Key points.
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Clinical examination is inaccurate in the setting of blunt abdominal trauma and physiological evaluation has limited sensitivity to detect hypovolaemia.
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Early imaging is crucial to the identification of traumatic injuries.
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Extended focused assessment with sonography for trauma (eFAST) is a non-invasive point-of-care test, which can guide clinical decision making.
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Clinicians should be mindful of the strengths and limitations of eFAST, and interpret results in the context of the mechanism of injury and the evolving clinical situation.
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Guidance from The National Institute for Health and Care Excellence should be followed, and immediate computed tomography or operative intervention considered where appropriate.
Trauma is an epidemic of our time with disproportionate morbidity and mortality affecting young adults. Of all deaths from trauma, 30–40% is caused by haemorrhage.1 However, the evaluation of thoracic and abdominal trauma can be a challenge. Clinical assessment, including physical examination, is inaccurate in the setting of blunt abdominal trauma2 and physiological evaluation has limited sensitivity to detect hypovolaemia.3 In view of this, early radiological imaging is crucial to the identification of traumatic injuries. For every 3 min of delay in patients who require a laparotomy, mortality increases by approximately 1%.4 The focused assessment with sonography in trauma (FAST) examination is a non-invasive point-of-care test whose role is to guide clinical decision making and direct angiographic or surgical interventions. It can be used by clinicians to diagnose haemopericardium or haemoperitoneum secondary to clinically significant injuries on ultrasound (US). Extended FAST (eFAST) is an evolution of the traditional FAST examination and incorporates thoracic window assessment to identify haemothorax and pneumothorax.
Feasibility
The FAST examination can be performed in 3–4 min at the bedside.5 It avoids the risks associated with transport, does not involve the use of ionising radiation and may be repeated. Operator training and experience affect performance and the number of supervised examinations required to acquire competence is debated. In learning how to perform FAST, most errors occur in the first 10 examinations and thereafter accuracy improves.6 Further reading and recommendations concerning competencies, methods of training, and maintenance of skills can be found at https://www.rcemlearning.co.uk/references/ultrasound-in-emergency-medicine-level-1-instruction.7
Utility
In adult trauma patients, a meta-analysis demonstrated FAST to have a pooled sensitivity of 78.9% and a specificity of 99.2%.8 Sensitivity was increased with greater volumes of blood loss. Significant heterogeneity in sensitivity was observed, whereas specificity remained reasonably constant across trials. Such variation in sensitivity can be explained by differences in the comparator reference standard used, such as computed tomography (CT), laparotomy or patient outcome. It confirmed that FAST is accurate for injury when positive and more readily identifies blood loss in haemorrhagic shock, but cannot be used to rule out intra-abdominal injury. In the setting of blunt torso trauma associated with hypotension and penetrating praecordial wounds, FAST is most discriminatory with a sensitivity of 100%.9
It has been shown that the use of US in adult trauma may decrease CT utilisation10 and the time to operative intervention; reduce the incidence of a composite measure comprising of haemorrhagic or septic shock, multisystem organ failure or death; shorten hospital stay; and lower costs.11 The National Institute for Health and Care Excellence (NICE) advocates FAST in patients who are haemodynamically unstable and not responding to volume resuscitation.12 FAST should not be used as a screening modality to determine the need for CT in patients with major trauma. Immediate CT should be considered in patients with suspected haemorrhage if their haemodynamic status is normal or they are responding to resuscitation. However, it should be noted that the evidence supporting clinical pathways based on FAST remains poor in suspected abdominal or multiple blunt trauma.10 In major trauma, where diagnosis and intervention are time critical, FAST can be a relative minor adjunct compared to the emphasis on the use of CT and operative intervention.
In the evaluation of traumatic haemothorax, US appears to have an increased sensitivity and a similar specificity compared to chest radiography (CXR).13 Up to 76% of pneumothoraces detected on CT are not visible and therefore occult on supine CXR,14 but US could overcome these shortcomings as it is more sensitive than CXR in detecting pneumothorax.15 Occult pneumothoraces do not however tend to progress or result in respiratory distress and a conservative approach is being increasingly supported.16 NICE recommends that patients with chest trauma and severe respiratory compromise undergo immediate CXR or eFAST.12 If severe respiratory compromise is not present and the haemodynamic status is normal, immediate CT should be considered.
Physics of US
In US, once the mechanical waves are emitted from the transducer, they pass through various body tissues and are reflected back as echoes to the transducer, creating an image on the screen. Acoustic impedance (Z) is a measure of the resistance of particles in a medium to mechanical vibrations and is proportional to the density of the tissue and speed of the sound wave. The effect of Z becomes noticeable at the interfaces between tissue types of different acoustic impedances. As the difference in Z increases, more US is reflected back and less is transmitted. Intraperitoneal fluid, such as fresh blood, has a constant Z resulting in no echoes (anechoic) and so appears as a black stripe. Blood however which has begun to clot, can become more echogenic and heterogeneous. In cases of a delayed presentation, this should be considered if no obvious free fluid (FF) is evident. Soft tissues have similar values of Z and so minimal reflection occurs. Interfaces between air and soft tissue reflect most of the US wave and so are poorly penetrated by US, producing artefact and no structural imaging.
How to perform a FAST
Traditionally, the FAST examination consists of four basic sonographic views: the right upper quadrant (RUQ), left upper quadrant (LUQ), pelvic, and cardiac. In order to learn how to acquire the appropriate views for lung US for the thoracic component of eFAST, readers are referred to the recent article by Miller.17
Probe selection
An US probe with a lower frequency, such as that of the curvilinear (3–5 MHz) or phased array (3–4.5 MHz) type, should be used to facilitate the evaluation of deeper structures. Compared to the phased array probe, the curvilinear transducer provides better resolution in the abdomen but is not ideal for imaging the heart. In contrast, the phased-array probe has a smaller footprint and can be used to scan between the ribs, such as for the cardiac parasternal long axis (PLAX) view. Linear transducers have a higher frequency (8–12 MHz) and should be avoided although they result in good resolution of superficial structures, depth penetration is limited. Conventionally, in standard emergency and general US imaging, the marker on the probe is directed to the patient's head for longitudinal views or the patient's right for transverse views and it should correspond to the left side of the image on the monitor. Conversely, in echocardiographic imaging, the convention is reversed and an opposite marker direction and screen orientation are used for most windows.
RUQ view
In the RUQ view, the perihepatic area and the potential space between the liver and kidney, otherwise known as Morison's pouch, are assessed using the liver as the sonographic window. It is the most sensitive view for free intraperitoneal fluid, as dependent fluid tends to distribute here in the supine patient, and thus should be the first view obtained in blunt trauma. Trendelenburg positioning can further enhance the detection of FF. The probe should be placed in a longitudinal orientation anterior to the right mid-axillary line between the seventh and eighth intercostal spaces and used to fan through the entire interface of the liver and right kidney (Fig. 1). Small fluid collections start near the caudal tip of the liver, which is the beginning of the right paracolic gutter, and should not be missed.
Fig 1.
US image of the RUQ view demonstrating anechoic FF in the pouch of Morison between the liver (L) and kidney (K) and around the liver.
LUQ view
In the LUQ view, the perisplenic and the potential space between the spleen and kidney are assessed using the spleen as the sonographic window. The probe should be placed in a longitudinal orientation near the left posterior axillary line between the seventh and eighth intercostal spaces. Moving the probe to a more posterior and superior approach means that gas in the stomach and colon is not encountered, which can otherwise obscure the view. Interference from the rib shadows can be avoided by turning the probe into a more oblique orientation parallel to the ribs. Compared to the RUQ, fluid flows differently in the LUQ, as the phrenicocolic ligament limits the passage of fluid down the left paracolic gutter. Small fluid collections may be found superior to the spleen and the interfaces between the diaphragm, spleen and kidney should be seen (Fig. 2).
Fig 2.
US image of the LUQ view demonstrating anechoic FF around the spleen (S) and its relationship to the kidney (K).
Pelvic view
In the pelvic view, an assessment is made for FF using the bladder as the sonographic window. Reverse Trendelenburg positioning and the presence of a fluid-filled bladder can further enhance the detection of fluid. If the bladder is emptied consequent to the insertion of an urinary catheter, the detection of fluid can be compromised. The probe should be placed just above the pubic symphysis and angled inferiorly towards the feet to fan through the bladder in both longitudinal and transverse orientations (Fig. 3). In males, FF is usually seen in the retrovesical space whilst in females, FF will first be visualised posterior to the uterus and then anterior to it as well once enough fluid collects. Clinical correlation is needed as, in female patients of reproductive age, FF of up to 50 ml is physiological in the pouch of Douglas between the rectum and the uterus.
Fig 3.
US image of the pelvic view demonstrating a moderately filled bladder (B) with the typical box-like appearance and rounded edges of a negative FAST examination. In a positive scan, the bladder is more likely to have irregularly shaped outlines, often with sharp edges.
Cardiac view
In the cardiac view, an assessment is made for FF within the pericardium to evaluate for effusion and tamponade. The probe should be placed in a transverse orientation just inferior to the xiphoid process and angled towards the left shoulder. If this subxiphoid view is difficult to obtain because of body habitus or pain, the PLAX view can be used. The probe should be placed between the second and fourth intercostal spaces on the anterior chest wall just to the left of the sternum. For this window, the marker is orientated towards the left hip rather than the right shoulder, as would usually be the case in standard emergency imaging, in order to obtain an image that is consistent with the reversed echocardiographic convention performed by cardiologists. It is essential that all of the heart is visualised as pericardial effusions can start at the posterior aspect of the pericardium (Fig. 4). This can be achieved by asking the patient to breathe in deeply in the case of the subxiphoid view, or increasing the depth on the US machine for both views. If a substantial amount of fluid is found in the pericardial space, cardiac tamponade is likely if collapse is present in any chamber during the cardiac cycle.
Fig 4.
US image of the subxiphoid cardiac view demonstrating a pericardial effusion (PCE) surrounding the heart causing tamponade. Right ventricle has collapsed. Left ventricle (LV), left atrium (LA) and right atrium (RA) are visualised.
Limitations
FAST cannot reliably grade solid organ injuries that do not result in significant haemoperitoneum. For the FAST examination to be positive, a critical volume of fluid should be present. The mean minimum volume of fluid needed for US detection is 668 ml when supine and 444 ml when Trendelenburg18 in the RUQ view and 157 ml in the pelvic view19 compared to 100–250 ml with CT. Because of this, FAST cannot be used as a diagnostic test to exclude small amounts of intraperitoneal haemorrhage. It has been noted to have poor accuracy in the early post-injury phase when sufficient haemoperitoneum has not yet accumulated, leading to a false-negative FAST. Delayed presentation after trauma is a further risk factor for a false-negative result as when blood begins to clot it can be difficult to differentiate from the surrounding tissue. Retroperitoneal haemorrhage, which can be secondary to a pelvic fracture or an injury to the aorta, inferior vena cava (IVC), or kidneys, is not well visualised on US unless it flows into the abdominal or pelvic compartments.20 Full US visualisation can be obstructed by bowel gas, obesity, and subcutaneous emphysema.
A FAST examination which is negative should not be regarded as conclusive, and clinicians should consider further imaging with CT. Mild abdominal and severe head injuries are associated with a false-negative FAST.21 Severe head injuries are related to a lack of patient cooperation and could distract the evaluating clinicians from performing a thorough FAST. In such cases, the liberal use of whole body CT could alternatively result in the increased detection of incidental FF. Patients with a false-negative FAST have been found to be less likely to undergo a therapeutic laparotomy but do not incur increased adverse outcomes related to mortality or length of stay. Serial FAST scans should be considered in non-major trauma or when access to CT is limited and can decrease the false-negative rate by 50% and increase sensitivity from 69 to 85%.22
FF detected with FAST is assumed to be haemoperitoneum, although it can also represent injury-related bile, bowel contents or urine. Fluid filled bowel can be differentiated from FF where appropriate by observing for peristalsis and repeating the examination to ensure the fluid pockets are in the appropriate tissue planes. Gallbladder or renal cysts, if prominent, can be misinterpreted but fluid is free flowing, unlike, the contained and circular appearance of body structures and cysts. Fluid as a result of non-traumatic conditions, such as ascites, ovarian hyperstimulation or rupture, peritoneal dialysate or ventriculoperitoneal shunt overflow, can result in false positives. Massive intravascular volume resuscitation can rarely cause fluid transudation from the intravascular to intra-peritoneal compartment after a prolonged period of time and hence a false-positive FAST.23 In some patients who are obese, perinephric fat can widen the hepatorenal and splenorenal interface and be misinterpreted as FF. Comparison views of each kidney and evaluation for the double line sign, a wedge-shaped hypoechoic area bounded on both sides by echogenic lines, caused by fat around the kidneys can be beneficial in these cases.24
In the US assessment for pneumothorax, bullae, adhesions and contusions can contribute to false-positive results. Loculated pneumothoraces can be difficult to identify and subcutaneous emphysema can significantly interfere with visualisation.
Special circumstances
Traumatic cardiac arrest
The focus of resuscitation in traumatic cardiac arrest (TCA) is the rapid and simultaneous identification and treatment of reversible causes, such as cardiac tamponade, hypovolaemia or tension pneumothorax.25 US assists in the diagnosis of these reversible causes and should be prioritised. In such circumstances, chest compressions are unlikely to be as effective as in normovolaemic cardiac arrest and take a lower priority compared to medical cardiac arrest. Lack of cardiac motion on US in traumatic cardiac arrest has been associated with a negative predictive value of 99% for survival to hospital admission but further studies are needed.26
Pregnant patients
For pregnant patients who have sustained trauma, US is advantageous in that there is no contrast or radiation exposure to the mother or fetus. Patients should be positioned in the left lateral position to avoid hypotension from uterine IVC compression. In blunt abdominal trauma, FAST in pregnant patients has similar sensitivities and specificities to that in non-pregnant patients.27 The most common pattern of FF accumulation is in the RUQ, LUQ, and pelvis.28 Careful technique is required as the gravid uterus can distort the usual US landmarks in the pelvic view. It can be difficult to distinguish between intrauterine and extrauterine fluid; free intraperitoneal fluid can be secondary to haemorrhage, amniotic fluid from uterine rupture or both. US can also be used to assess for fetal heart motion, fetal activity and injury, approximate gestational age, amniotic fluid volume and placenta. For patients with negative or equivocal US findings, continuous cardiotocographic monitoring should commence as early as possible to screen for placental abruption.
Children
Similar to practice in pregnant patients, FAST has been used in children to decrease radiation exposure to CT. A meta-analysis has demonstrated paediatric FAST to have a sensitivity of 66% and a specificity of 98%.29 Negative tests have questionable utility as the sole diagnostic modality to rule out the presence of intra-abdominal injury. In a recent randomised controlled trial, the use of FAST compared to standard care alone after blunt torso trauma in haemodynamically stable children did not improve outcomes.30 NICE does not advocate the use of FAST in children, but supports the consideration of CXR and US, rather than the routine use of CT, for first-line imaging in chest trauma.12
Current and future directions
Clinicians are now incorporating the eFAST examination into prehospital protocols as it has the potential to influence significantly trauma management at the scene. eFAST has similar sensitivities and specificities in the prehospital environment compared to when performed in hospital. It could cause a delay in prehospital transfer time, but can be undertaken in parallel with other procedures or during transport of the patient where the examination can be truncated once adequate diagnostic information has been obtained. US images acquired by paramedics can be transmitted to expert reviewers in trauma centres for evaluation and triage, which can then facilitate the targeted preparation of the resuscitation room and/or operating theatre by the receiving hospital.
Other US protocols include an evaluation of the IVC. Changes in the diameter of the IVC correlate with intravascular volume status and a flat IVC, usually assessed in the subxiphoid view, has been shown to be an indicator of poor prognosis in unintubated trauma patients.31 The role of contrast-enhanced US for trauma is as yet not clear, but it appears to be more effective in the detection of solid organ injury.
Conclusions
Evaluation with eFAST can provide critical information during the real time assessment of complex trauma patients. It can diagnose haemothorax and pneumothorax and identify FF suggestive of haemopericardium or haemoperitoneum. However, clinicians should be mindful of the inherent strengths and limitations of the US examination and follow guidelines from NICE, remembering that eFAST can be a relative minor adjunct in major trauma.
Biographies
Neel Desai, FHEA FRCA MRCP MRCS PGCert Medical Education, is a specialty registrar at St George's Hospital.
Tim Harris, FACEM FRCEM FFICM PGCert Ultrasound, is Professor in Emergency Medicine at Barts Health NHS Trust and the Queen Mary University of London. He is educational lead for ultrasound at the International Federation for Emergency Medicine and was previously research lead for the subcommittee on ultrasound at the Royal College of Emergency Medicine.
Matrix codes: 1A03, 2A02, 3A10
Footnotes
Supplementary data related to this article can be found at https://doi.org/10.1016/j.bjae.2017.10.003.
Declaration of interest
None declared.
MCQs
The associated MCQs (to support CME/CPD activity) can be accessed at www.bjaed.org/cme/home by subscribers to BJA Education.
Supplementary material
The following are the supplementary data related to this article:
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