Focused Assessment with Sonography for Trauma (FAST) scan

Focused Assessment with Sonography for Trauma (FAST) scan is a point-of-care ultrasound examination performed at the time of presentation of a trauma patient.

It is invariably performed by a clinician, who should be formally trained, and is considered as an 'extension' of the trauma clinical assessment process, to aid rapid decision making. Some studies have shown no significant difference in diagnostic accuracy between radiologists and non-radiologists .

The chief aim of the study, in a trauma patient, is to identify intraperitoneal free fluid (assumed to be hemoperitoneum in the context of trauma) allowing for an immediate transfer to theater, CT or other. Solid organ injury is seldom identified, and when present may warrant further investigation.

Many papers have been published detailing the pros and cons of this investigation . FAST scanning has a reported sensitivity of ~90% (range 75-100%) and a specificity of ~95% (range 88-100%) for detecting intraperitoneal free fluid . Sensitivity for detecting solid organ injuries is much lower.

Most studies in the emergency medicine literature dictate that peritoneal free fluid will not be identified by ultrasonography until more than 500 mL is present. Therefore, a negative exam will not preclude a bleed which will eventually become significant. Moreover, mesenteric vascular injuries, solid organ injuries, hollow viscus injuries, and diaphragmatic injuries may not result in free intraperitoneal fluid, and thus may not be detected .

It has replaced diagnostic peritoneal lavage as the preferred initial method for assessment of hemoperitoneum.

In several recent studies, the sensitivity and specificity of thoracic ultrasonography use for the detection of pneumothorax after blunt injury was 86-98% and 97-100%, respectively, outperforming the supine AP chest x-ray .

Technique

• patient in supine position
• 3.5-5.0 MHz convex transducer
• five regions may be scanned :
  • pericardial view: commonly referred to as the subcostal or subxiphoid view
    • to examine the pericardium, the liver in the epigastric region is most commonly used as a sonographic window to the heart
    • the potential space between the visceral and parietal pericardium is examined for a pericardial effusion
    • if anatomical factors preclude epigastric probe placement, parasternal or apical four-chamber views may be used
  • right flank view
    • commonly referred to as the perihepatic view, Morison pouch view or right upper quadrant view
    • four potential spaces are sequentially examined for the accumulation of free fluid
    • the hepatorenal interface (Morison pouch) is first identified, with subsequent assessment of the more cephalad subphrenic and pleural spaces
    • visualization of the inferior pole of the kidney, which is a continuation of the right paracolic gutter, defines the caudad extent of an adequate view
  • left flank view
    • commonly referred to as the perisplenic or left upper quadrant view
    • four potential spaces are sequentially examined in an analogous fashion to the right flank, albeit the splenorenal interface is assessed on the left
  • pelvic view
    • commonly referred to as the suprapubic view, this space is the most dependent peritoneal space in the supine trauma patient
    • a transverse sweep, using the bladder as a sonographic window, the pouch of Douglas or rectovesical space is explored for free fluid

An extended FAST or "eFAST" scan is now standard of care, and is performed by incorporating two views assessing the anterior thorax :

• anterior pleural views
  • the anterior pleura is assessed for the presence or absence of lung sliding as a sensitive, but non-specific, indicator of a traumatic pneumothorax
  • the probe is placed in a sagittal orientation in the midclavicular line between the clavicle and diaphragm
  • anterior and lateral interrogation of interspaces 5-8 bilaterally is recommended

Causes of false negatives

• obesity: severely limits assessment of the peritoneal cavity
• subcutaneous emphysema
• posterior acoustic enhancement caused by the fluid-filled bladder can result in free fluid being missed in the pelvic view

Causes of false positives

• epicardial fat pads, the descending aorta, and pericardial cysts have been mistakenly identified as an effusion
• pre-existing ascites, pleural, and pericardial effusions due to medical conditions
• seminal vesicles mistaken for pelvic free fluid in the young male patient

History and etymology

Ultrasound was first utilized for the examination of trauma patients in the 1970s in Europe