Chapter 5. Trauma Ultrasound: The FAST Exam

Evaluation of blunt trauma patients with ultrasound (US) has been described for over 30 years.1,2 Its use in the United States in the early assessment of blunt abdominal trauma patients rapidly increased in the 1990s.1 It is currently taught as an adjunct to the secondary survey in the Advanced Trauma Life Support (ATLS) course.1,3 US evaluation of the trauma patients decreases the time to operative care, resource utilization, and the costs of blunt trauma patients.4,5 The focused assessment with sonography in trauma exam, also known as the FAST exam, can be completed within 5 minutes.1 It has essentially replaced the need for a diagnostic peritoneal lavage in the initial assessment of all but a few trauma patients.6,7 This chapter reviews the technique and interpretation of the FAST exam.

The FAST exam evaluates four anatomical areas or potential spaces for the presence or absence of intraperitoneal fluid. These include the hepatorenal recess (Morrison's pouch), the splenorenal recess, the rectovesical or rectouterine space, and the pericardial space. The assumption is that fluid represents blood in the setting of trauma. The presence of ascites, urine, and bowel fluid can appear similar. These spaces represent the most dependent areas in the supine patient (Figure 5-1). The volume of fluid accumulation required for visualization by US ranges from 250 to 620 mL.8,9 More experienced sonographers are able to visualize volumes closer to 250 mL.8

The FAST exam is performed after the primary survey. It can be performed in conjunction with ongoing resuscitative efforts. It is indicated when evaluating for the presence of intraperitoneal or pericardial blood in the setting of acute thoracoabdominal trauma. It is useful in determining resource allocation in the setting of multipatient trauma scenarios.1

Blunt Thoracoabdominal Trauma

The most studied use of the FAST exam has been in adult patients with blunt abdominal trauma. The early diagnosis of hemoperitoneum and/or hemopericardium in the setting of blunt trauma is critical in the management of these patients. The sensitivity and specificity for the presence of hemoperitoneum varies depending on the comparison “gold standard,” and is generally reported between 78% to 90% and 98% to 100%, respectively.7,10–14 It has been reported to approach 100% sensitivity and 100% specificity in hypotensive patients.6,10 One study by Miller et al. reported a sensitivity of 42% and a specificity of 98%.15 The difference between these studies was how CT was utilized as the gold standard. Pediatric blunt trauma patients have been less well studied, but the FAST exam has shown a similar sensitivity and specificity.16–19 When used to predict the need for operative intervention in pediatric trauma patients, its sensitivity approached 90%, but its specificity dropped.20 Current guidelines recommend evaluation of the pericardial space in patients with blunt thoracoabdominal trauma.21

Penetrating Thoracoabdominal Trauma

Decreased mortality using US in the assessment of trauma was first described in relation to penetrating chest trauma.22 Its application in patients with penetrating abdominal trauma is less clear. The sensitivity ranges from 48% to 100%, but its specificity remains high at 98% to 100%.13,14,23,24 Assessment of the pericardial space can be helpful in guiding operative intervention in the unstable patient.

US is a noninvasive diagnostic modality. In the setting of trauma, it is contraindicated only if it would delay and negatively impact a clinically obvious need for emergent operative intervention. The FAST exam should not be performed unsupervised by providers who have not been adequately trained.21

• US machine
• US gel
• Abdominal US probe
• US probe cover or glove

A complete discussion of US equipment is beyond the scope of this chapter. Decisions regarding machines and probes depend on the user, cost, and intended applications. The FAST exam can be performed with a good-quality general abdominal probe. Probes with smaller footprints allow for easier viewing between the ribs.

The US probe is generally used directly against the patient's skin, with gel between them. Place the probe in a probe cover or glove to prevent contamination of the probe if the patient's skin is covered with blood, urine, feces, or other substances. Place US gel in the probe cover or glove before inserting the probe. Squeeze out any air in the space between the tip of the probe and the probe cover or glove.

Little to no preparation is required to perform the FAST exam. Wipe any debris and liquids from the patient's skin in the areas to be scanned. Place the US probe in a probe cover or glove, as described above, if the patient's skin is contaminated with blood, vomit, other body fluids, or other substances that may contaminate or damage the probe.

Performance of any US exam assumes the probe marker orientation is generally cephalad or toward the right relative to the patient. The FAST exam traditionally demonstrates two-dimensional grayscale images of four views reflective of their anatomic potential spaces. These are the hepatorenal recess (Morrison's pouch), the splenorenal recess, the rectovesical or rectouterine space, and the pericardial space. View the rectovesicle space, the area between the rectum and the bladder, in the male patient. View the rectouterine space, the area between the rectum and the uterus, in the female patient.

The overall exam time should be less than 5 minutes, and frequently can be completed in 1 or 2 minutes. If complete views of either the liver or spleen are not visualized with a single image, more than one image of the area is required. The order of image acquisition depends on the mechanism of injury. In blunt trauma, Morrison's pouch is generally imaged first (the area where fluid will most likely first accumulate), followed by the splenorenal view, the rectovesical or rectouterine view, and then the pericardial space. In penetrating trauma, the pericardial space is often imaged first followed by Morrison's pouch, then the remaining two views.

Hepatorenal Recess (Morrison's Pouch)

The hepatorenal recess is located between the inferior margin of the liver and inferior pole of the right kidney (Figure 5-1A). Place the probe on the midaxillary line at the level of the 8th to 11th ribs (Figure 5-2A). The liver and kidney should be visualized from the diaphragm to the inferior tip of the liver (Figure 5-2B). It is important to visualize the space between, and including, the inferior liver tip and the inferior pole of the right kidney as fluid tends to accumulate near the tip of the liver first. A black, echolucent, stripe between the liver and the kidney represents blood and a positive FAST exam (Figure 5-3). Rib shadows obscuring portions of the image may be prevented by a slight counter-clockwise rotation of the probe so it lies between the ribs.

Splenorenal Recess

The splenorenal recess is located between the inferior margin of the spleen and the inferior pole of the left kidney (Figure 5-1B). Place the probe on the posterior axillary line at the level of the sixth to ninth ribs (Figure 5-4). The spleen and kidney should be visualized from the diaphragm to the inferior tip of the spleen. A black, echolucent, stripe between the spleen and the kidney represents blood and a positive FAST exam (Figure 5-5). Rib shadows obscuring portions of the image may be prevented by a slight clockwise rotation of the probe so it lies between the ribs.

Rectovesical or Rectouterine Space

The rectovesical or rectouterine space represents the most inferior–posterior reflection of the peritoneum. Place the probe just superior to the pubic ramus (Figure 5-6). The probe can be oriented in either the sagittal or transverse plane. View the rectovesicle space (Figures 5-6A & B) or the rectouterine space (Figures 5-6C & D) in both the sagittal and transverse planes. A black, echolucent, stripe in either space represents blood and a positive FAST exam (Figures 5-7 & 5-8).

Pericardial Space

In the supine trauma patient, fluid accumulates initially in the inferior–posterior portion of the pericardial space. The probe and screen image orientation are different than traditional echocardiographic views. Place the probe with the marker oriented toward the patient's right and the tip of the probe aimed toward the patient's left shoulder (Figure 5-9). Use the liver as an acoustic window and increase the depth of the US beam to increase image quality. Fluid will initially accumulate near the top of the US image, representing the inferior portion of the pericardial space (Figure 5-9 inset). Visualize both the inferior and superior areas of the pericardial space. A black, echolucent, stripe in either the inferior or superior areas of the pericardial space represents blood and a positive FAST exam (Figure 5-10).

The incorporation of thoracic imaging coupled with the traditional FAST exam has been referred to as an extended FAST or EFAST exam. This additional imaging assesses for the presence of a hemothorax or a pneumothorax. A hemothorax is best assessed by visualizing the inferior–posterior aspect of the plural cavity in the supine patient (Figure 5-11). This area is often visualized in routine views of Morrison's pouch, but moving the US probe superior one rib interspace may be required. The lung edges are not normally visualized on US. However, the lung can be seen “floating” in fluid in the presence of a hemothorax (Figure 5-11B). Another sign suggestive of a hemothorax is the “spine” sign (Figure 5-11B). In the presence of a hemothorax, the portion of the thoracic spine above the diaphragm can be visualized with US. The thoracic spine is not normally well visualized on US superior to the diaphragm (Figure 5-11C).

A pneumothorax is best assessed in the supine patient with either a linear or a curvilinear probe. Place the probe on the anterior chest wall in the area bounded by the clavicle, sternum, nipple, and anterior axillary line. Start with the probe on the midclavicular line between the nipple and the clavicle. It may be moved within the area to obtain the best image possible. Position the probe perpendicular to two adjacent ribs to visualize the intercostal space (Figure 5-12). The normal sliding of the visceral pleura against the parietal pleura during respiration can be visualized (Figure 5-12A). This interface between the two pleura is known as the “sliding-lung” sign and is absent in a pneumothorax.

Use M-Mode to visualize the “sea-shore” sign (Figure 5-12B). This is visualized as slightly wavy parallel lines from the thoracic wall over an echogenic line (the pleural line), under which is a sandy pattern produced by the lung parenchyma. The parallel lines at the top of the screen represent the waves of the “sea.” The sandy pattern at the bottom of the screen represents the “shore.” Together they form the “sea-shore” sign. The echogenic line between the “sea” and the “shore” represents the pleural lines. The “sea-shore” sign indicates a normal chest wall–lung interface. Air interposed between the chest wall and lung forms parallel, horizontal echogenic lines and the loss of the “sea-shore” sign (Figure 5-12C).

The interpretation of each of the four views, and of the overall FAST exam, is classified into one of the three categories: positive, negative, or indeterminate. Positive exams demonstrate echolucent (seen as black on the image) fluid accumulations in any one of the four views. Negative exams and scans demonstrate no fluid with complete visualization of all structures of each view. Indeterminate exams are uncommon but may occur when fat or bowel cannot be distinguished from peritoneal fluid, or if there is incomplete visualization of any of the four views. Women and children may normally have a small amount of physiologic fluid in their pelvis. This finding may lead to an indeterminate exam in the setting of trauma.

Multiple algorithms exist for the management of trauma patients.1,3 In general, these are differentiated into blunt trauma versus penetrating trauma and hemodynamically stable versus unstable. In the hemodynamically stable blunt trauma patient, a positive FAST exam is typically followed with an abdominal cat exam. In the hemodynamically unstable adult blunt trauma patient, a positive FAST exam indicates the need for emergent operative intervention without further imaging. In adult patients with penetrating trauma, a positive FAST exam has good correlation with need for a therapeutic laparotomy.23 Management of these patients may vary between institutions. An unstable pediatric patient with a positive FAST exam may not necessarily proceed to laparotomy as the indications for operative management differ between adults and children.3

A negative FAST exam needs to be evaluated within the patient's clinical context (history, exam findings, vitals, etc.). In the hemodynamically stable or unstable penetrating trauma patient, a negative FAST exam is a reliable screen for the presence of intrapericardial blood. It is not a reliable screening test for the presence of abdominal injury. In the unstable patient, its use is not well defined. In the hemodynamically stable blunt trauma patient, a negative FAST exam should be followed by serial clinical exams and a repeat FAST exam.25 A negative FAST exam in the presence of hemodynamic instability has a high sensitivity, and an extraperitoneal source of bleeding or the instability should be considered. Indeterminate FAST exams require a follow-up alternative diagnostic modality, usually a CAT exam.

Performance of the actual FAST exam has no associated complications. However, inaccurate interpretation or inappropriate application of US findings in clinical decision making may complicate its use.

The FAST exam is a useful adjunct in the management of blunt and penetrating trauma patients. It can be performed rapidly, it is very specific, and it can positively impact patient outcomes. Proper training is required to ensure good-quality images and appropriate interpretation.