The focused assessment with sonography for trauma (FAST) examination is a safe, portable, noninvasive diagnostic tool to investigate for hemoperitoneum, hemopericardium, and hemothorax in trauma victims. The FAST exam can help detect free fluid (ie, hemoperitoneum in the hypotensive patient); however, the FAST exam cannot exclude solid organ injury. Hemoperitoneum in the pediatric trauma patient is usually a result of hepatic or splenic injury and will appear as an anechoic (dark) collection on ultrasound (US). The primary indications for use of the FAST exam are blunt or penetrating thoracic or abdominal trauma.
Use a low-frequency (1–5 MHz) curvilinear abdominal or phased-array probe or higher-frequency (6–12 MHz) linear transducers. The latter will provide better resolution as pediatric patients have thinner abdominal walls and less subcutaneous fat. Perform the scan with the patient in the supine position. Direct probe indicator toward the patient’s head or the patient’s right side. When imaging the right upper quadrant (RUQ) and the left upper quadrant (LUQ), rotate the probe counterclockwise during scanning to allow for successful imaging between intercostal rib spaces.
Obtain the following four views:
Subxiphoid view of the heart
Hepatorenal space (Morison pouch) in the right upper quadrant (RUQ)
Splenorenal recess, as well as the subphrenic space, in the left upper quadrant (LUQ)
Rectovesicular pouch (males)
Pouch of Douglas (females)
RUQ and pelvic views are routinely performed first in the pediatric trauma patient as these are most sensitive in detecting intraperitoneal free fluid
Figure 21.1 ▪ FAST Exam Ultrasound Windows.
(A) The 4 US windows of the FAST exam: (1) The right upper quadrant view of the hepatorenal recess; (2) the left upper quadrant view of the splenorenal recess; (3) the subxiphoid cardiac view; and (4) the suprapubic view. (B) Probes Used for Ultrasound. Few probes that one may use in their clinical practice are shown. A. Low-frequency curved array or convex probe. B. Low-frequency phased array or sector probe. C. High-frequency linear array probe. D. High-frequency endocavitary probe. (Reproduced with permission from Ma OJ, Mateer JR, Blaivas M: Emergency Ultrasound, 2nd ed. McGraw-Hill, New York, 2008 [A].) (Photo contributor: Dimitrios Papanagnou, MD [B].)
Figure 21.2 ▪ A Positive Right Upper Quadrant FAST View.
The liver (L) is visualized on the left side of the image with the right kidney (K) on the right side of the image; between the 2 organs, an anechoic stripe is visualized, representing free fluid (FF or hemoperitoneum) in a patient sustaining blunt trauma. (Photo contributor: James Tsung, MD.)
Figure 21.3 ▪ A Positive Left Upper Quadrant FAST View.
(A) Blood from a splenic injury will first be visualized above the spleen, in the left subphrenic space (double arrows). Note the spleen on the left side of the image with the hyperechoic (bright) diaphragm overlying it on the left; multiple anechoic pockets can be identified above the spleen (arrows) and between the spleen and the left kidney (cursors), representing free fluid. (B) This is an image taken from the LUQ during a FAST exam. Notice the anechoic fluid surrounding the spleen in the splenophrenic recess (red arrow), with a small amount of fluid at the tip of the spleen (small arrow) and no distinguishable fluid in the splenorenal recess. (Photo contributors: James Tsung, MD [A] and Michael Secko, MD, RDMS [B].)
Free fluid in RUQ appears as an anechoic collection between the liver and the right kidney. This dependent space is the most sensitive view to detect the hemoperitoneum, and it is the most important view. On the LUQ view, blood from a splenic injury appears first above the spleen in the left subphrenic space. If the volume is large enough, blood will collect in the splenorenal space and soon flow to other dependent spaces in the abdomen (ie, hepatorenal space, suprapubic space).
Use suprapubic imaging to visualize the bladder in both transverse and sagittal views. The bladder is a fluid-containing, anechoic structure on US, and serves as an acoustic window to detect free fluid in the dependent space below it. Use the pubic symphysis as an anatomic landmark for probe placement. Obtain a transverse view with the probe indicator directed toward the patient’s right side and a sagittal view with the probe indicator directed toward the patient’s head. Suprapubic imaging also helps to assess bladder volume before the placement of Foley catheters or sterile catheterizations.
Obtain a subxiphoid view to detect pericardial fluid, particularly in penetrating trauma. Direct the probe marker to the patient’s right side with the transducer below the xiphoid process and aim toward the patient’s left shoulder. Pericardial fluid appears as a dark, anechoic stripe between the heart and the liver.
Figure 21.4 ▪ A Positive Suprapubic FAST View.
Note the anechoic, urine-filled bladder (transverse view) with anechoic pockets inferior to it (arrows), representing free fluid in this pediatric patient with hemoperitoneum. (Photo contributor: Dimitrios Papanagnou, MD, MPH.)
Figure 21.5 ▪ A Positive Subxiphoid FAST View.
Note the echoic liver in the near field (top) of the image, with the heart in the far field (bottom) of the screen. Between the 2 organs, an anechoic stripe is visualized, representing pericardial fluid. (Photo contributor: James Tsung, MD.)