitivity).

Serial examinations after initial negative FAST. Non-trauma uses of the same views: undifferentiated hypotension or shock, ruptured ectopic pregnancy (free pelvic fluid), ruptured aneurysm, ascites, and cardiac tamponade. Ultrasound control of procedures (pericardiocentesis, paracentesis, thoracentesis, chest drain placement).

Not for ruling out injury in a stable patient with a high risk mechanism.

Contraindications and precautions. None; it must never delay resuscitation or necessary surgery. Subcutaneous emphysema, dressings, wounds and obesity limit views. The patient is supine, and full bladder views are best before catheter insertion (fluid filled bladder).

If the bladder is empty or a catheter is in place, the pelvic window is less sensitive and the bladder can be filled with 200 to 300 mL saline through the catheter (if urethral injury is excluded). A trained operator (documented competence) is needed; findings are recorded as positive, negative or indeterminate.

Preparation and technique. Each window is scanned in two planes. The right upper quadrant: probe at the mid to posterior axillary line in the 8th to 11th intercostal spaces; the hepatorenal interface (Morison pouch), diaphragm, subphrenic space and inferior liver tip are inspected, sweeping through the kidney and liver.

The left upper quadrant: probe in the posterior axillary line at the 6th to 9th intercostal spaces (higher and more posterior than the right); the spleen, kidney, splenorenal recess and subphrenic space are inspected. The suprapubic window: transverse and longitudinal views through the bladder.

The subxiphoid window: probe flat against the abdomen aimed at the left shoulder, to show the four chambers and pericardium (alternatively, parasternal long axis).

eFAST: probe in the 2nd to 3rd intercostal spaces in the midclavicular line, in the longitudinal orientation. Look for the pleural line between two ribs ("bat" sign), lung sliding, comet tail artefacts (B-lines), seashore sign on M-mode versus barcode sign in pneumothorax, and the costophrenic angles for pleural effusion.

Normal appearances. The liver, spleen and kidneys are outlined, with echogenic Gerota fascia and perinephric fat and no anechoic stripe between organs. Hepatorenal and splenorenal interfaces are tightly apposed, with the diaphragm seen as a bright curvilinear line, and the mirror image artefact of the liver across the diaphragm.

The bladder is a symmetrical, anechoic structure with posterior acoustic enhancement. The pericardium is a thin echogenic line hugging the myocardium, with the heart contracting. Pleural sliding is present on the anterior chest (the "sliding lung sign"), with the "seashore" pattern on M-mode.

Interpretation.

• Positive FAST (free intraperitoneal fluid): anechoic (black) stripe or wedge in the hepatorenal space, splenorenal space, around the bladder or in the pelvis (posterior to the bladder or uterus), or between bowel loops.
• Fresh blood may appear anechoic, and clotted blood is echogenic or heterogeneous. As little as 200 to 500 mL of fluid can be detected (more in the pelvis; small amounts of fluid in the pelvis may be physiological in women).
• The first collection typically forms in the hepatorenal space (the most dependent intraperitoneal space in the supine position), then spreads to the paracolic gutters and pelvis. Fluid in the splenorenal space or over the spleen appears later.
• Quantification: a small stripe (under 1 cm in the hepatorenal space) suggests small volume, a wide stripe (over 2 cm) or fluid in multiple windows suggests a large haemoperitoneum (over 1000 mL) and correlates with the need for laparotomy.
• Pericardial effusion/haemopericardium: anechoic stripe between the pericardium and the heart (more than 1 cm is moderate to large); signs of tamponade include right atrial and right ventricular diastolic collapse, dilated inferior vena cava, a small hyperdynamic left ventricle, and swinging heart.
• Pneumothorax (eFAST): absent lung sliding, absent B-lines, barcode (stratosphere) sign on M-mode, and the lung point (the transition between sliding and non-sliding lung, which is specific), seen in the anterior chest in a supine patient.
• Haemothorax or pleural effusion: anechoic space over the diaphragm with loss of the mirror image, with a floating lung, spine sign (visualisation of the vertebral column over the diaphragm), and atelectatic lung.
• Solid organ injury: sometimes seen directly (heterogeneous echotexture, capsular disruption, subcapsular haematoma), but FAST is not reliable for this.
• Non-trauma findings: ascites, aortic aneurysm, hydronephrosis, full bladder, gallstones, ectopic pregnancy (free fluid with empty uterus in a woman with positive pregnancy test), and cardiac activity.
• Indeterminate: obscured views, bowel gas, and fat; repeat.

Critical values and findings needing immediate action.

• Positive FAST in a hypotensive or unstable trauma patient: immediate transfer to the operating room for laparotomy, with continued resuscitation and activation of the massive transfusion plan.
• Haemopericardium or tamponade in penetrating or blunt trauma: immediate pericardiocentesis (as a bridge) or thoracotomy/sternotomy.
• Tension pneumothorax (absent sliding with haemodynamic compromise): immediate needle decompression and chest drain; do not wait for imaging.
• Large haemothorax: chest drain, with consideration of thoracotomy if over 1500 mL initial output or over 200 mL each hour.
• Positive FAST in a stable patient: CT of the abdomen and pelvis with contrast to define injuries, or observation depending on local practice.
• Negative FAST in an unstable patient: look for other sources of shock (retroperitoneal or pelvic bleeding, long bone fracture, chest, spinal shock, external bleeding); repeat FAST.
• Free fluid with no solid organ injury (hollow viscus injury): surgical review.
• Pregnant trauma patient with free fluid: urgent obstetric and surgical review, foetal monitoring.

Limitations and artefacts.

• A negative FAST does not exclude injury: sensitivity for solid organ injury without haemoperitoneum is low (a third to a half of significant injuries have no free fluid early).
• It does not reliably detect injury to hollow viscera, diaphragm, pancreas, retroperitoneal structures (kidney, pelvic fracture bleeding), or small volumes (under 200 to 500 mL). A repeat FAST after 15 to 30 minutes and CT in stable patients are used.
• Operator dependence is large; sensitivity falls with inexperienced operators and in obese patients, subcutaneous emphysema, bowel gas, and previous abdominal surgery.
• False positives result from non-traumatic fluid (ascites, peritoneal dialysis fluid, ovarian cyst, physiological pelvic fluid, ruptured cyst, pregnancy, bladder diverticulum, fluid filled bowel, seminal vesicles, unrelated pleural effusion).
• False positives also result from normal structures mimicking fluid (gallbladder, inferior vena cava, aorta, stomach contents, fat pad, perinephric fat, the pericardial fat pad), and artefacts (side lobe artefact, refraction).
• A pericardial fat pad can be mistaken for effusion, and a pleural effusion can be mistaken for pericardial effusion (the descending aorta is a landmark).
• In the pelvis, an empty bladder reduces sensitivity; the bladder should be full for views.
• Pneumothorax assessment requires lung sliding to be absent over the intercostal space, and false absence of sliding occurs with main stem intubation, pleural adhesions, bullae, apnoea, severe emphysema, acute respiratory distress syndrome, or pleurodesis.
• Penetrating trauma: FAST is less sensitive for hollow viscus and diaphragm injuries; the pericardium window is the highest yield view for precordial wounds.
• Paediatric trauma: lower sensitivity (about 50 to 60%) because children may have solid organ injury without free fluid; small amounts of fluid can be physiological.
• It does not quantify fluid precisely or show the source of bleeding; it also does not replace a CT.

Special groups.

• Children: FAST is limited in sensitivity; clinical examination, laboratory markers, and CT (if stable and indicated by criteria) are used; avoid unnecessary radiation.
• Pregnancy: FAST is the preferred initial study; the uterus, placenta and foetal heart activity can be seen; free fluid in the pouch of Douglas can be normal.
• Older adults and anticoagulated patients: delayed bleeding and solid organ injury; lower threshold for CT.
• Patients with cirrhosis or ascites: free fluid is not specific; compare with prior studies and examine for echogenic blood.
• Obese patients: reduced sensitivity; low frequency probe and repeated examinations.
• Cardiac arrest: modified subxiphoid view during pulse checks to identify reversible causes (tamponade, hypovolaemia, pulmonary embolism) with minimal interruption of compressions.
• Regions: trauma burden is high (road traffic injuries, violence), CT may be unavailable, and FAST is widely used for triage and referral. Training and quality assurance are key. Abdominal and cardiac conditions such as ruptured ectopic pregnancy, ruptured amoebic abscess, typhoid perforation, and splenic rupture in malaria also are detected.

Follow up tests. Repeat FAST, CT of the head, neck, chest, abdomen and pelvis with intravenous contrast in stable patients, chest radiograph and pelvic radiograph, urgent blood tests (haemoglobin, group and cross-match, coagulation, lactate, base deficit), viscoelastic tests (thromboelastography), diagnostic peritoneal lavage when ultrasound and CT are unavailable,

echocardiography for cardiac injury, troponin and electrocardiogram in blunt chest trauma, urine dipstick for haematuria, angiography and embolisation for solid organ or pelvic bleeding, serial abdominal examination and observation, and surgical referral.