Abstract
Ultrasound offers a quick, repeatable, and moderately sensitive assessment for an important subset of traumatic injuries. The Focused Assessment with Sonography for Trauma (FAST) exam is performed at the bedside without the need to transport a potentially unstable patient away from a monitored setting. The investigation is now incorporated into ATLS guidelines.
Introduction
Ultrasound offers a quick, repeatable, and moderately sensitive assessment for an important subset of traumatic injuries. The Focused Assessment with Sonography for Trauma (FAST) exam is performed at the bedside without the need to transport a potentially unstable patient away from a monitored setting. The investigation is now incorporated into ATLS guidelines.
FAST and EFAST Exams
The FAST exam involves views of the right upper quadrant, left upper quadrant, suprapubic area, and heart. An assessment is made to determine the presence or absence of intraperitoneal fluid (i.e., hemoperitoneum) and hemopericardium. The extended FAST, or EFAST, also includes thoracic views looking for hemothorax and pneumothorax. The goal of these exams is to detect life-threatening injuries in the primary survey requiring immediate intervention. The FAST/EFAST exams do not assess the retroperitoneum, nor are they designed to detect injuries to specific organs.
Advantages
The FAST exam is performed at the patient’s bedside during the primary survey. Unlike CT or comprehensive ultrasound, the patient is not transported out of the trauma bay, thus allowing for uninterrupted monitoring of the patient (Figure 25.1).
Figure 25.1 How various diagnostic modalities in trauma compare in terms of their ability to detect injuries in each of the main compartments of the torso, as well as in their depiction of anatomic detail.
The FAST exam is performed quickly, often in under one minute, and can be repeated as often as necessary should the patient’s condition deteriorate, or to detect the evolution of the patient’s injuries.
Unlike CT, the FAST exam contains no ionizing radiation – an important concern particularly in vulnerable populations such as pediatrics or pregnancy.
A positive FAST exam leads to more rapid transport of the hemodynamically unstable patient to the operating room, and earlier use of blood transfusion, thus expediting patient care.
Limitations
The FAST is user-dependent; thus an experienced operator might identify a small amount of free fluid that would be overlooked by a novice user.
The FAST exam is not designed to directly identify organ injury. Instead, the FAST looks for the presence or absence of free fluid – a frequent but not universal finding with many specific organ injuries. Furthermore, FAST does not directly assess the retroperitoneum and thus is not designed to evaluate potential blood loss in that compartment.
A number of patient factors can limit the views of the FAST exam, including obesity, bowel gas, and subcutaneous emphysema. In addition, preexisting medical conditions (e.g., ascites, pleural effusions, fluid from a ruptured ovarian cyst) can be detected by the FAST exam and mistakenly assumed to be caused by trauma, prompting surgical intervention when none was warranted. Even small physiologic free fluid such as in a female patient of childbearing age can be seen and misattributed to trauma.
While a positive FAST in an unstable patient is very useful information prompting operative intervention, a negative FAST is not sufficient to exclude important injury. Thus a negative FAST in a stable patient may require CT scan; a negative FAST in an unstable patient may require DPA or further resuscitation in the trauma bay until the patient is stable for CT scan (Figure 25.2).
Equipment and Protocol
Both cart-based and boom-mounted ultrasound machines can be used to perform the FAST exam. The boom-mounted option is attractive due to its decreased footprint in the trauma bay, leaving room for other critical tasks such as intravenous access and tube thoracostomy (Figure 25.3).
Figure 25.3 Boom-mounted equipment with a “zero footprint” leaves more space at the patient’s bedside to perform life-saving procedures and is preferred during a crowded resuscitation. Mounting the machine near the monitoring equipment allows all providers in the room to view and interpret images in real time.
A low-frequency transducer (e.g., curvilinear, phased, or microconvex) with a typical frequency range of 1–5 or 2–5 MHz ensures adequate penetration to visualize intra-abdominal and intra-thoracic structures. Depth settings vary, depending on the patient’s size, but typically are set to 20–25 cm.
The sequence of FAST views depends on mechanism of injury sustained by the patient. For penetrating trauma, it is (1) cardiac, (2) right upper quadrant (RUQ), (3), left upper quadrant (LUQ), (4) suprapubic, and (5) lungs. For blunt trauma, it is (1) RUQ, (2) LUQ, (3) suprapubic, (4) cardiac, and (5) lungs. The rationale for this approach has to do with the types of cardiac injuries that are encountered (Figure 25.4).
In penetrating trauma, the heart is the priority because a patient with hemopericardium can rapidly decompensate; thus these injuries must be identified as soon as possible. In blunt trauma, the incidence of blunt cardiac injury producing a survivable injury that can be detected with the FAST is very small.
Individual FAST Views
Right Upper Quadrant View
The RUQ view images the right hemidiaphragm, liver, right kidney, hepatorenal space (Morison’s pouch), and caudal liver edge (Figure 25.5 A,B).
Figure 25.5 A,B Normal RUQ FAST view with diaphragm, liver, and kidney, with Morison’s pouch located between liver and kidney. The liver edge is off the screen to the right in this view and requires the operator to slide the transducer caudally (A). Illustration depicting the typical location of free fluid in the RUQ FAST view, starting at the caudal liver tip and progressing into Morison’s pouch. A subphrenic fluid collection is rare in trauma and more commonly seen with the large amounts of fluid, as with medical ascites (B).
The earliest place for fluid to appear is at the caudal liver edge, but fluid is often easier to visualize in Morison’s pouch. Rarely, fluid may accumulate between the liver and the right hemidiaphragm, but this finding is unusual in trauma and much more likely to be due to preexisting ascites (Figure 25.6 AC).
