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STATE-OF-THE-ART REVIEW ARTICLE |
a Division of Emergency Medicine, Children's Hospital Boston and Department of Pediatrics, Harvard Medical School, Boston, Massachusetts
b Division of Emergency Medicine, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
ABSTRACT
Bedside emergency ultrasound has been used by emergency physicians for >20 years for a variety of conditions. In adult centers, emergency ultrasound is routinely used in the management of victims of blunt abdominal trauma, in patients with abdominal aortic aneurysm and biliary disease, and in women with first-trimester pregnancy complications. Although its use has grown dramatically in the last decade in adult emergency departments, only recently has this tool been embraced by pediatric emergency physicians. As the modality advances and becomes more available, it will be important for primary care pediatricians to understand its uses and limitations and to ensure that pediatric emergency physicians have access to the proper training, equipment, and experience. This article is meant to review the current literature relating to emergency ultrasound in pediatric emergency medicine, as well as to describe potential pediatric applications.
Key Words: emergency ultrasound • bedside ultrasound • ultrasound • sonography
Abbreviations: EUS—emergency ultrasound • ED—emergency department • EP—emergency physician • FAST—focused assessment with sonography in trauma • CT—computed tomography • DPL—diagnostic peritoneal lavage • CI—confidence interval • IV—intravenous • PEA—pulseless electrical activity • ETT—endotracheal tube
Bedside emergency ultrasound (EUS) has been used in the emergency department (ED) for >2 decades. Its use has grown rapidly as it has gained widespread acceptance among emergency physicians (EPs), and the range of diagnostic and procedural applications has continued to expand. The American College of Emergency Physicians Section of Emergency Ultrasound now has >500 members. In addition, the American Board of Emergency Medicine, American College of Emergency Physicians, and Society of Academic Emergency Medicine have deemed bedside ultrasound part of the core content for training residents in emergency medicine,1 and EUS is now a required component of emergency medicine residency training.2 In recent years, pediatric EPs have increasingly begun to use this modality. One recent study found that 39% of pediatric emergency medicine fellowship directors reported routine use of ultrasound by faculty.3
Many of the initial uses for EUS were applicable to disease states found primarily in adult patients, for example, evaluation for abdominal aortic aneurysm or biliary tract disease. However, the increased use of EUS to guide invasive procedures and the ever-broadening range of diagnostic applications reviewed in this article suggest that its use should not be limited to adult patients. As use of this imaging tool in pediatric EDs grows, there will be an increased need for access to training and proper equipment for pediatric emergency medicine physicians. The incorporation of EUS into pediatric emergency practice necessitates that the general pediatrician appreciate current capabilities, limitations, and areas of active research for this promising technology. This review outlines current and potential uses for EUS in the pediatric ED population. We additionally describe recent advances and novel applications of EUS in acutely ill or injured children.
Traditional Uses
Trauma
Emergency bedside sonography for victims of blunt abdominal trauma was one of the first applications for ultrasound in emergency medicine and remains one of the most frequent indications. The most commonly used test is the focused assessment with sonography in trauma (FAST) examination. The guiding principle behind the FAST scan is that the free fluid of hemoperitoneum or hemopericardium will appear anechoic (black) against the more hyperechoic (gray) background of solid organs.
The FAST scan possesses several advantages over other methods used to identify significant intraabdominal injury. Abdominal computed tomography (CT) is currently the study of choice in the evaluation of the stable pediatric patient with suspected traumatic abdominal organ injury. However, abdominal CT exposes the patient to ionizing radiation and necessitates the presence of other specialists to perform and interpret the test. The FAST scan can be performed at the bedside immediately by the treating physician and has no harmful radiation effects. Furthermore, it can be repeated as often as clinically indicated as the patient's condition changes. More importantly, if the patient is not stable, it may be unsafe to transport the patient to the radiology suite, and, thus, bedside FAST scanning offers a distinct advantage. Although diagnostic peritoneal lavage (DPL) has been the procedure traditionally used to evaluate unstable patients with injuries requiring operative intervention, it is now rarely used in children, because it is invasive and is contraindicated in selected patients. In addition, DPL has such a high sensitivity for intraabdominal bleeding that the rate of nontherapeutic laparotomy is unacceptably high.4 Like DPL, the FAST scan can be done rapidly at the bedside in critically injured patients but is noninvasive and can better predict the need for laparotomy.5
This application of EUS has generated a large body of research. Many EPs and surgeons now consider it a standard part of the trauma evaluation. Because the number of studies assessing the FAST scan is quite large, a review of them here is beyond the scope of this article. However, a brief, critical review is provided.
Results of studies examining the value of FAST in adults have been mixed. In 2005, a Cochrane review of the literature examining the use of EUS in blunt abdominal trauma was published.6 Only 6 studies were included in the final analysis. The authors concluded that there is insufficient evidence to justify the use of EUS-based pathways in assessing organ injury in victims of blunt abdominal trauma. Subsequent to this report however, Melniker et al7 published the First Sonography Outcomes Assessment Program Trial, a randomized, controlled clinical investigation. The authors found that use of ultrasound significantly reduced the time from ED presentation to operative care (57 vs 166 minutes). Patients in the ultrasound group were also found to have shorter lengths of stay, fewer complications, and decreased total charges, which the authors hypothesize may have been the result of a more rapid diagnosis in both operative and nonoperative patients, thus facilitating other types of care. With growing evidence regarding its benefit, the FAST examination remains one of the most commonly used and widely accepted applications of ED bedside ultrasound.
The use of the FAST scan as a screening tool specifically in children is more challenging for 2 major reasons. First, children with intraabdominal injuries are more frequently managed nonoperatively and, thus, the need for rapid decision-making regarding operative management, although still crucial, is less common. Second, children have a relatively higher incidence of solid organ injury without free fluid8–10; consequently, a negative FAST scan may not obviate the need for an abdominal CT when there is clinical suspicion of significant injury.
Despite these challenges, there is extensive data addressing the use of the FAST scan in children. Comparing results must be done carefully, however, because these investigations have used varying definitions of positive and negative FAST scans, used different scanning personnel, and measured different outcomes. In addition, few studies have addressed the use of FAST in assessing the need for laparotomy in children. Reported sensitivities have ranged from 40% to 93%, with associated specificities of 79% to 100%.8,9,11–26 One meta-analysis pooled pediatric data from 4 trials encompassing 501 patients.27 The authors found that a positive FAST examination had a likelihood ratio for organ injury of 17.35 (95% confidence interval [CI]: 8.97–33.53) but that a negative FAST scan did not have a satisfactory negative likelihood ratio (0.32; 95% CI: 0.22–0.45) for ruling out organ injury. Another review concluded that the FAST scan can identify intraabdominal damage but that a negative ultrasound does not rule out important injury.28
To account for the reported low sensitivity while using the apparent high specificity of the FAST scan in children, 1 study proposed using FAST in combination with physical examination.25 In the study protocol, an abdominal CT scan was obtained in the presence of the following: (1) a positive FAST scan, (2) a physical examination suggesting abdominal injury, (3) hypotension, (4) a hematoma or contusion on the torso, or (5) neurologic impairment. Patients were taken to the operating room if they were hypotensive and had a positive FAST scan. FAST scan alone had a sensitivity of 70% and specificity of 100%, but when combined with physical examination the sensitivity rose to 100%. Importantly, 5 children who had positive FAST scans and confirmed solid-organ injury by CT did not have physical examination findings suggesting intraabdominal injury. Of note, 2 children had positive FAST scans for evidence of injury but falsely negative CT scans. Similarly, another study found the accuracy of the FAST examination to be 100% in 13 hypotensive pediatric patients.18
Collectively, these data suggest that the FAST examination can be a valuable and reliable diagnostic tool in children with blunt abdominal trauma and hypotension who may be too unstable to be moved from the resuscitation bay. In addition, in the multitrauma patient, FAST may help to prioritize operative management. In the stable patient, the FAST scan should be used not as a screening tool but rather in conjunction with the clinical history and physical examination.
Procedural Applications of Ultrasound
Vascular Access
Although ultrasound-guided vascular access is not a new application, EUS-directed vessel cannulation is an important new procedure for the EP and may soon represent a standard of care for central venous access. In a 2001 Agency for Healthcare Research and Quality report,29 the US Department of Health and Human Services stated that ultrasound use for central venous catheterization is "a clear [opportunity] for safety improvement." The urgent need to reduce patient injury from the complications of necessary procedures has been one of the most powerful forces driving the placement of ultrasound in the ED.
Ultrasound use for central venous access has been shown to increase success rates and decrease the number of procedural complications.30–39 Initial investigations were performed largely in the adult population, particularly in the critical care or anesthesia arena. In the pediatric population, Verghese et al40 demonstrated that use of ultrasound for internal jugular vein cannulation was superior to the use of anatomic landmarks in terms of overall success rate, speed, and the incidence of inadvertent carotid artery puncture. Likewise, another study reported a 95% success rate on the first attempt when performing ultrasound-guided central venous cannulation in 42 children.41 A more recent study published in 2007 described using ultrasound for subclavian vein catheter placement in children <10 years of age.42 In none of these studies, however, was access obtained by an EP.
In the last several years, literature has emerged examining ultrasound-guided vascular access specific to emergency settings. One study showed that, during cardiopulmonary resuscitation, use of EUS by EPs for femoral vein catheterization improved success rates and lowered complications compared with landmark techniques in adults.32 In 2 separate studies, both Milling et al38 and Leung et al43 found that use of EUS improved first-attempt and overall success rates while producing fewer complications than landmark techniques when placing internal jugular vein catheters. Milling et al38 found that, when comparing dynamic ultrasound to landmark technique, the odds ratio for successful catheter placement using ultrasound was 53 (95% CI: 6.6–440). Further supporting the use of EUS in pediatric patients, research has shown that anatomic landmarks do not always predict femoral vein and artery anatomy; in 1 study, for example, in 12% of children, the femoral artery either completely or partially overlapped the femoral vein.44 Another investigation found that, after identifying the femoral vascular anatomy with ultrasound, the landmark method located the correct site of the femoral vein in only 9 of 30 attempts.45
EUS has also been shown to be useful in peripheral intravenous catheter placement. One study showed that EUS-guided brachial and basilic vein cannulation was safe, rapid, and had a high success rate in adult patients with difficult intravenous access.46 Costantino et al47 found that, in adults with difficult access, ultrasonographic-guided peripheral venous access was more successful, required less time, decreased the number of punctures, and was associated with greater satisfaction than traditional methods. A recent pilot study reported that lack of ultrasound visualization of peripheral veins was associated with unsuccessful peripheral intravenous (IV) placement in children <7 years old.48
Ultrasound has also been used to confirm intraosseous needle placement in adult cadavers. Stone et al49 showed that ultrasound correctly identified the location of all of the intraosseous placements, whereas examination of the IV tubing drip chamber for flow had 88% sensitivity and 25% specificity for correct needle position.
The current federal emphasis on patient safety and the abundant available literature both support the notion that EUS should be used when obtaining central venous access in the ED. Whether EUS can improve peripheral IV catheter placement in selected children remains an important area warranting additional investigation.
Identification and Drainage of Abnormal Fluid Collections
Differentiating cellulitis from abscess is a common reason for referral to the pediatric ED. Although abscesses may be clinically obvious in many patients, identification of some presents a diagnostic challenge. Moreover, the best area for incision and drainage may not be clear from physical examination. Of equal significance, there may be structures near the abscess, such as vessels or nerves, posing significant risk to the procedure. EUS plays a critical role both in abscess identification and in the safe performance of incision and drainage.
In 1 study of adult patients, the authors showed that clinicians were able to accurately identify subcutaneous abscesses after only a 30-minute training session and that EUS changed the clinical impression correctly in 17 of 18 cases.50 Another study found that EUS examination changed physician management in approximately one half of ED patients who presented with cellulitis.51 As pediatric emergency medicine physicians gain ultrasound skill and experience, this technology should aid in abscess identification and safe incision and drainage.
In a related application, Blaivas et al52 recently reported use of EUS in the evaluation of 6 patients with suspected peritonsillar abscess. Another study found that ultrasound could reliably identify peritonsillar abscesses and be used for their drainage.53
Numerous studies have examined ultrasound for identification and drainage of pleural and pericardial effusions. Although aspiration of these fluids in the ED has traditionally relied on using anatomic landmarks alone, EUS is being increasingly used for these procedures. Research is needed to define how EUS can be used safely and effectively in children requiring aspiration of pleural and pericardial fluid.
Bladder Catheterization and Suprapubic Aspiration
EUS can be used to determine the presence of fluid in the bladder before attempting urine collection. If the bladder appears empty, the clinician may defer the procedure until the child's bladder contains urine. EUS has been shown to increase the success rates in obtaining urine in both bladder catheterization and suprapubic aspiration. One study found that, using a predetermined calculation for bladder size, EUS could identify infants for whom urinary catheterization was likely to be unsuccessful.54 Two other studies showed that use of EUS significantly improved success rates of bladder catheterization.55,56 EUS has been shown to increase rates of success in the performance of suprapubic aspirations as well. Both Gochman et al57 and Munir et al58 reported that EUS significantly improved the success rate of suprapubic aspirations, limiting the number of failed attempts, whereas another study demonstrated similar success using ultrasound during the procedure.59
Foreign Body Identification
Ultrasound has long been used by radiologists to identify subcutaneous foreign bodies and has some advantages over radiography. First, it permits direct, real-time localization for the EP. Second, it has been shown to be more sensitive than radiograph for certain types of materials.60–62 Foreign bodies such as wood and plastic (and sometimes glass) are rarely detected radiographically, and EUS may, therefore, be better suited for these materials.
Studies of EUS used by EPs have demonstrated significant accuracy in the identification and localization of certain types of foreign bodies. One study reported a 93% sensitivity for the detection of wood foreign bodies and a 73% sensitivity for plastic when EUS was performed by EPs.63 Another study found that EPs' ability to detect various foreign bodies did not differ statistically from that of radiologists or ultrasound technicians.64 In the pediatric population, Friedman et al65 reported a small case series in which ultrasound was equally sensitive and more specific than radiography for detection of foreign bodies and that combining ultrasound with patient perception resulted in the highest sensitivity. These studies offer early data supporting the use of EUS in the evaluation and management of select soft tissue foreign bodies.
Obstetric and Gynecologic Emergencies
Abdominal pain in the adolescent girl is a common presenting complaint and one that is associated with a range of life-threatening and organ-threatening gynecologic conditions. EUS has been demonstrated in multiple studies to be especially valuable in the evaluation of pregnancy, specifically in ruling in those patients with normal intrauterine pregnancy and, thus, ruling out ectopic pregnancy in those patients with no increased risk of heterotopic pregnancy.
In the pregnant patient with abdominal pain, ectopic pregnancy must be identified and treated rapidly. Ultrasound has been shown to be an effective tool as part of a protocol to diagnose intrauterine and ectopic pregnancy.66 One prospective study demonstrated 100% sensitivity and 95% specificity for the diagnosis of intrauterine pregnancy and, thus, ruling out ectopic pregnancy by EPs.67 Another showed that EUS, when performed by EPs, can be used to rule out ectopic pregnancies and make accurate diagnoses in most patients with complicated first-trimester pregnancies.68 In relation to the time to diagnosis and lengths of stay, several studies have shown that EUS performed in the ED significantly reduced the time to diagnosis and treatment in patients with ruptured ectopic pregnancy when compared with patients who had ultrasound performed in the radiology suite.69–72 Another investigation reported that pelvic sonography performed by EPs was sensitive, specific, and safe and could shorten ED length of stay in women with early pregnancy.73 Similar results were demonstrated by Blaivas et al74 in women found to have intrauterine pregnancies by EUS. These studies imply that ultrasound may have a role in the emergent and urgent evaluation of the pregnant female.
The value of EUS in the nonpregnant female patient with abdominal pain is less clear. Diagnoses such as ovarian torsion, ovarian cyst, and tubo-ovarian abscess require advanced sonographic skill and experience. Further research with EUS in these conditions is needed to determine the range of its use.
Cardiac
Cardiac ultrasound, as part of the FAST examination, was one of the first applications of ED sonography. More focused cardiac examinations have become common in adult EDs and have been reported in pediatric settings as well. The 2 most common indications for EP-performed echocardiography are detection of pericardial effusion and evaluation of cardiac activity in patients with cardiac arrest and pulseless electrical activity (PEA). The uses of cardiac ultrasound in the patient with undifferentiated shock or to assess cardiac function are less established and are discussed later.
One of the earlier studies reporting the use of emergency bedside cardiac ultrasound retrospectively described 156 cases of EP-performed echocardiography. The authors reported that patient care was enhanced in all (n = 7) of the positive cases (4 traumatic pericardial effusions, 1 malignant pericardial effusion, 1 pulmonary embolism, and 1 case of PEA).75 Another study found that ED echocardiography changed decision-making in 37% of patients, changed treatment in 25% of patients, and changed disposition in 11% of patients.76 In this study, however, scans were not performed by EPs.
The diagnosis of pericardial effusion and tamponade is often difficult to make clinically, and classic physical examination findings such as Beck's triad are unreliable.77 The use of bedside ultrasound to diagnose pericardial effusions has been shown to be accurate in several different scenarios in the adult literature. In the setting of blunt trauma, EPs have shown the ability to use echocardiography to diagnose cardiac rupture and subsequent pericardial effusions.78–80 A retrospective review showed that EP-performed echocardiography led to reduced time to diagnosis and disposition to the operating room and to increased survival rates in adults with penetrating chest trauma.81 Finally, Mandavia et al82 found that EPs, with limited training, could detect or exclude pericardial effusions with an overall accuracy of 97.5% in patients with a variety of presenting complaints.
The pediatric literature is limited. Milner et al83 reported 3 cases of children with altered mental status and tachycardia who were subsequently diagnosed with pericardial effusions and tamponade by cardiologist-performed echocardiography. The authors implied that EP-performed cardiac sonography may have led to earlier diagnosis and improved outcome. Another study found that noncardiology-trained critical care physicians diagnosed pericardial effusions correctly in 91% of pediatric patients using a hand-carried ultrasound device.84 There have been no other published prospective studies using EUS to specifically diagnose pericardial effusions in the pediatric emergency medicine literature.
Bedside cardiac ultrasound has been shown to be useful in adults in cardiac arrest. One study reported that sonographic identification of cardiac activity in pulseless patients undergoing resuscitation was associated with increased survival when compared with those without cardiac activity by ultrasound.85 Another study found that, in 169 adults who presented in cardiac arrest, 19% of patients with cardiac activity by echocardiography survived to leave the ED, but none who had cardiac standstill survived.86 The authors concluded that EUS provided valuable information and may guide resuscitation efforts in patients with cardiac arrest.
There are no published studies that address the use of ultrasound to assess the pediatric patient in cardiac arrest. Because this event is rare in children, a single-institution, prospective, controlled study is not feasible. The results of the adult literature, however, suggest that ultrasound could contribute to the evaluation and management of children presenting in PEA or cardiac arrest by differentiating PEA from asystole and by potentially identifying life-threatening causes of PEA, such as pericardial tamponade.
Novel Uses
Skeletal Fractures
There have been several studies examining the use of EUS in the diagnosis of skeletal fractures in children and adults. In a prospective study of 163 children, Hübner et al87 were able to diagnose several types of fractures, with a reported sensitivity of 91%. Similarly, another study demonstrated 100% sensitivity and specificity in a small, pilot study using EUS to diagnose greenstick and nondisplaced forearm fractures.88 Marshburn et al89 showed that, with limited training, ED physicians were able to diagnose long bone fractures in adult trauma patients with 93% sensitivity. In addition, a case series reported 3 patients with toddler fractures that were not visualized on plain radiographs but were identified by EUS.90 Another study reported that ultrasound was useful for both guiding fracture reduction and avoiding radiation associated with follow-up films after the reduction of pediatric forearm fractures.91 Finally, EUS seems to identify sternal and rib fractures with equivalent or greater accuracy than plain radiographs.92–95 How EUS may be used in the pediatric patient with an occult or Salter Harris 1 fracture or in the reduction of displaced fractures warrants additional investigation.
Hip Effusions
A child who presents with hip pain or limp frequently requires evaluation for the presence of a hip effusion. Purulent effusions signal the diagnosis of septic arthritis, an infection requiring emergent operative intervention. When a significant effusion is identified, hip aspiration is typically required for fluid analysis and culture to distinguish septic arthritis from the more common and benign transient synovitis.
Traditionally, the diagnosis of hip effusion and subsequent hip aspiration occur in the radiology department. However, this protocol often requires transferring the patient out of the ED, which can prolong patient length of stay and necessitate the reallocation of ED staff, who may be needed to administer procedural sedation and provide appropriate monitoring. Smith96 reported a case in which EUS was used to correctly identify a hip effusion and to guide arthrocentesis successfully after multiple failed attempts using anatomic landmarks alone. Another case series described 5 patients in which ED personnel used EUS, accurately identifying the presence of a hip effusion.97 These reports imply that hip sonography by the pediatric EP is a desirable and achievable skill that can significantly improve the evaluation and treatment of hip effusion.
Undifferentiated Hypotension
There have been numerous reports describing the use of EUS in the evaluation of undifferentiated hypotension. In the adult literature, studies have shown that EP-performed bedside ultrasound in hypotensive patients leads to shorter, more accurate differential diagnosis lists98 and allows for a reasonably accurate assessment of cardiac function.99,100
There have been several case reports in the pediatric literature on the role of bedside ultrasound in the diagnosis of shock.101–103 In addition, in a prospective study, Pershad et al104 showed good agreement between pediatric EPs and experienced pediatric echocardiographers with respect to the estimation of shortening fraction and inferior vena cava volume. The authors concluded that, although there was a statistical difference, pediatric EP sonographers are capable of accurately assessing left ventricular function in pediatric patients. There are no other published studies regarding the accuracy of EP-performed bedside echocardiography to assess cardiac function and evaluate undifferentiated hypotension in the pediatric population. More research is needed to evaluate this potential useful application.
Other Applications and Future Directions
Numerous other applications for EUS have been reported recently and may prove beneficial in the pediatric ED. Studies have evaluated EP- and non-EP-performed bedside ultrasounds for detecting increased intracranial pressure by measuring the optic nerve sheath diameter.105–109 How this application may apply to pediatric head trauma patients, as well as those presenting to the ED with pseudotumor cerebri or ventriculoperitoneal shunt malfunction, warrants additional investigation, but early research suggests that optic nerve sheath diameter is a sensitive and specific noninvasive way to assess intracranial pressure. Another study revealed that EUS has excellent accuracy in adult patients presenting with an acute scrotum.110 Two studies have evaluated the use of ultrasound for performing lumbar punctures and found that EPs were able to visualize bony structures111 and spinal cord anatomy.112 In addition, Abo et al113 used ultrasound to find the optimal position for children undergoing lumbar puncture. Several studies have looked at the inferior vena cava diameter as a proxy for patient fluid status,114–118 and how this application may be used in pediatric patients with dehydration needs additional study. There have been several reports using ultrasound for endotracheal tube (ETT) placement confirmation.119–122 Weaver et al122 reported a positive predictive value of 100% for confirming tracheal versus esophageal ETT location and found that this modality had good accuracy, even for right mainstem ETT placement. Finally, considerable literature exists on the use of ultrasound in the diagnosis of pneumothorax,123–133 and some have suggested that EUS is more sensitive than the supine chest radiograph for detecting traumatic pneumothoraces.123,132
The State of the Art
Although EUS has been used in EDs for >20 years, only recently has it been embraced by pediatric EPs. Chen and Santucci3 reported that 94% of pediatric emergency medicine fellowship directors expressed a desire for pediatric-specific EUS training. Currently, numerous courses and "minifellowships" exist for EPs, ranging from 1 day to 2 weeks. None of these are pediatric specific, however.
Numerous obstacles still exist in the implementation of an ultrasound program in the ED. First, despite abundant literature, many radiology departments still oppose the use of EUS for any indication. Second, establishing a program requires a financial commitment from both hospital and ED administration. Finally, dedicated physicians must be properly trained and gain adequate experience before they can disseminate their skills and work toward the routine application of EUS in the pediatric ED. Despite these barriers, however, establishing a program in EUS is feasible. At our institution, EUS has facilitated patient care and, in some cases, played an integral role in establishing diagnoses and creating management plans. EUS has been used to help diagnose pericardial effusions and traumatic organ injuries. It has become almost routine for the placement of central venous lines and has aided in the incision and drainage of abscesses. Some clinicians are now using EUS for peripheral IV placement in selected patients with difficult venous access.
EUS is a safe technology with numerous applications. Its use in the pediatric ED is likely to increase as new trainees incorporate EUS into their routine practice. As the modality advances and becomes more available, it will be important to ensure that pediatric EPs have access to the proper training, equipment, and experience.
FOOTNOTES
Accepted Nov 8, 2007.
Address correspondence to Jason A. Levy, MD, Division of Emergency Medicine, Children's Hospital Boston, 300 Longwood Ave, Boston, MA 02115. E-mail: jason.levy{at}childrens.harvard.edu
The authors have indicated they have no financial relationships relevant to this article to disclose.
REFERENCES
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