Sonography for Trauma Christine Butts and Justin Cook 9 KEY POINTS Focused abdominal sonography for trauma (FAST) is sensitive and specific for the detection of intraperitoneal free fluid, but it has poor results when used in an attempt to localize solid organ injury. The indications for FAST have expanded to include the evaluation of patients with normotensive blunt trauma and penetrating trauma. FAST can be learned quickly by most emergency physicians, although its chief limitation is that it is operator dependent. FAST EXAMINATION INTRODUCTION Ultrasound for the evaluation of trauma patients was one of the first applications of bedside ultrasound used by emergency physicians (EPs). The FAST examination (originally, focused abdominal sonography for trauma; currently, focused evaluation with sonography for trauma) was developed initially as a noninvasive modality for the initial triage of patients with hypotensive blunt abdominal or thoracic trauma. Its purpose was to rapidly identify patients with free intraperitoneal fluid or with pericardial effusion. However, over the past 20 years, FAST has evolved considerably to include the evaluation of patients after normotensive blunt trauma and penetrating trauma. Implementation of other bedside ultrasound advances has led to the development of E-FAST (or extended FAST). E-FAST involves evaluation of the inferior vena cava (IVC) for overall intravascular fluid status and assessment of the chest for pneumothorax and pleural effusion. For more information on E-FAST, see www. expertconsult.com COMPARISON OF IMAGING MODALITIES FOR TRAUMA Before the development of FAST, either computed tomography or diagnostic peritoneal lavage was the standard method for evaluating patients with abdominal trauma. Each of these modalities has distinct advantages and drawbacks. FAST allows the EP to rapidly evaluate trauma patients at the bedside, frequently while other interventions are ongoing. It requires a minimum of training, is noninvasive, and can be repeated. In addition to evaluating the abdomen, ultrasound can be used to evaluate the pericardium and pleura. Another important advantage of this technique is that intravenous contrast material or ionizing radiation is not required, which allows safe use in a broad spectrum of patients, including pregnant women. FAST is not without limitations, however. Although it can be learned quickly by most, it is operator dependent. Achievement of the highest sensitivity relies on the sonographer obtaining adequate views, which can be hampered by patient habitus, bowel gas, or the presence of subcutaneous air. FAST is not as reliable for pinpointing the site of hemorrhage or for discerning solid organ injury. The retroperitoneum is also typically poorly visualized, and hemorrhage in this area may be missed. WHAT WE ARE LOOKING FOR Following trauma, particularly blunt trauma, patients may be hemodynamically unstable because of bleeding from multiple sites. It can frequently be difficult to ascertain where the patient s injuries lie based on the history and physical examination alone. FAST was designed to identify possible sources of bleeding that result in instability. It relies on the premise that free fluid (blood) within the peritoneum will accumulate within the most gravity-dependent areas. These areas are the right upper quadrant between the liver and kidney (pouch of Morison), the right paracolic gutter, the left upper quadrant between the spleen and kidney, the potential space between the spleen and diaphragm, the left paracolic gutter, and the pelvis. FAST seeks to evaluate these areas quickly for the presence of free fluid. It also seeks to evaluate the pericardial sac for the presence of pericardial effusion and possible cardiac tamponade. The presence of cardiac activity or the status of overall cardiac function can also be assessed. LITERATURE REVIEW Multiple studies have demonstrated the utility of FAST for the evaluation of patients after blunt abdominal trauma. One of the first studies to highlight FAST by EPs was performed by Ma and Mateer in 1995. This study evaluated ultrasound for detection of free fluid not only in the peritoneum but also in the pericardium, the retroperitoneal space, and the pleural 79
SECTION I RESUSCITATION SKILLS AND TECHNIQUES cavity. The authors evaluated a total of 975 cavities and calculated a sensitivity of 90%, a specificity of 99%, and an accuracy of 99%. 1 This study demonstrated that with training, EPs are capable of identifying free fluid with high sensitivity and specificity. Subsequent studies focusing on FAST have found variable results ranging from sensitivities of 79% to 100% and specificities of 95.6% to 100%. 2-5 Although calculated sensitivities and specificities have been variable across studies, one finding that seems consistent is that both sensitivity and specificity appear to increase in hypotensive patients. 6 Conversely, a Cochran review published in 2005 found insufficient evidence from RCTs [randomized controlled trials] to justify promotion of ultrasound-based clinical pathways in diagnosing patients with suspected blunt abdominal trauma. 7 These findings have been controversial, and a similar literature review by Melniker found the FAST examination, adequately completed, is a nearly perfect test for predicting a Need for OR in patients with blunt torso trauma. 8 One finding that has appeared consistently in most studies is that although FAST is sensitive and specific for the detection of intraperitoneal free fluid, it has poor results when used in attempts to localize solid organ injury. 9,10 Ultrasound has been shown to be a reliable study for the evaluation of traumatic pericardial effusions. Mandavia et al. found that EPs with training in echocardiography had a sensitivity of 96%, a specificity of 98%, and an overall accuracy of 97.5% for this indication. 11 Fig. 9.1 Proper placement of the transducer for a subxiphoid view of the heart. Note that the sonographer is holding the transducer overhand and pushing downward into the subxiphoid space while pointing the transducer toward the left side of the chest. HOW TO PERFORM A FAST EXAMINATION FAST consists of ultrasound views of four primary areas, the right and left upper quadrants of the abdomen, the pelvis, and a view of the pericardium from the subxiphoid area. A low-frequency transducer is typically used to ensure proper depth of penetration. SUBXIPHOID VIEW To evaluate the subxiphoid view of the heart, the transducer is placed just below the subxiphoid process and aimed toward the patient s left shoulder (Fig. 9.1). It is frequently necessary to apply some pressure to the upper part of the patient s abdomen to enable the sonographer to look up into the patient s chest. It is also helpful to think of the transducer as a flashlight and imagine shining it toward the left side of the patient s chest. Another helpful tip for beginning sonographers is to increase the depth if at first the heart is not seen in full. A four-chamber view of the heart should be sought (Fig. 9.2). Specifically, the bright white (or hyperechoic) outline of the pericardium should be sought to evaluate for the presence of pericardial effusion. RIGHT UPPER QUADRANT The right upper quadrant should be evaluated in both the coronal and transverse planes. To begin, the transducer is placed on the patient s midaxillary line between the 8th and 11th ribs (Fig. 9.3). This position should be adjusted as needed to overcome rib shadowing and to obtain the best image possible. Aiming the indicator toward the patient s head will yield a coronal image. The interface between the liver and kidney Fig. 9.2 Subxiphoid view of the heart. A four-chamber view of the heart surrounded by the hyperechoic (white) border of the pericardium is seen. At the top of the screen, the left lobe of the liver is adjacent to the right ventricle. (pouch of Morison) and the potential spaces around this area should be thoroughly evaluated for the presence of free fluid (Fig. 9.4). This can be done by sweeping the transducer anteriorly and posteriorly. Moving the transducer superiorly a rib space or two will usually allow a view of the echogenic diaphragm curving over the dome of the liver. The area superior to the diaphragm, the costophrenic recess, can be evaluated for the presence of pleural fluid as well. Once a coronal image has been obtained, the transducer should be rotated so that the indicator points toward the patient s right to obtain a transverse view. Although such placement frequently provides an adequate view, it is often helpful to angle the transducer on a 80
CHAPTER 9 Sonography for Trauma Fig. 9.3 Placement of the transducer for evaluation of the right upper quadrant. Note that the indicator on the transducer is pointing toward the patient s head. This will yield a coronal image. The transducer should be placed along the anterior midaxillary line between the 8th and 11th rib spaces. Fig. 9.5 Placement of the transducer for evaluation of the left upper quadrant. Note that the indicator on the transducer is pointing toward the patient s head, which will yield a coronal image. The transducer should be placed along the posterior midaxillary line between the 8th and 11th rib spaces. Fig. 9.4 Normal right upper quadrant as viewed in a coronal orientation. The liver is seen on the left of the image, with the kidney seen to the right and slightly inferior to the liver. The diaphragm is seen as a brightly echogenic arc on the far left of the image. Fig. 9.6 Normal left upper quadrant as viewed in a coronal orientation. The spleen is seen on the left of the image, with the kidney lying to its right. At the bottom left of the image, a portion of the brightly echogenic diaphragm can be seen overlying the spleen. slightly oblique plane so that it fits into the intercostal space and thus limits rib shadowing. Once the liver and kidney are seen, sweeping the transducer superiorly and inferiorly offers a full evaluation of areas in which free fluid may collect. LEFT UPPER QUADRANT The left upper quadrant is evaluated in much the same manner as the right upper quadrant. One important distinction is that the left kidney is usually found in a more posterior and superior location. Therefore, to obtain a coronal image, the transducer is placed in the posterior midaxillary line between the 8th and 11th ribs (Fig. 9.5). The indicator should be pointing toward the patient s head. It is particularly important not only to evaluate the interface between the kidney and spleen but also to seek the interface between the spleen and diaphragm (Fig. 9.6). This aids in viewing the costophrenic recess for free pleural fluid, as well as the subphrenic recess, where free peritoneal fluid frequently collects. Again, once the coronal view has been obtained, the transducer should be swept anteriorly and posteriorly to fully assess the left upper quadrant. After the coronal plane has been viewed, the transducer should be rotated so that the indicator faces the patient s right. It may be helpful to place the transducer at a slight angle to avoid any artifact created by the ribs. Once the interface between the kidney and spleen is found, the transducer should be swept inferiorly and posteriorly to evaluate this region in full. 81
SECTION I RESUSCITATION SKILLS AND TECHNIQUES PELVIS The final component of the basic FAST examination is evaluation of the pelvis. The transducer should be placed just superior to the pubic symphysis. Beginning in the transverse plane, the indicator on the transducer should be facing toward the patient s right (Fig. 9.7). The bladder is easily identified in this orientation as a rectangularly shaped object filled with dark, anechoic urine, especially if the ultrasound can be performed before placement of a urinary catheter (Fig. 9.8). Although the bladder is generally identified quickly, the evaluation should proceed further and the bladder be used as an acoustic window to view the dependent portions of the pelvis. This can be done by tilting the transducer toward the patient s feet and back upward or more superiorly. The transducer can then be rotated toward the patient s head to view the same area in a sagittal orientation (Fig. 9.9). In this plane the bladder has a triangular appearance (Fig. 9.10). Complete evaluation of the potential spaces of the pelvis can be achieved by tilting the transducer from side to side. Fig. 9.7 Placement of the transducer for evaluation of the pelvis in a transverse orientation. Note that the indicator is pointing toward the patient s right. The transducer should be placed just superior to the pubic symphysis. Fig. 9.9 Placement of the transducer for evaluation of the pelvis in a sagittal orientation. Note that the indicator is pointing toward the patient s head. The transducer should be placed just superior to the patient s pubic symphysis. Fig. 9.8 Normal pelvis as viewed in a transverse orientation. In this image the bladder is clearly seen as a rectangularly shaped fluid collection. Its clearly defined walls distinguish it as the bladder as opposed to free fluid. Fig. 9.10 Normal pelvis as viewed in a sagittal orientation. In this image the bladder is seen as a triangular fluid collection. Just deep to the bladder, the fundus of the uterus can be seen pushing into its posterior wall. 82
CHAPTER 9 Sonography for Trauma NORMAL AND ABNORMAL FINDINGS The primary purpose of FAST is to identify free fluid either within the peritoneum or within the pericardium. Free fluid, although it may accumulate in a number of dependent areas, is fairly reliable in its appearance. Fluid is identified as black on ultrasound, and most free fluid, in this case blood, will have a black appearance. Bleeding that has begun to form a clot is less fluid-like in its consistency and may appear as varying shades of gray. When evaluating the subxiphoid view of the heart, the bright white, echogenic outline of the pericardium should be sought (see Fig. 9.2). Normally, the pericardium should closely abut the ventricle. Free fluid will be seen as a black collection between the pericardium and the ventricles of the heart (Fig. 9.11). Typically, fluid will first accumulate in the most dependent part of the pericardial sac and can thus be seen deep to the left ventricle. It should be noted that the amount of fluid seen may appear underwhelming on first inspection. It is important to realize that a smaller amount of fluid is needed to cause cardiac tamponade in a trauma patient. This is due to the rapid accumulation of free fluid within the pericardial sac, which quickly overcomes the ability of the fibers of the pericardium to stretch to accommodate increasing pressure. In the right upper quadrant, free fluid most commonly accumulates in the area between the liver and kidney (pouch of Morison). Acute hemorrhage will be seen as an anechoic (black) stripe of varying size in this potential space (Fig. 9.12). Free fluid in the left upper quadrant appears much the same as in the right upper quadrant. It may appear as an anechoic stripe between the spleen and kidney. However, the potential space between the spleen and the diaphragm is a common location for free fluid to accumulate and may be overlooked without careful evaluation (Figs. 9.13 and 9.14). In the pelvis, free fluid will accumulate in the gutters surrounding the bladder (Fig. 9.15). The bladder can be distinguished from free fluid by noting the rectangular shape of the bladder. Free fluid is amorphous and will appear to seep into the gutters of the pelvis, whereas the bladder is either a rectangular (transverse) or triangular (sagittal) shape, depending on the imaging plane chosen. PITFALLS The bedside sonographer may encounter both technical and diagnostic challenges while performing FAST. Diagnostic challenges frequently arise from misuse or incorrect interpretation of the FAST examination. FAST was designed to evaluate the presence of free fluid and pericardial effusion and is most sensitive when used for this purpose, particularly in hypotensive blunt trauma patients. It does not MORRISON Fig. 9.12 Right upper quadrant image demonstrating free fluid in the pouch of Morison. This large amount of fluid is represented by the anechoic (black) area both surrounding the tip of the liver and in the space between the kidney and liver. Fig. 9.11 Subxiphoid image of the heart with a pericardial effusion demonstrated by the arrow. The pericardium can be seen at the bottom of the image as a bright white boundary surrounding the dark fluid collection. This is a large pericardial effusion. Fig. 9.13 Transverse image of the left upper quadrant demonstrating an anechoic (black) fluid collection surrounding the spleen. The kidney can be seen at the bottom right of the image. 83
SECTION I RESUSCITATION SKILLS AND TECHNIQUES fare as well when used to either diagnose the location of injury or rule out injury in normotensive or penetrating trauma patients. The primary technical challenge faced by the sonographer is overcoming artifact. Subcutaneous air, bowel gas, or air in the stomach may make performing a complete FAST examination difficult. Misidentification of normal structures may also cause falsely positive FAST findings. A full stomach, the IVC, and the gallbladder may appear similar to fluid collections. Knowledge of anatomy and experience with the normal appearance of the structures may minimize this mistake. Fig. 9.14 Coronal image of the left upper quadrant demonstrating an anechoic (black) free fluid collection between the spleen, on the right of the image, and the diaphragm, on the left of the image. BLDDR SUGGESTED READINGS Blaivas M, Lyon M, Sandeep D. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med 2005;12:844-9. Ma OJ, Mateer JR, Ogata M, et al. Prospective analysis of a rapid trauma ultrasound examination performed by emergency physicians. J Trauma 1995;38:879-88. Melniker LA. The value of focused assessment with sonography in trauma examination for the need for operative intervention in blunt torso trauma: a rebuttal to emergency-ultrasound based algorithms for diagnosing blunt abdominal trauma (review), from the Cochrane Collaboration. Crit Ultra J 2009;1:73-8 Stengel D, Bauwens K, Sehouli J, et al. Emergency-ultrasound based algorithms for diagnosing blunt abdominal trauma. Cochrane Database Syst Rev 2005;2:CD0044446. REFERENCES References can be found on Expert Consult @ www.expertconsult.com. Fig. 9.15 Pelvic image demonstrating free fluid overlying the bladder. The bladder is seen as a well-defined fluid collection surrounded by echogenic walls at the right of the image. The free fluid is seen to the right of the bladder and is distinguished by its lack of clear boundaries. At the superior aspect of the free fluid, bowel loops are seen within it, a finding that further distinguishes it from the bladder. 84
CHAPTER 9 Sonography for Trauma E-FAST EXAMINATION The extended FAST, or E-FAST, is a relatively new addition to the standard FAST protocol and includes views of the lungs for pleural fluid and pneumothorax and views of the IVC to estimate the volume status of the patient. LITERATURE REVIEW Ultrasound of the chest shows potential as a rapid tool for diagnosing the presence of pleural fluid and pneumothorax. A prospective study of 240 trauma patients was retrospectively analyzed and published by Ma and Meteer in 1997. This study found an overall sensitivity of 96.2%, specificity of 100%, and accuracy of 99.6% for detection of hemothorax. Interestingly, the authors concluded that Ultrasonography is comparable to the initial chest radiograph for accuracy in detection of hemothorax and that its use may result in more rapid diagnosis in trauma patients. 12 Similarly, the use of ultrasound for detection of pneumothorax has also yielded positive results, with multiple studies reporting a range of sensitivities from 95.3% to 100% and specificities from 78% to 99.2%. 13-15 Specifically, the study of Blaivas et al. evaluated ultrasound and supine chest radiography head to head and found ultrasound to have higher sensitivity (98.1% for ultrasound versus 75.5% for chest radiography) and similar specificity (99.2% for ultrasound versus 100% for chest radiography). 15 Ultrasound of the IVC has also been examined as an indirect measure of central venous pressure (CVP). A study by Randazzo et al. in 2008 compared CVP estimated by EPs using ultrasound of the IVC with that determined by cardiologists using formal echocardiography. Overall, they found a 70.2% rate of agreement, with most agreement occurring in patients with high CVP (83.3%). 16 A later study by Nagdev et al. in 2010 compared the caval index (percent collapse of the IVC with inspiration) with CVP measured directly with an indwelling catheter and found that a caval index of greater than 50% was 91% sensitive and 94% specific in predicting a CVP of less than 8 mm Hg. 17 Both the overall size of the IVC and the percentage of collapse with inspiration have also been found to correlate with blood loss, an important finding in trauma patients. 18,19 HOW TO SCAN AND SCANNING PROTOCOLS The lungs and pleural recesses can be evaluated for the presence of fluid, such as occurs with hemothorax, and for the presence of pneumothorax. The pleural recesses should be viewed while the right and left upper quadrants of the abdomen are viewed in the coronal orientation as part of the traditional FAST examination. Moving the transducer more superiorly, from the 8th through the 11th rib interspaces to the 6th to 9th rib interspaces, will usually allow a clear image of the bright white, echogenic diaphragm arcing over the liver or spleen (Fig. 9.16). To the left of the diaphragm, the costophrenic recess can be studied for the presence of fluid. Fig. 9.16 Coronal image of the right upper quadrant. The diaphragm is visible as a hyperechoic (white) arc to the left of the liver. The area to the left of the diaphragm is the thoracic cavity and should be evaluated for the presence of an anechoic or hypoechoic fluid collection. The anterior surface of the lung can also be evaluated for pneumothorax. A high-frequency transducer is best suited for this purpose because it offers a clearer image of the interface between the visceral and parietal pleura. The transducer should be placed in the sagittal orientation, with the indicator pointing toward the patient s head, in the second or third intercostal space in the midclavicular line. In this view the ribs can be seen with acoustic shadowing extending behind them. Slightly deep to the ribs, the interface of the parietal and visceral pleura can be seen as an echogenic, white horizontal line that slides back and forth with respiration (Video 1). The IVC can be viewed as it courses behind the liver toward the right atrium. The size and movement of the IVC can be used to estimate the overall fluid status of the patient. Again using a low-frequency transducer to obtain adequate depth of penetration, the IVC is typically best seen in the epigastric area. The transducer should be placed in a sagittal orientation, just to the patient s right, with the indicator pointing toward the patient s head. Tilting the transducer toward the patient s head will best use the liver as an acoustic window. The IVC can be seen as a longitudinal tube terminating in the right atrium of the heart (Fig. 9.17). It should be evaluated for overall size and for changes with respiration. NORMAL AND ABNORMAL FINDINGS Pneumothorax is chiefly identified by watching the pleural interface for the presence of sliding. When the lung is viewed with a high-frequency transducer, the visceral and parietal pleura interface is seen as a bright white line deep to the ribs. It should normally be seen sliding back and forth with respiration. When pneumothorax is present, the appearance of sliding is disrupted and the pleura appears as a stationary hyperechoic line (Video 2). A pleural effusion is typically seen from the costodiaphragmatic recesses as an anechoic (black) fluid collection superior to the diaphragm (Fig. 9.18). This is in contrast to a normal 84.e1
SECTION I RESUSCITATION SKILLS AND TECHNIQUES Fig. 9.17 Longitudinal image of the inferior vena cava (IVC) as it courses behind the liver and toward the right atrium. The overall size and respiratory variation of the IVC provide clues to the overall intravascular status of the patient. Fig. 9.18 Coronal image of the right upper quadrant demonstrating a pleural effusion. The diaphragm can be seen as a hyperechoic line arcing over the liver. To the left of the liver (more superior), an anechoic fluid collection is seen. Table 9.1 Summary of the Relationship Between the Size and Percent Collapse of the Inferior Vena Cava and Central Venous Pressure IVC SIZE CHANGE WITH INSPIRATION (%) CENTRAL VENOUS PRESSURE <1.5 cm Total collapse 0-5 cm 1.5-2.5 cm >50% collapse 5-10 cm 1.5-2.5 cm <50% collapse 11-15 cm >2.5 cm <50% collapse 16-20 cm spectrum, depending on the patient s overall intravascular fluid status (Videos 3 and 4). Table 9.1 summarizes the relationship between IVC size and percent collapse with respiration correlated with CVP. These findings are applicable to trauma patients when attempting to establish their overall stability. A patient with a finding of low CVP following blunt abdominal trauma may cause the EP to be more aggressive in resuscitation or more cautious in delaying definitive management. >2.5 cm No change >20 cm IVC, Inferior vena cava. lung, which will appear as a hazy, ill-defined area superior to the diaphragm. The hazy appearance is caused by the presence of the normally air-filled lungs. No set normal definition of the IVC has be established. Rather, both its size and percent collapse are situated on a Video 1 Video of the interface between the visceral and parietal pleura demonstrating the slide sign. Video 2 Video of the interface between the visceral and parietal pleura showing lack of movement (lack of a slide sign ), which is consistent with pneumothorax. Video 3 Video of the IVC before fluid resuscitation. The overall size of the IVC is small and there is nearly complete collapse with respiration. Video 4 Video of the IVC after fluid resuscitation. The IVC now appears larger and does not collapse with respiration. 84.e2
CHAPTER 9 Sonography for Trauma REFERENCES 1. Ma OJ, Mateer JR, Ogata M, et al. Prospective analysis of a rapid trauma ultrasound examination performed by emergency physicians. J Trauma 1995;38:879-85. 2. Healey M, Simons RK, Winchell RJ, et al. A prospective evaluation of abdominal ultrasound in trauma: is it useful? J Trauma 1996;40:875-85. 3. Boulanger BR, McLellan BA, Brenneman FD, et al. Emergent abdominal sonography as a screening test in a new diagnostic algorithm for blunt trauma. J Trauma 1996;40:867-74. 4. Rozycki GS, Ochsner MG, Jaffin JH, et al. Prospective evaluation of surgeon s use of ultrasound in the evaluation of trauma patients. J Trauma 1993;34:516-27. 5. Rozycki GS, Ballard RB, Feliciano DV, et al. Surgeon performed ultrasound for the assessment of truncal injuries: lessons learned from 1540 patients. Ann Surg 1998;228:557-67. 6. Lee BC, Ormsby EL, McGahan JP, et al. The utility of sonography for the triage of blunt abdominal trauma to exploratory laparotomy. AJR Am J Roentgenol 2007;188:415-21. 7. Stengel D, Bauwens K, Sehouli J, et al. Emergency-ultrasound based algorithms for diagnosing blunt abdominal trauma. Cochrane Database Syst Rev 2005;2:CD0044446. 8. Melniker LA. The value of focused assessment with sonography in trauma examination for the need for operative intervention in blunt torso trauma: a rebuttal to emergency-ultrasound based algorithms for diagnosing blunt abdominal trauma (review), from the Cochrane Collaboration. Crit Ultra J 2009;1:73-84. 9. Chiu WC, Cushing BM, Rodriguez A, et al. Abdominal injuries without hemoperitoneum: a potential limitation of focused abdominal sonography for trauma (FAST). J Trauma 1997;42:617-25. 10. Kendall JL, Faragher J, Hewitt GJ, et al. Emergency department ultrasound is not a sensitive detector of solid organ injury. West J Emerg Med 2009;10:1-5. 11. Madavia DP, Hoffner RJ, Mahaney K, et al. Bedside echocardiography by emergency physicians. Ann Emerg Med 2001;38:377-82. 12. Ma OJ, Mateer JR. Trauma ultrasound examination versus chest radiography in the detection of hemothorax. Ann Emerg Med 1997;29:312-16. 13. Lichtenstein DA, Meziere G, Lascois N, et al. Ultrasound diagnosis of occult pneumothorax. Crit Care Med 2005;33:1231-8. 14. Lichtenstein DA, Menu Y. A bedside ultrasound ruling out pneumothorax in the critically ill: lung sliding. Chest 1995;108:1345-8. 15. Blaivas M, Lyon M, Sandeep D. A prospective comparison of supine chest radiography and bedside ultrasound for the diagnosis of traumatic pneumothorax. Acad Emerg Med 2005;12:844-9. 16. Randazzo MR, Snoey ER, Levitt MA, et al. Accuracy of emergency physician assessment of left ventricular ejection fraction and central venous pressure using echocardiography. Acad Emerg Med 2008;10:973-7. 17. Nagdev AD, Merchant RC, Tirado-Gonzalez A, et al. Emergency department bedside ultrasonographic measurement of the caval index for noninvasive determination of low central venous pressure. Ann Emerg Med 2010;55:290-5. 18. Lyon M, Blaivas M, Brannam L. Sonographic measurement of the inferior vena cava as a marker of blood loss. Am J Emerg Med 2005;23:45-50. 19. Yanagawa Y, Nishi K, Sakamoto T, et al. Early diagnosis of hypovolemic shock by sonographic measurement of inferior vena cava in trauma patients. J Trauma 2005;58:825-9. 84.e3