Emergency Ultrasound: Basic Trauma Training
Authors: David P. Bahner MD, RDMS, Assistant Professor and Ultrasound Director, Department of Emergency Medicine, Ohio State University, Columbus; Robert Moskowitz MD, Assistant Director, Emergency Medicine, Skyline Medical Center, Nashville, TN; and Robert Falcone, MD, President, Grant Medical Center, Columbus, OH.
Peer Reviewers: Michael Blaivas, MD, Chief, Section of Emergency Ultrasound, Associate Professor, Department of Emergency Medicine, Medical College of Georgia, Augusta; and Christopher L. Moore, MD, RDMS, RDCS, Assistant Professor, Yale University School of Medicine, New Haven, CT. What did the ultrasound show? This may become commonplace at the 21st century bedside. Emergency ultrasound has been highlighted on television, in everyday trauma bays, and in a variety of emergency scenarios. It begs the question: Will ultrasound become the primary bedside diagnostic of the 21st century? This idea has echoed in the medical community for the past 20 years but ultrasound is turning into something much more. Portable ultrasound provides clinicians visual patterns to help them make critical decisions. Ultrasound imaging has established a definitive role during the evaluation of patients in a trauma setting. This article will describe the basic principles of ultrasound imaging, the process on how to acquire a good image, and how best to implement ultrasound in the emergent trauma setting.
—The Editor
Introduction
Ultrasound utilizes frequencies well above the range of human hearing to penetrate and visualize structures in the body. While human hearing is generally in the range of 20-20,000 Hz (cycles/second), diagnostic ultrasound is typically in the range of 2-12 mega-hertz (MHz), or 2-12 million cycles per second. Ultrasound quality has advanced as computer technology, microcircuitry, and advanced algorithmic signal processing have become cheaper and smaller. These complex ultrasound machines have evolved to have the capacity to produce high-resolution images quite easily at the patient bedside. The market for sonographic professionals has grown as ultrasound equipment has become more accessible. Trauma ultrasound has become a staple in evaluating the acutely injured patient. For those caring for the trauma patient, refining basic ultrasound skills is simply following a few key steps.
Pick a Probe. Lower frequencies penetrate better, while higher frequencies provide improved resolution. Probes vary in size and frequency (see Figure 1), yet all retain some main shared characteristics. The piezoelectric crystals within the probe, their size and shape will help the operator identify which probe to use. For a FAST scan, a curvilinear probe is able to produce a picture with a wide field of view while a linear probe is better for vascular (e.g., central line placement) or small parts evaluation (e.g., pneumothorax). Some operators like the phased array probes with the smaller footprint to image more effectively between ribs. More modern microconvex transducers allow a smaller footprint yet still provide superior abdominal imaging. The probe produces a window into the patient’s anatomy and each probe produces a different shaped window. Practitioners should practice obtaining ultrasound images on colleagues and volunteers until they decide on their preference of probes. Typically the FAST scan can be performed using a probe in the range of 3-5 MHz.
| Figure 1. Transducers |
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| Three transducers are shown: A curvilinear array (far left), a linear array (middle), and a phased array (far right). |
Probe Orientation, Picture Orientation. Each transducer has some palpable indentation, protuberance or lighted indicator that marks the leading edge. In typical body (trauma) ultrasound, the indicator corresponds to the left side of the screen as one looks at it. Placing a small amount of gel on the side of the indicator and touching a finger to the probe can verify which side of the probe corresponds with the image on the screen. When scanning during a FAST examination, the indicator generally is pointed to the patient’s right (in transverse views) or toward the head (in longitudinal views) to standardize display patterns. Thus in a transverse scan, the indicator is directed toward the patient’s right, corresponding to the left (leading) side of the screen displaying sonographic echoes of right-sided body structures. The image displayed in anterotransverse fashion is similar to a CT image (looking at the patient from the feet up). In longitudinal (sagital) and coronal views, the indicator is directed toward the patient’s head, and superior (cephalad) structures, such as the diaphragm, will be seen on the left side of the screen and inferior (caudal) structures, such as the inferior pole of the kidney, will be seen on the right side of the screen. The top of the screen corresponds to the near field of the beam emanating from the transducer face of the probe. Superficial structures closest to the probe face will be displayed at the top of the screen (e.g., skin, SQ, muscle), while deeper structures (e.g., liver, kidney, psoas muscle) will be displayed in the far field near the bottom of the screen.
Proper Scanning. Once the operator becomes comfortable with the orientation of the patient, probe, and picture, proper scanning can take place. Each probe should be held with a comfortable grip and basic movements are used to scan through organs of interest. The dominant hand holds the probe with the indicator serving as a direction arrow. Basic hand movements allow the operator to visualize the human anatomy from various sectional planes. Basic scanning movements include sweeping from side to side in the short axis, rocking and sliding in the long axis, and turning the probe clockwise and counterclockwise obliquely around its axis. Novices will use all three movements without discrimination and order, while more experienced operators develop a systematic approach to getting the image. Broadly sweeping through the organ of interest allows a general survey of the underlying anatomy and allows for identification of key sonographic landmarks. Rocking and sliding allows the operator to position the area of interest (e.g., the right kidney) into the middle of the screen. The probe can be rotated to image through difficult sonographic structures such as the rib cage. When rotating the probe, think of it as a clock with the hands turning. The FAST scan images two organs (liver and kidney) surrounded by a protective rib cage. The operator rotates the probe counterclockwise for the perihepatic image and clockwise for the perisplenic image. Thus the alignment of the probe, the beam, and the intercostal muscles allows for a proper acoustic window, between ribs, in which to image the internal organs.
The Use of Ultrasound in Trauma
Significant morbidity and mortality are associated with the delayed or missed diagnosis of intraabdominal injury due to blunt abdominal trauma (BAT). While computed tomography (CT) scanning is the test of choice in a stable patient with suspected intraabdominal injury, it may not be appropriate in a patient who is hemodynamically unstable. In addition, there are risks associated with ionizing radiation, which may make CT less than desirable in certain patients. Diagnostic peritoneal lavage (DPL) first was described in 19651 but is an invasive test (complication rate 1-5%) with some contraindications as well as the potential for both false positives and false negatives. In centers that use bedside ultrasound, DPL largely has been replaced by bedside ultrasound.2,3 Ultrasound was first described to detect free fluid in the abdomen in 1970, As technology improved in the 1970s and ’80s, ultrasound developed a niche by addressing these issues of rapidity and portability, subsequently becoming a diagnostic adjunct in trauma.
The FAST Scan
The Focused Assessment with Sonography for Trauma (FAST) exam is a goal directed ultrasound examination performed at the bedside,5 typically as part of the secondary survey of the trauma patient. During the past two decades it has evolved from just the abdomen to include cardiac and pleural spaces. It is primarily directed at looking for fluid (assumed to be bleeding in acute trauma) in three spaces: peritoneal, pericardial, and pleural. The most widely accepted FAST exam includes four primary windows. (See Figure 2.) The FAST exam has been widely studied in Germany,6,7 Japan,8 and the United States9,10 and proven to be sensitive (63-99%),11 specific (88-100%)6,12 and accurate (85-99%).13 The sensitivity and specificity have remained high when performed by trained operators searching for free fluid within the torso. The FAST scan is not sensitive for solid organ injury without hemoperitoneum, and further imaging should be sought if this is suspected. It further has been demonstrated that FAST training readily can be taught to physicians, as they have shown a distinct learning curve14 with recommended training (8 hour didactics, 50 exams).15 Training and proficiency depend on the individual and mainly are institution-dependent.
Emergency ultrasound in trauma should focus on detection of free fluid in the potential spaces of the torso and is not intended for more detailed exams of specific organs. The entire FAST exam should take fewer than 2-4 minutes and can be remembered as the 4 Ps, addressing the optimal number of views: Pericardial, Perihepatic, Perisplenic, and the Posterior cul de sac view. The subxyphoid cardiac view can be performed first to adjust the gain according to the anechoic (black) inferior vena cava (IVC). Many operators begin with interrogation of the hepatorenal space known as Morison’s pouch, as abdominal fluid collects in this area preferentially. Then the perisplenic space is imaged including the superior pole of the spleen, and finally the pelvic view that interrogates the posterior cul de sac. The most sensitive view in detecting hemoperitoneum is the hepatorenal space,16 and Trendelenberg may assist in increasing the sensitivity17 although this is not always practical in the trauma setting. While operator dependent, the FAST scan has been reported to detect a volume of intraperitoneal fluid as little as 30 mL when next to the site of injury,18 while most operators are able to detect volumes between 200 and 400 mL. Sensitivity in Branney et al did not reach 97%, however, until almost 1 L of free fluid was infused to the intraperitoneal cavity.19 The pericardial view (see Figure 3), obtained from probe placement in the subxyphoid region, allows for imaging of the pericardial sac and possible tamponade physiology. The right upper quadrant view examines Morison’s pouch (see Figure 4), with additional limited visualization of the subphrenic, retroperitoneal, and supradiaphragmatic spaces. The perisplenic view (see Figure 5) investigates the contours of the spleen and left kidney while evaluating the corresponding subdiaphragmatic, supradiaphragmatic, and subsplenic area. Finally, transverse probe placement along the suprapubic region explores the rectovesicular space. (See Figure 6.)
| Figure 4. Normal Ultrasound of Hepatorenal Interface |
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| Figure 5. Normal Ultrasound of Splenorenal Interface |
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| Figure 6. Normal Ultrasound of the Male Posterior Cul de Sac |
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Ma et al demonstrated that the four views combined to offer superior sensitivity over any single view for detection of hemoperitoneum due to hepatic or splenic injury.20 A positive FAST scan exhibits a fluid stripe with sharp angles as exhibited in Figure 7 of the hepatorenal space or Figure 8 of the pelvis. Positive fluid collections around the spleen can be more subtle than those found in the right upper quadrant window. Instead of distending the splenorenal space, some fluid collections will be perisplenic and display separation of the normal splenoperitoneal interfaces. (See Figure 9.)
| Figure 7. Positive FAST Scan |
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| Figure 8. Positive Pelvic Fluid |
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| Figure 9. Positive Ultrasound of Perisplenic Fluid |
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The Role of FAST Scans
The role of the FAST scan developed in response to the limitations of DPL and CT scans. DPL, while highly sensitive, is highly invasive and can cause significant injury. Fischer et al pointed to complication rates of about 1.7%.21 There are the relative contraindications for DPL, including pregnancy (third trimester), preexisting coagulopathy, advanced cirrhosis, and previous laparotomies in the performance of DPL. DPLs registered false positive studies necessitating non-therapeutic laparotomies in a range of 6-19%.21,22 CT scanning is an excellent modality for the evaluation of blunt trauma and detecting solid organ injury, a well-recognized limitation of the FAST scan. However, CT scanning involves ionizing radiation, higher cost, and removes the patient away from the resuscitation area.
Ultrasound offers a rapid, non-invasive bedside tool for the evaluation of the injured patient. However, the FAST scan does have limitations. Ultrasound is operator-dependent and may be limited by subcutaneous emphysema, extensive bowel gas patterns, and obesity. In patients with known ascites or peritoneal dialysis, the FAST exam may be of little utility. Furthermore, the view of the retroperitoneum is limited23 and sensitivity is diminished in detecting hollow viscus injuries.14 Repeat serial FAST exams have been shown to improve accuracy.7,23
Head-to-head comparisons in the trauma setting have been limited to examining ultrasound only with respect to DPL. Rose et al did demonstrate that the routine use of ultrasound in BAT evaluations significantly reduced the number of CT scans ordered.24 Study outcomes, with respect to DPL and ultrasound, reinforced the conclusions that DPL maintained higher sensitivity but resulted in more non-therapeutic laparotomies. If the FAST scan achieves good visualization of the intended structures, the clinician should search for other sources of blood loss in the unstable patient before performing a DPL.
The algorithm for trauma care should include FAST scans via standard approach based on the hemodynamic stability of the patient. Primary survey and management issues regarding the ABCs are first accomplished, with the FAST exam incorporated into the secondary survey, especially when there is a suspicion for thoraco-abdominal injury. The clinical algorithm (see Figure 10) incorporates ultrasound into the decision making process regarding operative intervention or further diagnostic evaluation. An unstable patient with a positive ultrasound should be taken to the operating room for exploration. A positive ultrasound scan in a stable patient may have a CT scan to help delineate the injuries. A negative or indeterminate scan in an unstable patient warrants consideration of a DPL or other sources of hemodynamic instability. The stable patient with a negative FAST or indeterminate scan can have repeat FAST exams or investigation by CT scan depending on mechanism of injury and clinical suspicion. The fact remains that trauma ultrasound can miss many intraabdominal injuries and, if the suspicion for injury is high and the FAST is indeterminate or negative, further testing should be performed to delineate the abdominal anatomy.
The issue of classifying an ultrasound as positive has been defined by different scoring systems. The finding of an anechoic stripe usually is measured and subsequently classified as small, medium, or large. Different scoring systems have described the smaller level usually designated as 0.5 to 1.0 cm.25,26 It has been found that larger fluid levels (greater than 3.0 cm) in conjunction with unstable vital signs (pulse greater than 100, systolic blood pressure less than 90) is sensitive for categorizing patients who necessitate exploratory laparotomy.25
Mild variations on the clinical pathway above have been studied, concluding acceptable patient outcome and suggested cost effectiveness.12 Branney et al used an ultrasound-based algorithm and demonstrated an expected decrease in the use of DPL (from 17% to 4%) and CT (56% to 26%).27 Additionally, there was not an increased risk to the patients examined via their ultrasound pathway. Injury severity scores increased in patients with DPLs (11.6 to 21.5) and CT scans (4.6 to 8.3). While formal analysis was not done, there was an estimated savings of $450,00 per year.27 Arrillaga et al supported this provision, demonstrating almost a one-third decrease in cost to patients in an ultrasound pathway with a corresponding shorter time to disposition.28
Both DPL and CT scan are problematic in pregnant patients. Goodwin et al examined the sensitivity of ultrasound in detecting hemoperitoneum after BAT in the pregnant patient population. Of the 6 patients with hemoperitoneum (out of 127 studied), 5 were detected by FAST exam, with the one missed not having an adverse outcome. There were 3 false positive exams, one of which was found to be serous intraperitoneal fluid at laparotomy. The other two had uneventful clinical courses of observation. This study demonstrated similar sensitivities and specificities to non-pregnant populations.29
The clinical pathway of ultrasound has not found a definitive role with respect to children suffering BAT. Numerous studies have not shown expected accuracy nor sensitivity when used in all patients.30-32 Coley et al found that the FAST scan missed 10 of 22 patients with free fluid confirmed by CT scan.30 In addition, injuries to the liver and spleen in pediatric patients are managed non-operatively more commonly than in the adult population. While ultrasound likely still is very specific for free fluid in the pediatric trauma patient, its role needs to be defined by further research.
Cardiac. Is there cardiac activity? Is there depressed cardiac activity? Is there normal cardiac activity? Is there presence of pericardial fluid? Focused cardiac ultrasound in the critical emergency patient can have high diagnostic yield in the patient experiencing arrest, shock, or having dyspnea.33 Beck’s triad of hypotension, JVD, and muffled heart tones34 only occurs in one-third of patients with tamponade. Focused cardiac ultrasound can allow the operator to visualize pericardial fluid directly and may provide evidence of tamponade physiology, expediting patient care and definitive therapy.35,36 In addition, cardiac imaging may be used to help clarify underlying rhythm,37 determine the presence of pericardial fluid,38,39 and determine gross evaluations of ejection fraction40 in those patients with hemodynamic instability.
Approaches to the critical patient have utilized adaptations of the FAST exam to systematically investigate specific anatomy to answer clinical questions. Hendrickson et al developed an algorithm using ultrasound in pulseless electrical activity (PEA) to assess first the heart (for gross cardiac motion) and then for signs of hypovolemia (right heart collapse). If hypovolemia was suspected, the aorta and potential spaces (FAST) were interrogated in search of blood loss.41 This model has been described with similar approaches to the patient with decreased blood pressure and signs of hypotension.42,43 Bedside ultrasound can provide rapid focused answers to the hemodynamically unstable patient and help guide therapy by addressing cardiac function and fluid collections in the torso. The subxiphoid view is used during the FAST scan to interrogate the pericardial space. In Figure 11, the pericardial space is distended with free fluid separating the myocardium and the hyperechoic pericardial sac.
| Figure 11. Positive Subxiphoid Ultrasound for Pericardial Fluid |
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Evaluation for Pneumothorax and Hemothorax. Both pneumothorax and hemothorax commonly accompany both blunt and penetrating injuries in trauma patients. The supine chest radiograph (CXR) has significant limitations for detection of these injuries compared to the upright film due to gravity and the dispersion of fluid/air across the thorax. Ultrasound is significantly more sensitive for pleural fluid than CXR, detecting as little as 20 cc of pleural fluid.13 Pleural effusion, assumed to be hemothorax in the acute trauma patient, is seen as an anechoic space above the hyperechoic diaphragm in either the right upper quadrant or left upper quadrant views. (See Figures 12, 13.)
| Figure12
(top). Positive Right Upper Quadrant Ultrasound for Hemothorax Figure 13 (bottom). Positive Left Upper Quadrant Ultrasound for Hemothorax |
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The diagnosis of pneumothorax is based upon the absence of normal respiratory ultrasound findings. In a normal patient, the phenomenon of "ung sliding" or "gliding" is appreciated and can be compared to the adjacent side. This term is used to describe the to-and-fro sliding motion between the visceral and parietal pleura, typically seen at the posterior edge of the rib shadows. In addition to lung sliding, the presence of a comet tail artifact at the level of the pleural line indicates the absence of pneumothorax. (See Figure 14.) Comet tail is an artifact created by reverberations from the interface of the parietal and visceral pleural layers. While not present in all patients (and perhaps pathologic in some), its presence indicates that the lung is opposed to chest wall. However, the comet tail artifact has limited utility in the trauma setting as this also may be produced falsely in the presence of subcutaneous air or subcutaneous bullet fragments. The use of Doppler has been described to demonstrate flow or movement of the visceral and parietal pleura in the normal lung. (See Figure 15.)
| Figures 14, 15. Comet Tail Artifact |
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Dulchavsky et al demonstrated that ultrasound diagnosis of pneumothorax was comparable to that of x-rays.44 This prospective study had images collected by surgery and trauma attendings who had undergone training and were blinded to the subsequent portable chest film. Of the 382 patients evaluated, 39 patients had pneumothoraces identified by radiography, 37 of which had been identified by ultrasound. The two missed had subcutaneous air and did not provide adequate ultrasound imaging. There were no false positives and the ultrasound evaluation took, on average, 2-3 minutes. Ma and Mateer addressed the utility of ultrasound in detecting hemothoraces.45 The diagnosis of hemothorax was based upon identification of free pleural fluid, appearing on ultrasound as an anechoic space proximal to the hyperechoic diaphragm. In their retrospective chart review of trauma patients who had sonograms of the pleural cavity (recorded and interpreted by the emergency physician) prior to further imaging and interventions, they found comparable results between the ultrasound interpretation and initial radiographs. Twenty-six of the 240 patients studied had hemothoraces, of which both modalities had 0 false positives, 1 false negative, 25 true positives, and 214 true negative findings. CT or tube thoracostomy findings were used as criterion standard for pathology. Both of these studies suggest that there may be a role for ultrasound in evaluation of thoracic injury.
Beyond the FAST. In addition to expansion of the FAST exam to include the pleural spaces, ultrasound may have other roles in the acutely injured patient. Fracture reduction of long bone injuries focuses on identifying the hyperechoic periostium and identifying disruptions in the bony cortex. Using a high frequency transducer, Durston described reducing forearm fractures in the injured child,46 yet the availability of these specialized transducers in the typical trauma setting is variable. Dulchavsky combined the FAST exam with the detection of long bone extremity fracture and pneumothorax detection into a FASTER exam.47 Additionally, the use of contrast agents to augment the FAST exam has been utilized to examine solid organ injury but remains less sensitive than CT in detecting these injuries. Agitated saline injected into a peripheral line has been used in the past to highlight various organs and improve the sensitivity for detecting abnormal flow patterns and disease. However, in a recent study of second-generation contrast agents used in a cohort of 210 blunt abdominal trauma patients, 88 with solid organ injury, contrast-enhanced sonography missed 18% of solid organ injuries compared to CT.48 Blaivas et al reported using an FDA-approved cardiac contrast agent during a FAST exam compared with the effective half-life of the agent. The authors found that a FAST can be performed in a feasible time period (mean 1 minute 42 seconds) during the effective life of the contrast agent, thus enhancing visualization of solid organ parenchyma.49 This study was in a simulated population, and future studies with advanced contrast agents will have to address the limitations of sonography for detecting solid organ injury in the clinical setting.
Vascular access in trauma is imperative for resuscitation and can be challenging in those with larger body mass index and distorted anatomy. Ultrasound-guided central lines have been proposed by the Agency for Healthcare Research and Quality in an attempt to decrease errors and improve on the chasm between published research of this safe intervention and current clinical practice.50 Additionally, ocular ultrasound has been described to interrogate the posterior structures of the eye in those patients too swollen to examine clinically. In a cohort of trauma patients, measurement of the optic nerve sheath 3 mm behind the globe greater than 5 mm correlated with increased elevation of intracranial pressure compared with CT scanning of the head.51 Significant care is needed to avoid applying any pressure (use copious amounts of gel) to the eye when scanning in case there is a ruptured globe. Obviously ultrasound can visualize specific anatomical relationships if the right equipment and trained personnel are available in the trauma setting. Future research and training may very well enable diagnostic ultrasound interventions to further define bedside trauma evaluation and care.
Conclusions
Focused ultrasound evaluation of the trauma patient has become the standard of care in many centers. Ultrasound training is incorporated into many training programs and is part of the Residency Review Committee’s (RRC) requirements for procedural competency in emergency medicine residency programs. The injured patient can be assessed quickly with ultrasound after a primary survey and management. The identification of free fluid in key areas within the torso can help the clinician risk stratify the patient and decide on further treatment based on hemodynamics. The FAST exam has proven to be a reliable bedside diagnostic for 21st-century trauma care, reducing the number of DPL’s and CAT scans. Future applications of ultrasound in trauma will become more commonplace after educational training is standardized and proficiency strategies are optimized. For now, many programs teach the FAST exam and this bedside ultrasound protocol serves as an introduction to approach the critical trauma patient.
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Ultrasound utilizes frequencies well above the range of human hearing to penetrate and visualize structures in the body. While human hearing is generally in the range of 20-20,000 Hz (cycles/second), diagnostic ultrasound is typically in the range of 2-12 mega-hertz (MHz), or 2-12 million cycles per second.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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