10 Things I hate about you: FAST edition
/FAST - Focused Assessment with Sonography in Trauma. A tool used daily by emergency medicine providers to assess the unstable trauma patient. While touted as an easy and quick way to look for hemorrhagic causes of instability in the trauma patient, the FAST exam can be deceptively tricky. The exam has many pitfalls that can cloud the picture or make image acquisition difficult. When doing a time sensitive exam in high stakes situations there are opportunities for interpretation errors. We all love the FAST – it helps us expedite dispositions, activate resources quickly, and provide better care for our patients; but when it all doesn’t come up roses the FAST exam can really drive you nuts.
Below is a list of 10 common mistakes that occur while performing the FAST exam, along with quick tips to mitigate these errors…
1. Failure to image the entire right upper quadrant.
When obtaining the Morison’s pouch, or right upper quadrant (RUQ), view be sure to include the caudal edge of the liver, or liver tip, and the subdiaphragmatic space. While free fluid most commonly layers evenly throughout the hepatorenal recess, without including the caudal edge of the liver small volumes of free fluid can be easily missed. Traditional teaching has been that Morison’s pouch was the only view needed for assessing the RUQ as that was the most likely place for free fluid to accumulate. A study completed by Lobo et al. revealed that the caudal edge of the liver is the most common location for free fluid to accumulate when the RUQ was broken down into subquadrants. Furthermore, the caudal edge of the liver and superior right paracolic gutter is the most sensitive location for finding free fluid (1).
Figure 1. Incompletely imaged RUQ on the left, excluding the caudal most portion of the liver. A small amount of fluid is visible in the clip on the left, which becomes quite obvious when the liver tip is captured in the clip on the right.
2. Ignoring the left upper quadrant.
Blunt splenic injury accounts for 25% of injuries in blunt abdominal trauma and of these, 25% do not have intraperitoneal hemorrhage (2). Thus, it might be tempting to skip over this view. Obtaining an adequate LUQ , which includes the left subdiaphragmatic space, splenorenal recess, and left paracolic gutter, is also more challenging than the RUQ view. O’Brien et al. demonstrated that 32% (n = 32) of patients with positive FAST exams had fluid in the LUQ, and of these patients 6% had free fluid isolated to the LUQ. None of these patients demonstrated fluid isolated to the splenorenal fossa, as traditionally taught, and five of the six did not demonstrate any fluid in the splenorenal fossa despite having free fluid in at least one other subquadrant of the LUQ (3). So, the LUQ cannot be ignored and all of the areas, the splenorenal space, subdiaphragmatic space and the left paracolic gutter should all be assessed. Ignoring the LUQ in its entirety would be a grave mistake and potentially increase morbidity and mortality for the patient.
3. Forgetting the subdiaphragmatic space.
As we’ve already seen above, free fluid follows no rules and as such may not layer in the hepatorenal or splenorenal recesses, as traditionally taught, but rather in the subdiaphragmatic spaces above the liver and spleen. Notably free fluid on the left abdomen preferentially flows into the subdiaphragmatic space, not the splenorenal space. Lobo et al. demonstrated that the left subdiaphragmatic space is the most sensitive left subquadrant of the abdomen for detection of free fluid though the data is not as strong for the right subdiaphragmatic space (1). By moving the probe cephalad in both the RUQ and LUQ views to include the diaphragm you will be able to evaluate for fluid in the subdiaphragmatic spaces as well as in the thorax. [Pro tip: Consider placing the patient in Trendelenberg positioning to detect smaller volumes of fluid than would be needed in the supine position.]
Figure 2. Right upper Quadrant clip that captures a strip of free fluid in the subdiaphragmatic space, which is not visible in the initial portion prior to fanning cephalad.
4. Misinterpretation of the stomach
The stomach is tricky, as it is a hollow viscous that will vary in its appearance depending on the nature of its contents. Trauma patients always seem to have consumed a huge meal and a six pack right before suffering an injury, often leading to a fluid-filled stomach. This gastric fluid can be misinterpreted as a positive FAST. Intraperitoneal free fluid should be anechoic, may appear spikey or irregular in contour, and fill the space between the spleen and kidney. Fluid contained within the stomach will appear round and will not appear in the subdiaphragmatic space because the stomach is not in a dependent portion of the abdomen (4). By fanning the probe cephalad and posteriorly to include the subdiaphragmatic space, misinterpretation of gastric fluid can be avoided.
Figure 3. Left upper quadrant view with a glimpse of the stomach as the operator fans anteriorly from spleno-renal recess. The stomach often appears as an irregularly outlined structure with variable echo texture depending on its current contents.
5. Skipping assessment of the bladder in two orthogonal planes
Free fluid will settle in the most dependent portions of the pelvis; in men this is the potential space inferior to the posterior wall of the bladder and in women this is the Pouch of Douglas, or the space just posterior to the uterus. Given that the pelvis is the most dependent portion of the abdomen this is the view that is most likely to deem your FAST positive. The transverse view, while providing excellent views of the bladder, can miss small amounts of free fluid in the pelvis that become obvious on the sagittal view, which provides a slightly better view of surrounding anatomy. One view is no view, so consider the exam incomplete without both the sagittal and transverse planes.
Figure 4. Transverse view of the pelvis with small area of plausible free fluid, however, difficult to completely exclude underlying vessel without color doppler. The addition of a transverse view demonstrates an irregular anechoic collection deep to the bladder, increasing the confidence for free fluid.
6. Pericardial effusion or fat pad?
With rising rates of obesity come further diagnostic dilemmas for medical providers as these patients have increasing incidence of pericardial fat and difficult ultrasound windows. Pericardial fat, especially on subxiphoid view, can appear anechoic and be easily mistaken for a pericardial effusion. In a study looking at the ability of attending and resident physicians to discern between fat pad and effusion the accuracy in difficult windows was a measly 30% (5). The key to determining whether the image contains a pericardial fat pad or an effusion is to obtain further cardiac views. An effusion is more likely to wrap around the apex of the heart, whereas a fat pad is more likely to be present in one view (6). The parasternal long view is considered the most accurate view for detecting pericardial effusion, so this would be a good second view. Another quick second view of the pericardial contents is done by obtaining an IVC view. The IVC view reveals the most dependent portion of the heart in the supine position, the right heart, where any non-loculated effusion should be seen (5).
Figure 5. Pericardial effusion versus fat pad. The image on the left demonstrates a pericardial effusion after blunt trauma to the chest, compared to an epicardial fat pad on the right. The effusion can be seen to wrap around the apex, while the fat pad is confined to the anterior surface on this view.
7. Mis-interpreting clot as a negative FAST.
Coagulated blood can have the same echogenicity as soft tissue, such as the liver or spleen, making interpretation of the images difficult if clot is present. It is important to understand abdominal anatomy and proximity of surrounding structures to aid in detection of intraperitoneal clot, and a positive FAST. Ongoing hemorrhage can appear as a swirling mix of clot and free fluid which appears as an heterogenous ill-defined area of mixed echogenicity or isoechoic (6). A good tip for learners: If you are having trouble identifying a structure, briefly consider clot as the source of confusion. Concern for clot on one view should encourage more thorough views of the remaining quadrants to assess for free fluid in other places.
Figure 6. Longitudinal pelvic view demonstrating a rounded body with mixed echogenicity posterior to the bladder concerning for a pelvic hematoma. The lack of a consistent anechoic texture can complicate interpretation of these sorts of findings.
8. Subcutaneous emphysema - the ultimate enemy.
Many trauma patients suffer thoracic injuries that result in the presence of subcutaneous emphysema. As air rapidly expands along subcutaneous tissue planes, ultrasound images will become uninterpretable - air is a strong reflector due to the mismatch in acoustic impedance of air where it meets tissue. Unfortunately, there are no tricks to foil the laws of physics. Recognizing displaced air as the cause of difficulty when obtaining images can save valuable time in the quest to get the patient to definitive care. Also, periodically, subcutaneous air in the chest wall may be mistaken for the pleural line or the curtain sign, thus mimicking or obscuring underlying pathology. Identifying ribs, then pleural line, every time when doing a thoracic exam will help you avoid this pitfall (7).
9. Hemoperitoneum or ascites?
What could be more confusing than a patient with liver failure injured in a car accident on his way to his therapeutic paracentesis presenting with hypotension in need of a FAST exam? The FAST exam is great for detecting free fluid in the abdomen and thorax, but does not differentiate between types of free fluid (such as ascites or blood) as they all may appear hypoechoic. There are also documented cases of patients receiving heroic resuscitations prior to arrival at definitive care and noted to have false positive FAST exams due to development of ascites (8). Clinicians can make the make the mistake of falsely assuming that the free fluid they are visualizing is blood. There is no fool-proof method of differentiating free fluid on ultrasound and in select cases, a diagnostic peritoneal aspiration may be indicated. It is generally best practice to assume free fluid is blood in the setting of trauma, while letting your clinical assessment, other clinical information and perhaps a diagnostic paracentesis guide you.
10. Failure to repeat
Trauma patients are dynamic and their exams change with time and resuscitation. The hypotensive trauma patient with an initially negative FAST that undergoes significant resuscitation and is persistently hypotensive warrants a second look, especially if other sources of hypotension are not identified. Free fluid takes times to accumulate and given the many pitfalls we’ve seen above, there’s a lot more nuance than meets the eye. The FAST is also gloriously easy to repeat, safe, and serial FAST exams are well supported across multiple organizations (9).
The FAST exam can be deceptively difficult at times. When images aren’t working out, try a different view and revert back to the anatomy you knew so well as a medical student. You have spent many hours crafting your clinical judgement and the FAST exam should be a well-honed, reliable accessory to this knowledge. Holding yourself and your skills to a high standard will keep you doing a great job every time.
Authored by Susan Owens, MD
Dr. Owens is a PGY-4 at the University of Cincinnati Emergency Medicine residency, and soon-to-be Global Health fellow in Washington D.C.
Faculty edits by Lori Stolz, MD RDMS
Dr. Stolz is faculty at the University of Cincinnati Emergency Medicine residency and director of the Ultrasound fellowship.
References
Lobo V, Hunter-Behrend M, Cullnan E, et al. Caudal Edge of the Liver in the Right Upper Quadrant (RUQ) View Is the Most Sensitive Area for Free Fluid on the FAST Exam. West J Emerg Med. 2017;18(2):270–280. doi:10.5811/westjem.2016.11.30435
El-Matbouly M, Jabbour G, El-Menyar A, et al. Blunt splenic trauma: Assessment, management and outcomes. Surgeon. 2016 Feb;14(1):52-8. doi: 10.1016/j.surge.2015.08.001. Epub 2015 Aug 30.
Obrien KM, Soltz L, Amini R et al. Focused Assessment With Sonography for Trauma Examination: Reexamining the Importance of the Left Upper Quadrant View. JUM. 2015; 34(8): 1429-1434. doi.org/10.7863/ultra.34.8.1429
Nagdev A, Racht J. The “gastric fluid” sign: an unrecognized false-positive finding during focused assessment for trauma examinations. Am J Emerg Med. 2008; 26(5): 630.e5-630.e7. doi.org/10.1016/j.ajem.2007.09.013
Blaivas M, DeBehnke D, Phelan MB. Potential Errors in the Diagnosis of Pericardial Effusion on Truama Ultrasound for Penetrating Injuries. Ac EMerg Med. 2000; 7(11): 1261-166
Section Links. Ultrasound in Trauma - The FAST Exam Focused Assessment with Sonography in Trauma. https://www.acep.org/sonoguide/FAST.html.
Ma O, Mateer J et al. Emergency Ultrasound. 3rd Edition. McGraw-Hill Education. 2014.
Tsui, C.L., Fung, H.T., Chung, K.L. et al. Focused abdominal sonography for trauma in the emergency department for blunt abdominal trauma. Int J Emerg Med 1, 183–187 (2008) doi:10.1007/s12245-008-0050-2
Blaivas M, Debehnke D, Phelan MB. Potential Errors in the Diagnosis of Pericardial Effusion on Trauma Ultrasound for Penetrating Injuries. Academic Emergency Medicine. 2000;7(11):1261-1266. doi:10.1111/j.1553-2712.2000.tb00472.x
