FAST of the Abdomen – Beyond the Basics: False Positives, Limitations, and Approach to the Unstable Patient


Introduction

The development of focused assessment with sonography for trauma (FAST) over 30 years ago enabled clinicians to rapidly screen for injury at the bedside of patients, especially those hemodynamically unstable for transport to the computed tomography (CT) suite. The identification of free fluid within the peritoneal cavity, pericardium, and pleural spaces can be accomplished with point-of-care ultrasound (US) immediately upon patient arrival to the hospital. Other applications of FAST include detection of solid organ injury, pneumothorax, fractures, and serial examinations, as well as use in prehospital transport and multiple casualty settings as a triage tool. Before FAST, invasive procedures such as diagnostic peritoneal lavage and exploratory laparotomy were commonly utilized to diagnose intraabdominal injury (IAI). Today the FAST examination has evolved into a more comprehensive study of the abdomen, heart, chest, and inferior vena cava, and many variations in technique, protocols, and interpretation exist.

US was first utilized for the examination of trauma patients in the 1970s in Europe. It was not widely adopted in North America until the 1990s, during which time the FAST acronym was defined as “focused abdominal sonography for trauma.” As FAST evolved into a more comprehensive examination, the acronym was changed to “focused assessment with sonography for trauma.” Since then, FAST has become the common initial screening modality in trauma centers worldwide, and it is included in the Advanced Trauma Life Support (ATLS) program for evaluation of the hypotensive trauma patient. A unique aspect of FAST is that it is routinely utilized by radiologists, emergency physicians, surgeons, and paramedics with variable training and experience. Over time, a new role for FAST has evolved, in which its use in the evaluation of unstable, hypotensive trauma patients is emphasized. The most effective use of FAST has been rapid triage of hemodynamically unstable trauma patients to definitive intervention.

In the mid-2000s, the addition of US evaluation of the thorax to detect pneumothorax to the traditional FAST examination resulted in a new acronym, “e-FAST,” or extended FAST. Several other protocols have been developed for evaluation of shock, respiratory distress, and cardiac arrest, some of which feature echocardiography. The number of different protocols for evaluation of the critically injured or ill patient is a source of confusion, especially as even more protocols are developed with creative acronyms and abbreviations. In this chapter we will focus on the abdominal portion of the overall FAST examination, as point-of-care evaluation of the heart and lungs is covered in separate chapters.

Ultrasound Technique

The original FAST scan included views of the right upper quadrant, which included the perihepatic area and hepatorenal recess (Morison’s pouch), of the left upper quadrant encompassing the perisplenic view, the suprapubic view (pouch of Douglas), and later a subxiphoid pericardial view ( Fig. 15.1 ). The preferred initial site for detection of free fluid with FAST is the right upper quadrant view using a lower-frequency (3.5–5 MHz) curved array transducer. The liver serves as an acoustic window during insonation of the hepatorenal space and liver parenchyma. In the left upper quadrant view, the spleen is the target during examination of the splenorenal fossa and perisplenic area. Cephalad scanning enables visualization of the left pleural space. Moving the probe caudally brings the inferior pole of the left kidney and paracolic gutter into view. The perisplenic area may be inadequately scanned due to difficult physical access. Rolling the patient to the right side and/or having the patient take a deep breath is helpful in evaluating this area, as small amounts of free fluid may collect superiorly to the spleen. The suprapubic view enables visualization of the most dependent space in the peritoneal cavity. The transducer is placed above the pubic symphysis in a sagittal plane and swept side to side, then rotated transversely and repeated. Reverse Trendelenburg positioning may enhance the detection of free fluid in the pelvis. The visualization of small amounts of free fluid in the pelvis is aided by the presence of a full bladder. When free fluid is present, it is most frequently located posterior or superior to the bladder and/or the uterus. Free fluid in the pelvis can be missed when a Foley catheter is placed to empty the bladder, as the acoustic window for examining the pelvis is removed, limiting the sonographer’s ability to discern small amounts of pelvic fluid. Bowel gas, subcutaneous emphysema, and obesity represent common obstacles to full sonographic visualization.

Fig. 15.1, Probe placement. The original FAST scan included views of the (A) right upper quadrant, which included the perihepatic area and hepatorenal recess (Morison’s pouch); (B) left upper quadrant encompassing the perisplenic view; (C) the suprapubic view (pouch of Douglas); and, later, (D) a subxiphoid pericardial view.

The volume of free fluid necessary to enable detection with FAST is a well-known limitation. In one early study, the mean minimum detectable free-fluid volume in 100 patients undergoing diagnostic peritoneal lavage was 619 mL in the right upper quadrant view. Trendelenburg positioning may improve visualization of free fluid in the splenorenal and hepatorenal interface. FAST performed in the Trendelenburg position enables detection of smaller amounts of hepatorenal free fluid than supine. Even smaller volumes of free fluid are required for detection in the pelvic views of FAST, with median minimal volume of fluid of 100 mL. Scoring systems have been described in the past, and the common theme among these studies is the larger the amount and number of sites of free fluid, the greater the likelihood of injury or need for surgical intervention.

Hemoperitoneum

Free fluid detected by US is assumed to be hemoperitoneum, which usually appears anechoic or hypoechoic compared with adjacent solid organs. During prolonged periods, this hemorrhage may organize into clots and become more echogenic-appearing on US. In the absence of injury or other pathology, free fluid should not be found within the peritoneal cavity on US, with the exception of the suprapubic view: only small amounts of rectouterine (pouch of Douglas) free fluid in women of childbearing age should be detectable. These small free-fluid collections of up to 50 mL in the pouch of Douglas are considered physiologic, and amounts exceeding 50 mL should be regarded as pathologic in the setting of trauma. As the initial FAST represents a “snapshot” in time, serial examinations performed on stable blunt trauma patients may be useful ( Fig. 15.2 ). Examination after stabilization gives the sonographer more time for a comprehensive scan. With active intraperitoneal hemorrhage, the amount of free fluid should theoretically increase with time. Serial FAST examinations have been reported to decrease the false-negative rate by up to 50% and may be a logical alternative for stable trauma patients, patients with a sudden change in hemodynamic status or physical examination, and pregnant patients to mitigate radiation exposure.

Fig. 15.2, Serial FAST in a 44-year-old man with blunt abdominal trauma from a motor vehicle accident with abdominal pain. (A) Initial CT scan was interpreted as normal. Slight inhomogeneity of the spleen was thought to be due to normal enhancement of splenic pulp. (B) Nine hours later, the patient developed hypotension, and a bedside FAST examination was performed, which demonstrated free fluid in the upper abdomen ( arrow ) and pelvis. K , Kidney; L , liver. (C) Real-time images showed marked heterogeneity to the spleen. (D) Color flow demonstrated a fairly avascular appearance of the spleen. (E) Patient was resuscitated and underwent CT, during which a large spleen laceration with subcapsular hematoma and free fluid was detected. Patient was rushed to the operating room for successful emergency splenectomy.

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