Focused Assessment with Sonography for Trauma (FAST) Exam: Image Acquisition

Summary

The Focused Assessment with Sonography for Trauma (FAST) exam is a diagnostic point-of-care ultrasound examination used to screen for the presence of free fluid in the pericardium and peritoneum. Indications, techniques, and pitfalls of the procedure are discussed in this article.

Abstract

Over the past twenty years, the Focused Assessment with Sonography for Trauma (FAST) exam has transformed the care of patients presenting with a combination of trauma (blunt or penetrating) and hypotension. In these hemodynamically unstable trauma patients, the FAST exam permits rapid and noninvasive screening for free pericardial or peritoneal fluid, the latter of which implicates intra-abdominal injury as a likely contributor to the hypotension and justifies emergent abdominal surgical exploration. Further, the abdominal portion of the FAST exam can also be used outside of the trauma setting to screen for free peritoneal fluid in patients who become hemodynamically unstable in any context, including after procedures that may inadvertently injure abdominal organs. These "non-trauma" situations of hemodynamic instability are often triaged by providers from specialties other than emergency medicine or trauma surgery who are not familiar with the FAST exam. Therefore, there is a need to promulgate knowledge about the FAST exam to all clinicians caring for critically ill patients. Toward this end, this article describes FAST exam image acquisition: patient positioning, transducer selection, image optimization, and exam limitations. Since the free fluid is likely to be found in specific anatomic locations that are unique for each canonical FAST exam view, this work centers on the unique image acquisition considerations for each window: subcostal, right upper quadrant, left upper quadrant, and pelvis.

Introduction

The Focused Assessment with Sonography for Trauma (FAST) exam is a diagnostic point-of-care ultrasound (POCUS) exam of the torso designed to rapidly assess potentially life-threatening hemorrhage in trauma patients¹. The FAST exam was one of the earliest POCUS techniques to achieve widespread adoption: it was first developed in the 1980s in Europe and spread to the United States in the early 1990s. As POCUS became more commonly utilized in the evaluation of trauma patients, a consensus conference was held in 1997, which standardized the definition of the FAST exam and its role in the care of trauma patients. Over time, some authors have advocated for adding a focused ultrasound exam of the lung to the traditional FAST exam and have termed this multi-organ exam the extended FAST (e-FAST) exam².

The primary role of both the classical FAST and its newer iteration, e-FAST, is in the initial evaluation of trauma patients³. Hemodynamic instability in traumatically injured patients is commonly caused by a limited number of conditions, including primary hemorrhage, cardiac tamponade, and tension pneumothorax^3,4. As a part of the ACBDE steps of the Advanced Trauma Life Support(ATLS) primary survey, the Circulation step looks to identify and treat the life-threatening causes of hemodynamic instability in trauma patients^3,5,6. This step includes ruling out cardiac tamponade and intracavitary bleeding in the pleural spaces and peritoneum, among other sources^6,7. The FAST exam allows for visualization of free fluid in the pericardium and peritoneum, and with e-FAST views, bilateral pleural spaces^3,6,7. In the clinical picture of hemodynamic instability after major trauma, this fluid is presumed to be blood until proven otherwise.

As a point-of-care ultrasound examination, the FAST/e-FAST exam offers several advantages. The exam can be performed using small portable ultrasound machines at the patient's bedside while other care is ongoing and without requiring the transport of the patient ³. The limited views using B-mode technique means that a complete examination can be obtained rapidly within a few minutes, and the noninvasive nature of the ultrasound exam means that the exam can be easily repeated if the patient's clinical picture changes^3,8,9.

At the same time, the simple nature of the FAST exam has several limitations. Like any ultrasound examination, it is operator dependent to obtain appropriate views and accurate interpretation of the images in real-time⁹. Various patient factors, including obesity, and subcutaneous emphysema, may limit the ability to acquire adequate images. Additionally, the simplified views of the FAST/e-FAST exams do not look for specific organ injuries but rather screen for free fluid in the various body compartments. In the appropriately selected trauma patient, this free fluid is likely to represent blood from ongoing hemorrhage but may represent other fluid from traumatic or non-traumatic medical conditions.

Given the advantages and limitations of the FAST/e-FAST exams, their primary indication is in evaluating hemodynamically unstable patients who have suffered blunt trauma. For this patient population, the primary goal is to identify traumatic sources of hemodynamic instability, such as cardiac tamponade and intracavitary hemorrhage, which require immediate operative intervention. In this role, it has replaced diagnostic peritoneal lavage (DPL) as the primary modality for diagnosing intraperitoneal hemorrhage and physical examination and challenges the chest X-ray for diagnosing intrapleural hemorrhage and pneumothorax¹. With their rapid and noninvasive nature, the FAST/e-FAST exams have been used in other trauma patients, including hemodynamically stable blunt trauma patients and penetrating trauma patients, both stable and unstable. However, the indications for and interpretation of these exams remain less clear.

Outside of the trauma setting, the FAST exam may have value in several different crisis management situations, including but not limited to any of the following: triaging the severity of obstetric hemorrhage¹⁰, searching for the location of perioperative bleeding, screening for peri-procedural bladder rupture, and as part of the preoperative assessment of patients with suspected but unconfirmed ascites scheduled for elective surgery^11,12,13. In these non-trauma contexts, the providers available to perform the FAST exam are likely to come from specialties like obstetrics, anesthesiology, internal medicine, and critical care, for whom FAST exam training is highly variable in residency/fellowship curricula^13,14,15,16. It is these non-trauma specialties that form the target audience of this review. Some of these non-trauma specialties tend to either have existing expertise in lung ultrasound (e.g., intensivists¹⁷) or have reasons to perform the abdominal views of the FAST exam in isolation (e.g., anesthesiologists and obstetricians)¹⁰. For these reasons and because the lung views of the e-FAST exam are already comprehensively covered in a separate manuscript¹⁸, this review will focus primarily on image acquisition for the abdominal views of the FAST exam. Despite this, it is worth emphasizing that, in the trauma setting, sonographic examination of the lung is, in many hospitals, considered a core part of the FAST protocol (i.e., e-FAST is the form of the FAST exam preferred by some trauma providers).

Protocol

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The patients provided written informed consent for participating in the study. Patient inclusion criteria: any patient with hemodynamic instability or abdominal pain/distension. Patient exclusion criteria: patient refusal.

1. Transducer selection

1. Select a low-frequency linear transducer (1-5 MHz) (see Table of Materials) to visualize organs deeper than 6 cm in the body^1,19.
2. If available, select the curvilinear low-frequency probe as its wide footprint maximizes spatial resolution of intra-abdominal organs.
3. If the curvilinear probe is not available, select any low-frequency probe such as a sector array probe (aka "phased array probe", see Table of Materials).
   NOTE: The sector array probe is sometimes colloquially called the "phased array probe". However, this latter colloquial term is misleading because all modern ultrasound transducers (including even linear high-frequency transducers) use electronic phasing to steer the ultrasound beam^20,21,22, so what distinguishes the sector array from other ultrasound probes is not that it is a "phased array" (all modern transducers are) but that this probe traces out a sector arc. But because many point-of-care ultrasound providers use the term "phased array" to refer to the sector array probe, this manuscript will mention both terms. However, to those interested in the mechanics of how ultrasound machines work, sector array probe is the technically accurate name and is already widely used by ultrasound experts outside of the point-of-care ultrasound world^{18,23,24,25,26}.
4. If the e-FAST exam is being performed and is being used to screen for pneumothorax, use a linear high-frequency probe (≥ 5 MHz, see Table of Materials) for that application and then resume using a low-frequency probe for the remainder of the FAST/e-FAST exam.

2. Machine settings and machine placement

1. Mode
   1. Select abdominal mode, which will place the indicator on the left screen and maximize spatial resolution while minimizing temporal resolution.
      NOTE: In contrast to "abdominal" mode, "cardiac" mode will maximize temporal resolution at the expense of spatial resolution, settings that are optimal for visualizing the fast-moving structures within the heart but unhelpful for visualizing the slow-moving structures in the abdomen or screening for gross fluid in the pericardial sac.
2. Machine placement
   1. Place the ultrasound machine on either the patient's left or right side but ensure that the sonographer has a direct line of sight to both the machine's screen and to the patient simultaneously so that the operator can manipulate both the ultrasound probe and the ultrasound machine settings concurrently.
3. Image acquisition preset
   1. Set the ultrasound machine's image acquisition technique to prospective collection. If the operator prefers "retrospective collection, " they will need to reverse the order of any paired steps involving fanning the ultrasound probe and click Acquire before image acquisition.

3. Patient positioning

1. Position the patient supine with the chest and abdomen exposed¹.
2. For Right Upper Quadrant (RUQ) and Left Upper Quadrant (LUQ) views, abduct the patient's arms at least 5 inches away from their body to allow access for the ultrasound probe to reach the patient's flanks.

4. Scanning technique

1. Apply gel to the ultrasound probe prior to attempting each view.
2. Point indicator mark cranially for coronal or sagittal views and toward the patient's right side for transverse views.

5. FAST exam cardiac views

1. Subxiphoid (aka subcostal) 4-chamber view
   1. Place the probe on the anterior abdominal wall just caudal to the xiphoid process in the midline or slightly to the patient's right¹.
   2. Orient the ultrasound beam transversely with the indicator to the patient's right and the probe nearly flat against the patient's abdomen and directed towards the patient's left shoulder¹ (Figure 1).
   3. Adjust probe positioning and screen depth to obtain a view of the four cardiac chambers visualized in the center of the ultrasound image (Figure 2; Video 1).
   4. Adjust the gain until the intracardiac blood appears uniformly black (anechoic) with just a few specks of grey²⁷.
   5. Click on Acquire.
   6. Inspect the circumference of the heart for a similar dark hypoechoic stripe around the myocardium (Figure 2; Video 2).
2. Parasternal long-axis view (optional)
   NOTE: In some patients, the subxiphoid window may provide ambiguous findings or inadequate visualization of the pericardium due to abdominal obesity or distended/gas-filled stomach¹. In these circumstances, the parasternal window may provide an alternative window to screen for pericardial effusion.
   1. Place the probe along the left sternal border just caudal to the clavicle with the indicator mark pointing toward the patient's left hip (Figure 3).
      NOTE: The transducer indicator mark is pointed toward the patient's left hip and not toward the right shoulder, as would be done when performing transthoracic cardiac ultrasound because the entire FAST exam is traditionally performed in "abdominal" rather than "cardiac" mode.
   2. While keeping the probe indicator pointed toward the patient's left hip, slide (translate) the probe caudally examining each rib interspace until the heart disappears and making a note of which interspaces provided a useful view of the heart.
   3. Slide (translate) the probe back cranially to the interspace or spaces that provide the best visualization of the heart.
   4. Adjust probe positioning to obtain a view with the following structures visible: descending thoracic aorta, left atrium, left ventricle, left ventricular outlow tract, right ventricle, and pericardium (Figure 4; Video 3).
   5. Adjust screen depth so that at least 3-6 cm of depth is visible deep to the descending thoracic aorta (Figure 4; Video 3; Video 4).
   6. Adjust gain as mentioned in step 5.1.4.
   7. Click on Acquire.
   8. Inspect the circumference of the heart for a dark hypoechoic stripe that dissects into the plane between the heart and the descending thoracic aorta (Figure 4).

6. FAST exam abdominal windows

1. Right upper quadrant (RUQ) window
   1. Place the ultrasound probe in the coronal plane on the patient's right side along the mid-to-posterior axillary line in the 7^th to 9^th intracoastal space with the probe indicator towards the patient's head (Figure 5)^1,28.
   2. Adjust the probe positioning to obtain a view containing the following structures: (1) liver; (2) right kidney; (3) hepato-renal interface (a potential space also called Morison's pouch) (Figure 6; Video 5)¹.
   3. Adjust screen depth so that the hepato-renal interface occupies the middle third of the screen (Figure 6; Video 5).
   4. Adjust the gain until the liver and kidney appear slightly hyperechoic (tissue echogenicity) but not so dark as completely black and not so bright that they are indistinguishable from their hyperechoic capsules (Figure 6; Video 5). Click on Acquire.
   5. Fan through the hepato-renal interface anteriorly to posteriorly and back during the video acquisition (Video 6).
   6. Inspect the hepato-renal recess for a hypoechoic or anechoic stripe between the caudal-most tip of the liver and the inferior pole of the kidney, as this is the most sensitive site for detection of free peritoneal fluid in both the RUQ and usually the entire FAST exam in a supine patient²⁹ (Figure 6; Video 7).
   7. If the initial view is negative, continue the search for fluid by sliding (translating) the probe caudally into the paracolic gutter and/or cranially to view the hepato-diaphragmatic space between the liver and the diaphragm^28,29 (Video 8).
      1. From the cranial-most RUQ view, visualize the right pleural space cranial to the diaphragm allowing the operator to easily perform this component of the e-FAST exam as a logical extension of the conventional FAST exam^1,28,29 (Video 9).
2. Left upper quadrant (LUQ) window
   1. Place the ultrasound probe in the coronal plane on the patient's left flank along the mid-to-posterior axillary line in the 5^th to 7^th intracoastal space with the probe indicator towards the patient's head^1,28(Figure 7).
   2. Adjust the probe positioning to obtain a view containing the following structures: (1) spleen; (2) diaphragm; and (3) if possible, the spleno-renal interface (Figure 8; Video 10).
   3. Adjust screen depth so that the spleno-diaphragmatic interface occupies the middle third of the screen (Figure 8; Video 10).
   4. Adjust gain as indicated in step 6.1.4, but replace liver with spleen in the instructions (Figure 8; Video 10). Click on Acquire.
   5. Fan through the interface between the spleen and diaphragm anteriorly to posteriorly and back during the video acquisition (Video 11).
   6. Inspect the interface for a hypoechoic or anechoic stripe between the spleen and diaphragm and between the spleen and left kidney (Figure 8; Video 12).
   7. If the spleno-renal interface was inadequately visualized in steps 6.2.5-6.2.7, slide (translate) the probe caudally until the spleno-renal interface is visualized and repeat steps 6.2.5-6.2.7 but this time focusing on the spleno-renal rather than the spleno-diaphragmatic interface (Video 13).
   8. To examine the left pleural space (i.e., if performing an e-FAST exam), slide (translate) the probe cranially until the view is centered on the diaphragm^1,28 (Video 14).
3. Suprapubic (pelvic) window
   NOTE: Since a fluid-filled bladder provides an excellent medium for the transmission of ultrasound waves, imaging the pelvis before insertion of a foley catheter or clamping the foley catheter to allow filling of the bladder can improve image acquisition^1,28.
   1. Transverse suprapubic (pelvic) view
      1. Position the ultrasound probe in the transverse plane with the indicator mark pointing to the patient's right side, place the probe just cranial to the pubic symphysis, and angle the ultrasound beam 10-20 degrees caudally into the pelvis^1,28 (Figure 9).
      2. Adjust the probe positioning to obtain a view containing the following sex-specific structures.
      3. If the patient is female:
         1. Adjust the probe to visualize the following structures: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas)² (Figure 10).
         2. Adjust screen depth so that the uterus occupies the middle third of the screen (Figure 10; Video 15).
         3. Adjust screen gain so that the urine in the bladder appears relatively anechoic (black) and the space deep into the bladder is distinct from the posterior bladder wall (Figure 10; Video 15).
      4. If the patient is male:
         1. Adjust the probe to visualize the following structures: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch)² (Figure 11; Video 16).
         2. Adjust screen depth so that the bladder occupies the middle third of the screen (Figure 11; Video 16).
         3. Adjust the screen gain so that the urine in the bladder appears relatively anechoic (black) and the space deep into the bladder is distinct from the posterior bladder wall (Figure 11; Video 16).
      5. Click on Acquire. Fan across the pelvis posteriorly to anteriorly during the video acquisition (Video 17).
      6. Inspect the view for an anechoic stripe in the peri-uterine/rectouterine space if the patient is female (Figure 10B; Video 18) and in the recto-vesical space if the patient is male (Figure 11B; Video 19).
   2. Sagittal suprapubic (pelvic) view
      1. Starting with the transverse view above (6.3.1.1), rotate the ultrasound probe 90 degrees clockwise until the ultrasound beam is in the sagittal plane with the indicator mark pointing to the patient's head and keep the ultrasound beam angled 10-20 degrees caudally into the pelvis^1,28 (Figure 12).
      2. Adjust the probe positioning to obtain a view containing the following sex-specific structures.
      3. If the patient is female:
         1. Adjust the probe to visualize the following structures: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas)² (Figure 13; Video 20).
         2. Adjust screen depth so that the uterus occupies the middle third of the screen (Figure 13; Video 20).
         3. Adjust screen gain so that the urine in the bladder appears relatively anechoic (black) and the space deep to the bladder is distinct from the posterior bladder wall (Figure 13; Video 20).
      4. If the patient is male:
         1. Adjust the probe to visualize the following structures: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch)² (Figure 14; Video 21).
         2. Adjust screen depth so that the bladder occupies the middle third of the screen (Figure 14; Video 21).
         3. Adjust the screen gain so that the urine in the bladder appears relatively anechoic (black) and the space deep into the bladder is distinct from the posterior bladder wall (Figure 14; Video 21).
      5. Click on Acquire. Fan across the pelvis left-to-right and back during the video acquisition (Video 22).
      6. Inspect the view for an anechoic stripe in the peri-uterine/rectouterine space if the patient is female (Video 23) and in the recto-vesical space if the patient is male (Video 24).

Representative Results

Four sonographic windows are typically used to obtain the traditional FAST exam views¹⁹. The windows are subcostal 4-chamber (SC4C), right upper quadrant (RUQ), left upper quadrant (LUQ), and suprapubic/pelvic. Although the windows can be imaged in any order, the exam is typically performed in the following order: SC4C, RUQ, LUQ, and then suprapubic/pelvic^1,19. This is because pericardial tamponade is usually more rapidly life-threatening than abdominal bleeding and because the RUQ view is the most sensitive site for the detection of fluid in the abdomen, regardless of the site of injury². If the SC4C view is indeterminate for free pericardial fluid, the parasternal long-axis view can be added to the exam protocol, as explained below.

Subxiphoid (aka subcostal) 4-chamber (SC4C) and Parasternal Long-Axis (PLAX) views
The SC4C view is part of the imaging sequence for both the focused cardiac ultrasound and the FAST/e-FAST exams^12,30. As part of the FAST/e-FAST exam, the main goal of the SC4C view is to screen for the presence of free pericardial fluid. When present, the fluid is typically present between the liver and the right ventricle, as shown in Figure 2B and Video 2. The presence of free pericardial fluid makes the SC4C FAST exam view "positive." In contrast, a "negative" SC4C FAST exam view is one that does not contain visible pericardial fluid, as shown in Figure 2A and Video 1.

Although traditionally, the SC4C view of the FAST exam has simply been scored either positive or negative for the presence of pericardial effusion^1,31, some operators with advanced training in cardiac ultrasound may also be qualified to screen for specific signs of cardiac tamponade (e.g., right atrial collapse during ventricular systole, right ventricular collapse during ventricular diastole, etc.)²⁷. But the minimum number of training studies required to accurately detect these specific signs of cardiac tamponade has yet to be defined³². And it is known that it typically takes a minimum of 30 cardiac ultrasound exams before a novice ultrasound user can even reliably detect the presence versus absence of pericardial effusion³². Given these issues, most FAST exam users should, at minimum, strongly consider cardiac tamponade when the following "sonographic Beck's triad" is present: (1) hypotension; (2) a moderate or larger pericardial effusion (> 1cm between the parietal and visceral pericardium); and (3) a fixed and dilated inferior vena cava (IVC)^27,33. For details on image acquisition of the IVC, please see the relevant article by Hoffman et al.²⁵.

Notably, at least two common conditions can be seen in the SC4C view that are easy to mistake for free peritoneal fluid: an epicardial fat pad and ascites. Classically, providers are taught that epicardial fat may be distinguished from pericardial fluid in the SC4C view as follows: (1) fat moves synchronously with the heart, whereas hemopericardium usually moves independently and (2) fat typically appears more "speckled" than does blood³³ (Video 25). However, these criteria are highly subjective and thus prone to error, even by experienced providers³⁴. Rather than relying on these subjective criteria, some authors have proposed adding the parasternal long-axis (PLAX) view to the FAST exam³⁴, a step that we have found highly useful (Figure 3,4; Video 3,4). In the PLAX view, hemopericardium and epicardial fat pads are usually very easy to distinguish: epicardial fat lies anterior to the right ventricle, whereas hemopericardium typically settles in the most gravity-dependent portion of the view: between the descending thoracic aorta and the heart³⁵ (Video 26).

In the SC4C view, ascites can also easily be misinterpreted as pericardial effusion. To differentiate the two conditions in the SC4C view, some sonographic clues can be helpful: pericardial fluid follows the contours of the heart, whereas ascites follows the contours of the liver and usually contains the undulating falciform ligament along the exact midline of the body³⁶. However, these heuristics do not always provide a clear answer when evaluating the SC4C view alone. In these situations, peri-cardiac fluid that was visible in the SC4C view, fails to manifest in the PLAX view, and is likely peritoneal rather than pericardial.

For all of the above reasons, the PLAX view is a useful adjunct to the FAST exam, which typically only visualizes the heart in the subcostal window. Further, the PLAX view tends to be the easiest view for novice providers to obtain consistently³⁷. In contrast, alternative cardiac views like the apical 4-chamber are consistently the hardest for ultrasound learners to obtain effectively³⁷.

Right Upper Quadrant (RUQ) view
The RUQ view is considered "negative" when it reveals no free peritoneal fluid (Figure 6A; Video 5). In contrast, a "positive" RUQ exam is one that shows free fluid (Figure 6B; Video 7). Notably, in a supine patient, the most sensitive portion of this view and the entire FAST for fluid is around the caudal tip of the liver, so it is more important for providers to examine this location than the hepato-diaphragmatic interface²⁹.

Left Upper Quadrant (LUQ) view
The LUQ view is considered "negative" when it reveals no free peritoneal fluid (Figure 8A; Video 10). In contrast, a "positive" LUQ exam is one that shows free fluid (Figure 8B; Video 12). Due to the presence of the spleno-colic ligament, free peritoneal fluid in the LUQ is more likely to be found in the spleno-diaphragmatic interface than in the spleno-renal interface^2,29 (Video 12). So the search for fluid in the LUQ centers on visualization of the spleno-diaphragmatic interface. However, because some patients may have abnormal anatomy, it is reasonable to examine the spleno-renal interface, which sometimes requires a second LUQ view from one rib interspace caudal to the best spleno-diaphragmatic view.

In the LUQ, an important false positive for free peritoneal fluid is the presence of a full stomach (Video 28). If the stomach is distended with fluid or solids, it can be visualized inadvertently while insonating the LUQ window. This is more likely to happen if the ultrasound beam is angled too far anteriorly from the usual LUQ FAST exam angle. Anterior beam angulation creates two problems: (1) increases the chance of inadvertently visualizing the stomach and misinterpreting its contents as free peritoneal fluid, and (2) moves the beam away from the more gravity-dependent portions of the LUQ, where true peritoneal fluid is likely to be found. To decrease the chance of this anterior angulation, providers should aim the ultrasound beam posteriorly enough to be able to visualize the ipsilateral kidney.

Pelvic transverse and sagittal views
The female pelvic views are considered "negative" when they reveal no free peritoneal fluid (Figure 10A and Figure 13A; Video 15 and Video 20) and "positive" when they show free fluid (Figure 10B and Figure 13B; Video 18 and Video 21). Notably, in the female pelvis, free fluid is most likely to be found in the rectouterine space (pouch of Douglas) posterior to the uterus as well as the spaces lateral to the uterus. Free fluid is less likely to be found in the recto-vesical space because the peritoneal reflection between the bladder and uterus in women is shallow, whereas the peritoneal reflection posterior to the uterus tends to be deep enough to allow fluid to collect².

Similarly, the male pelvic views are considered "negative" when they reveal no free peritoneal fluid (Figure 11A and Figure 14A; Video 16 and Video 22) and "positive" when they show free fluid (Figure 11B and Figure 14B; Video 19 and Video 23). In the male pelvis, free fluid is most likely to be found in the recto-vesical space posterior to the bladder. In this location, an important false positive for free fluid are the seminal vesicles, which are a normal finding³⁸ (Video 16).

Figure 1: Probe positioning for obtaining the FAST exam version of the subxipoid (aka subcostal) 4-chamber view. Note that the probe indicator mark is pointing toward the patient's right side. Please click here to view a larger version of this figure.

Figure 2: Subxiphoid (aka subcostal) 4-chamber view. (A) shows a grossly normal view. This view was obtained with a curvilinear probe in "abdominal" mode. The key structure that should be seen in the middle of this view is an image of the heart that includes the four cardiac chambers. In this example, there is no pericardial effusion seen around the heart (see Video 1). (B) shows the hemopericardium between the parietal and visceral pericardium. This view was obtained using a sector array probe (colloquially often referred to as a "phased-array" probe) and the view was obtained in "cardiac" mode, so the screen indicator is seen on screen right (see Video 2). Please click here to view a larger version of this figure.

Figure 3: Probe positioning for obtaining the FAST exam version of the parasternal long-axis view. Note that the probe indicator mark is pointing toward the patient's left hip. Please click here to view a larger version of this figure.

Figure 4: Parasternal long-axis view. (A) shows the grossly normal view. This view was obtained with a curvilinear probe in "abdominal" mode. The key structures that should be seen in this view are the following:descending thoracic aorta, left atrium (LA), left ventricle (LV), left ventricular outlow tract (LVOT), right ventricle (RV), and pericardium. There is no gross evidence of pericardial effusion between the pericardium and the descending thoracic aorta (see Video 3). (B) shows the fluid in the pericardial sac. The presence of the fluid between the heart and descending thoracic aorta identifies the fluid as in the pericardial rather than pleural space (see Video 4). Please click here to view a larger version of this figure.

Figure 5: Probe positioning for obtaining the FAST exam RUQ view. Note that the probe indicator mark is pointing cranially (i.e., toward the patient's head). Please click here to view a larger version of this figure.

Figure 6: RUQ view. (A) shows the normal appearance of the RUQ view. This view includes the following three structures:(1) liver; (2) right kidney; (3) hepato-renal interface (a potential space also called Morison's pouch). (B) shows positive RUQ FAST exam highlighting free fluid between the liver and right kidney. Please click here to view a larger version of this figure.

Figure 7: Probe positioning for obtaining the FAST exam LUQ view. Note that the probe indicator mark is pointing cranially (i.e., toward the patient's head). Please click here to view a larger version of this figure.

Figure 8: LUQ view. (A) shows the normal appearance of the LUQ view. This view includes the following three structures: (1) the spleen; (2) the diaphragm; and (3) the spleno-renal interface. (B) shows a positive LUQ FAST exam highlighting free peritoneal fluid between the spleen and diaphragm (see Video 12). Please click here to view a larger version of this figure.

Figure 9: Probe positioning for obtaining the FAST exam suprapubic (aka pelvic) transverse view. Note that the probe indicator mark is pointing toward the patient's right side. Please click here to view a larger version of this figure.

Figure 10: Pelvic transverse FAST exam view in a female. (A) shows normal appearance of the suprapubic (pelvic) transverse FAST exam view in a female. This view includes the following: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas). In women, the peritoneal reflection into the recto-vesical pouch is shallow. In contrast, the peritoneal reflection in the pouch of Douglas is relatively deep. Hence, the pouch of Douglas and the spaces lateral to the uterus are the most sensitive sites to screen for free peritoneal fluid in the female pelvis. (B) shows a positive pelvic transverse FAST exam view in a female showing free peritoneal fluid posterior to the uterus (pouch of Douglas). Please click here to view a larger version of this figure.

Figure 11: Pelvic transverse FAST exam view in a male. (A) shows normal appearance of the suprapubic (pelvic) transverse FAST exam view in a male. This view includes the following: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch). In men, the most sensitive site for detection of free peritoneal fluid is the recto-vesical space (i.e., the space just posterior to the urinary bladder). (B) shows a positive pelvic transverse FAST exam view in a male showing free peritoneal fluid posterior to the urinary bladder (recto-vesical space). Please click here to view a larger version of this figure.

Figure 12: Probe positioning for obtaining the FAST exam suprapubic (aka pelvic) sagittal view. Note that the probe indicator mark is pointing cranially (i.e., toward the patient's head). Please click here to view a larger version of this figure.

Figure 13: Sagittal FAST exam view in a female. (A) shows normal appearance of the suprapubic (pelvic) sagittal FAST exam view in a female. This view includes the following: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas). The sagittal pelvic view is an important feature of the exam because it is more sensitive to free fluid than the transverse pelvic view. (B) shows a positive pelvic sagittal FAST exam view in a female, highlighting free peritoneal fluid posterior to the uterus (pouch of Douglas). Please click here to view a larger version of this figure.

Figure 14: Sagittal FAST exam view in a male. (A) shows normal appearance of the suprapubic (pelvic) sagittal FAST exam view in a male. This view includes the following: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch). (B) shows a positive pelvic sagittal FAST exam view in a male highlighting free peritoneal fluid posterior to the urinary bladder (recto-vesical space). Please click here to view a larger version of this figure.

Video 1: Grossly normal subxiphoid (aka subcostal) 4-chamber view. This view was obtained with a curvilinear probe in "abdominal" mode. The key structure that is seen in the middle of this view is an image of the heart that includes the four cardiac chambers. In this example, there is no pericardial effusion seen around the heart. The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 2: Subxiphoid (aka subcostal) 4-chamber view showing hemopericardium between the parietal and visceral pericardium. This view was obtained using a sector array probe (colloquially often referred to as a "phased-array" probe) and the view was obtained in "cardiac" mode, so the screen indicator is seen on screen right. The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 3: Grossly normal parasternal long-axis view. This view was obtained with a curvilinear probe in "abdominal" mode. The key structures that are seen in this view are the following: descending thoracic aorta, left atrium (LA), left ventricle (LV), left ventricular outlow tract (LVOT), right ventricle (RV), and pericardium. No gross evidence of pericardial effusion between the pericardium and the descending thoracic aorta exists. The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 4: Parasternal long-axis view showing fluid in the pericardial sac. The presence of the fluid between the heart and descending thoracic aorta identifies the fluid as pericardial rather than pleural. Left pleural fluid can also occasionally be seen in this view. To distinguish pericardial from pleural fluid in this view, it helps to assess the potential space between the descending thoracic aorta and the heart: pericardial fluid can enter into this space, whereas pleural fluid cannot. Please click here to download this Video.

Video 5: Normal appearance of the RUQ view. This view includes the following three structures: (1) liver; (2) right kidney; (3) hepato-renal interface (a potential space also called Morison's pouch). The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 6: Demonstration of the operator fanning anteriorly to posteriorly and back in the RUQ view while centered on the hepato-renal interface. This fanning maneuver allows the operator to screen for fluid across a large three-dimensional interface between the liver and kidney, increasing the sensitivity of the exam for free peritoneal fluid. Please click here to download this Video.

Video 7: Positive RUQ FAST exam showing free fluid between the liver and right kidney. Please click here to download this Video.

Video 8: Cranial version of the RUQ FAST exam view. This view centers on the diaphragm and can be used to screen for fluid in the right pleural space as part of an e-FAST exam. Please click here to download this Video.

Video 9: Cranial version of the RUQ FAST exam view showing a large pleural effusion and lung consolidation cranial to the diaphragm. Please click here to download this Video.

Video 10: Normal appearance of the LUQ view. This view includes the following three structures: (1) the spleen; (2) the diaphragm; and (3) the spleno-renal interface.The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 11: Demonstration of the operator fanning anteriorly to posteriorly and back in the LUQ view while centered on the spleno-diaphragmatic interface. This fanning maneuver allows the operator to screen for fluid across a large three-dimensional interface between the spleen and diaphragm, increasing the sensitivity of the exam for free peritoneal fluid. Please click here to download this Video.

Video 12: Positive LUQ FAST exam showing free peritoneal fluid between the spleen and diaphragm. Note the second clip in this video showing that, in the same patient, the spleno-renal interface appears to be devoid of fluid despite the copious fluid seen in the first clip between the spleen and diaphragm. Please click here to download this Video.

Video 13: The caudal version of the LUQ view centered on the spleno-renal interface. Please click here to download this Video.

Video 14: Cranial version of the LUQ FAST exam view. This view centers on the diaphragm and can be used to screen for fluid in the left pleural space as part of an e-FAST exam. Please click here to download this Video.

Video 15: Normal appearance of the suprapubic (pelvic) transverse FAST exam view in a female. This view includes the following: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas). In women, the peritoneal reflection into the recto-vesicle pouch is shallow. In contrast, the peritoneal reflection in the pouch of Douglas is relatively deep. Hence, the pouch of Douglas and the spaces lateral to the uterus are the most sensitive sites to screen for free peritoneal fluid in the female pelvis. The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission.Please click here to download this Video.

Video 16: Normal appearance of the suprapubic (pelvic) transverse FAST exam view in a male. This view includes the following: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch). In men, the most sensitive site for detection of free peritoneal fluid is the recto-vesicle space (i.e., the space just posterior to the urinary bladder)6. The schematic seen at the beginning and end of this clip was reprinted with author's (DC) permission. Please click here to download this Video.

Video 17: Demonstration of the operator fanning anteriorly to posteriorly and back in the male pelvic transverse view while centered on the bladder. This fanning maneuver allows the operator to screen for fluid across a large three-dimensional section of the recto-vesicle space, increasing the sensitivity of the exam for free peritoneal fluid. Please click here to download this Video.

Video 18: Positive pelvic transverse FAST exam view in a female showing free peritoneal fluid posterior to the uterus (rectouterine pouch of Douglas). Please click here to download this Video.

Video 19: Positive pelvic transverse FAST exam view in a male showing free peritoneal fluid posterior to the urinary bladder (recto-vesical space). Please click here to download this Video.

Video 20: Normal appearance of the suprapubic (pelvic) sagittal FAST exam view in a female. This view includes the following: (1) the bladder in its maximal dimension; (2) the uterus (if present); and (3) the space just posterior to the uterus (rectouterine pouch of Douglas). The sagittal pelvic view is an important feature of the exam because it is more sensitive to free fluid than the transverse pelvic view. Please click here to download this Video.

Video 21: Normal appearance of the suprapubic (pelvic) sagittal FAST exam view in a male. This view includes the following: (1) the bladder in its maximal dimension and (2) the space just posterior to the bladder (recto-vesical pouch). Please click here to download this Video.

Video 22: Demonstration of the operator fanning left to right and back in the male pelvic transverse view while centered on the bladder. This fanning maneuver allows the operator to screen for fluid across a large three-dimensional section of the recto-vesicle space, increasing the sensitivity of the exam for free peritoneal fluid. Please click here to download this Video.

Video 23: Positive pelvic sagittal FAST exam view in a female showing free peritoneal fluid posterior to the uterus (rectouterine pouch of Douglas). Please click here to download this Video.

Video 24: Positive pelvic sagittal FAST exam view in a male showing free peritoneal fluid posterior to the urinary bladder (recto-vesical space). Please click here to download this Video.

Video 25: Subcostal 4-chamber view showing a prominent epicardial fat pad. Please click here to download this Video.

Video 26: Parasternal long-axis views of the epicardial fat pad and hemopericardium. Please click here to download this Video.

Video 27: Subcostal 4-chamber view showing an example of ascites which could easily be mistaken for free peritoneal fluid. Please click here to download this Video.

Video 28: Left upper quadrant (LUQ) view showing a gastric body distended with fluid, a false positive finding when searching for free peritoneal fluid in the LUQ. Please click here to download this Video.

Discussion

Traumatic injuries remain a leading cause of morbidity and mortality in the United States and worldwide. The rapid evaluation of the trauma patient and identification of injuries, including major hemorrhage, is a key component of reducing trauma morbidity. The FAST exam rapidly and non-invasively screens for potential sources of life-threatening hemorrhage. Critical steps to the success of the procedure are obtaining all of the views through the four primary ultrasonographic windows and, if necessary, using the alternative parasternal window to fully visualize the spaces.

The key to the successful use of the FAST exam in trauma remains the identification and interpretation of free fluid in the pericardium, peritoneal, and - if possible - pleural spaces¹. Both false positive and false negative exams are possible. False negative exams may occur for a variety of reasons but generally are due to an inability to obtain adequate visualization of the target anatomy or insufficient free fluid for detection^39,40,41. The inability to obtain images is related to patient factors such as obesity, subcutaneous air, movement, and injuries which preclude appropriate windows^39,41,42. False positive exams are typically related to the misinterpretation of anatomic information. In the pericardial space, the epicardial fat pad may appear hypo-echoic in relation to the myocardium and may be misinterpreted as a positive pericardial window³⁴. Similar fluid-filled normal anatomic structures such as gallbladder and bowel and pathology such as hepatic or renal cysts or ascites may be misinterpreted as free peritoneal fluid^39,42. Because of the potential to encounter all these false positives for pericardial fluid in the SC4C view, other authors and we recommend obtaining the PLAX view to determine whether peri-cardiac fluid seen in the SC4C view is truly located in the pericardial sac or not (see Representative Results section)³⁴.

In addition to false negative and positive exams, true positive and negative exams which correctly identify the presence or absence of free fluid may misinterpret the clinical significance of these findings^{1,39,40,41,42}. Though in the setting of trauma, free pericardial or peritoneal fluid is presumed to represent the blood, in the setting of hemodynamic instability, it may, in fact, represent other physiologic or pathologic conditions^39,43. Ascites is the prototypical example of the medical condition leading to a true positive FAST exam with indeterminate significance in the trauma evaluation. Finally, a true negative FAST exam, which correctly identifies no significant peritoneal fluid, does not rule out injuries requiring exploratory laparotomy^1,39. Hollow viscus injuries, which require surgical repair, as well as retroperitoneal organ injuries and hemorrhage are not adequately identified.

Given the many potential pitfalls, when used for the primary indication of blunt thoracoabdominal trauma, the FAST exam demonstrates good specificity and sensitivity. In a recent Cochrane review, overall sensitivity was estimated at 74%, and specificity was 96%⁴⁴. However, in a pediatric population, both sensitivity and specificity were lower at 63% and 91%, respectively. Similarly, there was a marked decreased sensitivity of 28%-100% with preserved specificity (94%-100%) when studies looking at penetrating abdominal trauma were reviewed. Outside of the trauma setting, the abdominal portion of the exam can be used to screen for free peritoneal fluid for any patient where such fluid could contribute to a patient's symptoms of hemodynamic instability and/or abdominal pain/distension. Thus, the abdominal portion of the FAST exam is sometimes relevant to the care of non-trauma patients, as any patient can develop clinically significant amounts of intra-abdominal fluid for a variety of reasons (e.g., peri-procedural abdominal organ injury, bladder rupture, uterine rupture, ascites accumulation, etc.). However, the sensitivity and specificity of the FAST exam in these non-trauma settings have not been rigorously studied. Therefore the primary indication for the FAST exam remains the evaluation of hemodynamically unstable blunt thoraco-abdominal trauma patients for whom rapid clinical decision-making is necessary and who cannot safely undergo a CT scan. Outside of the trauma setting, other applications include any of the following: triaging the severity of obstetric hemorrhage¹⁰, screening for a cause of abdominal pain/distension, and as part of the preoperative assessment of patients with suspected but unconfirmed ascites scheduled for elective surgery^11,12,13.

As the technology of portable ultrasonography improves and provider familiarity and comfort with ultrasonographic image acquisition increases, further applications in intensive care and even prehospital setting are inevitable.

Materials

Name	Company	Catalog Number	Comments
Affiniti  (including linear high-frequency, curvilinear, and sector array transducers)	Philips	n/a	Used to obtain a subset of the Figures and Videos
Edge 1 ultrasound machine (including linear high-frequency, curvilinear, and sector array transducers)	SonoSite	n/a	Used to obtain a subset of the Figures and Videos
M9 (including linear high-frequency, curvilinear, and sector array transducers)	Mindray	n/a	Used to obtain a subset of the Figures and Videos
Vivid iq  (including linear high-frequency, curvilinear, and sector array transducers)	GE	n/a	Used to obtain a subset of the Figures and Videos