Journal of Medical Ultrasound (2014) 22, 71e77
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Prehospital Ultrasound Jen-Tang Sun 1, Chun-Yen Huang 1, Yi-Shin Huang 1, Shyh-Shyong Sim 1, Kah-Meng Chong 2, Hsiu-Po Wang 3, Wan-Ching Lien 2* 1
Department of Emergency Medicine, Far Eastern Memorial Hospital, New Taipei City, Taiwan, Department of Emergency Medicine, and 3 Department of Internal Medicine, National Taiwan University and National Taiwan University Hospital, Taipei City, Taiwan 2
Received 9 April 2014; accepted 21 May 2014
Available online 27 June 2014
KEY WORDS emergency medical staff, prehospital ultrasound
Ultrasound is a commonly used diagnostic tool in clinical conditions. With recent developments in technology, use of portable ultrasound devices has become feasible in prehospital settings. Many studies also proved the feasibility and accuracy of prehospital ultrasound. In this article, we focus on the use of prehospital ultrasound, with emphasis on trauma and chest ultrasound. ª 2014, Elsevier Taiwan LLC and the Chinese Taipei Society of Ultrasound in Medicine. All rights reserved.
Introduction Ultrasound (US) is a useful diagnostic tool for use in hospitals. It is noninvasive and inexpensive, and causes no radiation exposure. Besides radiologists, many emergency physicians use US to assist in their decision making during critical conditions . With the current improvement in technology, US machines have become more portable and are available with a better resolution. Ziegler et al  reported that a portable device had approximately 90%
Conflicts of interest: All authors declare no conflicts of interest. * Correspondence to: Dr Wan-Ching Lien, Department of Emergency Medicine, National Taiwan University Hospital and National Taiwan University, Number 7, Chung-Shan South Road, Taipei, Taiwan. E-mail address: [email protected]
accuracy compared with high-end devices. US machines such as PRIMEDIC HandyScan, V-scan, and Sonosite are commonly used as portable devices in prehospital settings. US has been brought to prehospital settings as a result of the recent advances in technology . A prehospital setting is a unique, most likely noisy, and often limited space. Traditionally, diagnostic tools used in prehospital settings are based on history taking and physical examination. Physical examination alone cannot be sufficient to diagnose certain conditions . In addition, many studies suggested that prehospital US can change the final diagnosis and treatment [5,6]. Prehospital US has a variety of applications, such as focused assessment with sonography in trauma (FAST) , assessment of cardiac arrest , lung US (mainly in pneumothorax) [6,8], and others. Countries that have studied prehospital US extensively include Germany, France, Italy, and the United states . Literature was reviewed and discussed in the following sections.
http://dx.doi.org/10.1016/j.jmu.2014.05.008 0929-6441/ª 2014, Elsevier Taiwan LLC and the Chinese Taipei Society of Ultrasound in Medicine. All rights reserved.
J.-T. Sun et al.
Feasibility of US in a prehospital environment Because a prehospital space is unique and limited, a US machine should be smaller in size but should have better image quality. Some studies performed US at the scene, and others in a vehicle, such as an ambulance or a helicopter If performed at the scene, the delivery time to hospital may be prolonged, and if performed in a helicopter or an ambulance, the transporting environment may influence the scan. There are studies of prehospital US in a fixed wing and helicopter, which showed good results. However, Melanson et al  reported in their study that the lack of sufficient time during helicopter transport and a proper lighting system in the helicopter can compromise the results of FAST examination. Snaith et al  reported that FAST and abdominal aortic aneurysm (AAA) performed in a static and ground ambulance is of good quality due to the availability of sufficient time and is comparable to that performed at the emergency department. In Taiwan, emergency medical services mainly involve ground ambulances, and most of the ambulance beds are located at the left side; hence, left-hand-based practice may be helpful for performing the scan. Fixation of machines to the frontal areas of ground ambulances may be helpful in reducing shaking.
Fig. 1 Fluid accumulation in the Morrison pouch (arrow) on FAST examinations, indicating hemoperitoneum in traumatic patients. FAST Z focused assessment with sonography in trauma.
As discussed earlier, prehospital FAST shows good feasibility and accuracy if performed by trained personnel.
Prehospital chest US Educating paramedics about US Many studies have invested in the learning curve for US, especially in FAST. They concluded that a 1-day course, including lecture and hand-on practice, can generate good accuracy and competency . Heegaard et al  designed a FAST training course, which lasted 7 hours, for emergency nurses and paramedic flight crews; they reported 100% sensitivity and specificity in nontrauma patients, and 60% sensitivity and 93% specificity in trauma patients after 1 year of training. Kim et al  also reported that a 4-hour FAST training course for intermediate emergency medical technicians (EMT) resulted in 61% sensitivity and 96.3% specificity.
Focused assessment with sonography in trauma Abdominal injuries are frequent causes of early mortality in trauma patients. Early detection of internal bleeding plays a key role in the management of these patients. FAST is a standard procedure for evaluating trauma patients. Most studies revealed that FAST can detect hemoperitoneum (Fig. 1) and hemopericaridum accurately, and reduce time to operation as well as treatment costs . It is very important that trauma patients should reach a closed and appropriate facility. FAST may be used to guide hospital selection. It may also aid in the early activation of a trauma team and shortening the time to operation. Walcher et al  reported that, in a sample of 202 patients, prehospital FAST could alter prehospital management in 30% and change the final admitting hospital in 22%. They also reported that the sensitivity, specificity, and accuracy of prehospital FAST were 93%, 99%, and 99%, respectively.
Acute dyspnea is a major symptom of patients suffering from acute pulmonary or cardiac disorders. Possible and potentially life-threatening differential diagnoses of acute dyspnea include congestive heart failure (CHF), pulmonary embolism, pleural effusion, hemothorax, chronic obstructive pulmonary disease (COPD), pneumothorax, and pericardial effusion/cardiac tamponade . Accurate and fast differentiation is crucial for dyspneic patients in a prehospital environment because it may change the treatment plans and even alter the destination of definitive care . Currently, diagnostic tools used by prehospital care providers or paramedics are limited to physical examination, auscultation findings, and monitoring of hemoglobin oxygenation by pulse oximetry. However, these methods often lack both sensitivity and specificity, and are difficult to apply in a noisy and often chaotic prehospital environment [16,18]. Recently, there have been an increasing number of clinical studies focusing on the use of chest US in a prehospital environment [8,11,16,17,19e21]. Snaith et al  proved that examination of pneumothorax using US (extended FAST) can be performed in a stationary or moving ambulance, the outcomes being consistent with those performed in the hospital emergency department. Ketelaars et al  reported a chest US study that included 281 patients in a helicopter emergency medical service; 21% of patients had a change of treatment plans and 4% had to change their initially selected destination for definitive care. Neesse et al  made a prospective study of prehospital chest US and found that it provided an additional diagnostic value in 38 out of 56 cases (68%). That is to say, prehospital chest ultrasonography is a “helpful tool” for the emergency doctor. Interestingly, the finding of a normal
sonographic examination also guides the emergency doctor in the prehospital management of patients. For example, the treatment for 14 out of 56 patients (25%) was put on hold due to normal sonographic finding . We now present a review of prehospital chest US articles in the following sections.
Pneumothorax Prehospital care providers are important in rapidly recognizing and treating life-threatening tension pneumothorax. Fortunately, this condition can be treated effectively with needle decompression or tube thoracostomy . Traditionally, decisions to perform chest decompression have been taken based on physical examination. However, physical examinations are often insensitive when working in a noisy and, at times, austere environment . Furthermore, a suboptimal physical examination may lead to a delay in chest decompression or result in the patient receiving an unnecessary treatment when a pneumothorax is not present . The sliding lung sign (SLS) is the characteristic image of the movement of the parietal pleural surface relative to the visceral pleural surface. On US, it appears as two echogenic lines that slide during respiration. When pneumothorax is present, the air in the pleural space will mask the visceral pleural surface and the sliding motion will disappear. The presence of the SLS on US essentially rules out pneumothorax . Accuracy can be enhanced with the addition of secondary techniques that use M-mode or power Doppler. The presence and absence of the SLS in M-mode are visualized as the seashore sign (Fig. 2) and stratosphere sign (Fig. 3), respectively. On power Doppler, color will be visible at the pleural interfaces due to the relative motion of the pleural surface in the absence of pneumothorax (Fig. 4). The presence of the SLS, signifying the absence of pneumothorax, has been shown to have a sensitivity of 95e100%, which is superior to chest X-ray and comparable with computed tomography . Some studies use US artifacts to detect pneumothorax [26,27]. US artifacts arising from the lung-wall interface are either vertical (comet-tail artifacts) or horizontal. A comettail artifact is a US artifact that arises from the pleural line
Normal lung sliding with M-mode seashore sign.
Pneumothorax with M-mode stratosphere sign.
and spreads to the edge of the screen. The appearance of lung sliding and comet-tail artifacts rules out pneumothorax, due to a high negative predictive value (NPV) of 99% . Horizontal artifacts and absent lung sliding, when combined, has a sensitivity and an NPV of 100%, and a specificity of 96.5% . Chest US has been applied at a forward military health service support station to exclude pneumothorax in the setting of deep thoracic shrapnel wounds. Without US, many soldiers would have undergone unnecessary chest decompressions . This technique has also been successfully used in a high-altitude environment, where a portable US machine excluded pneumothorax in a patient with a stable vital sign but suffering from blunt chest trauma and hemoptysis at 4000 feet above sea level . In an in-flight helicopter environment, decision regarding the insertion of a chest tube changed in 13 of 281 (4.6%) patients following a US examination . Lyon et al  demonstrated that prehospital care providers could retain the skills necessary to acquire diagnostic-quality US images of a pneumothorax with 100% sensitivity and specificity [95% confidence interval (CI) 93.6e100% and 95% CI 93.6e100%, respectively] over a 9-month trial period.
Normal pleural image on power Doppler examination.
J.-T. Sun et al.
Fig. 5 (A) Tracheal ultrasound examination showing one tract sign in patients with tracheal intubation. (B) By contrast, ultrasound examination showing double tract signs in patients with esophageal intubation.
Verification of correct endotracheal tube positioning Endotracheal intubation is the “gold standard” for controlling airway patency . A rapid detection of esophageal intubation is essential because failure to do so may cause immediate mortality . To confirm endotracheal tube (ETT) positioning, clinical assessment and use of devices such as capnography are considered useful techniques. However, clinical assessment cannot be performed well in a noisy and, at times, chaotic environment. Moreover, capnography can provide false-positive and falsenegative results in some situations such as cardiac arrest, low cardiac output, acute pulmonary embolism, hypothermia, airway obstruction, and technical problems . Recently, two additional methods that use US for confirming ETT placement have been described. When correctly placed in the trachea, the air-filled ETT should be hidden within the air-filled trachea and should become invisible. In the setting of esophageal intubation, the tube becomes visible lateral and deep to the trachea (Fig. 5) . The second method uses US to detect bilateral pleural sliding after intubation. The presence of lung sliding confirms tracheal intubation. In addition, use of US can also detect one-lung intubation when pleural sliding is present on the right side and absent on the left (sensitivity 95e100%, specificity approaching 100%) . Although there have been only a few trials, US procedures can enhance a physician’s confidence and help in making decisions regarding airway management . This may especially be useful in areas where radiology is not readily available or auscultation may be inaccurate. Brun et al  reported a case in a prehospital setting: a 52-year-old female patient presented with asystole. During cardiopulmonary resuscitation, there was a sudden absence of end-tidal CO2 capnographic detection. Correct tube positioning could not be ascertained by auscultation because the environment was very noisy. However, lung US revealed bilateral pleural sliding during insufflation with the self-filling balloon, thus confirming correct ETT positioning.
Acute heart failure versus COPD Acute CHF is one of the main causes of acute dyspnea presented in prehospital emergency settings, and is associated with high morbidity and mortality . An early and accurate diagnosis presents a significant clinical challenge, as misdiagnosis can result in deleterious consequences to patients . Point-of-care bedside lung US has become a useful method for differentiating between acute CHF and COPD . Two methods are used in a prehospital environment: the first is recognizing the diffuse comet-tail artifacts and the second is recognizing the pleural effusion [16,21]. The first technique is based on the recognition and analysis of sonographic artifacts caused by the interaction between water-rich structures and air, called comet tails or B-lines. When such artifacts are widely detected on anterolateral transthoracic lung scans, diffuse alveolareinterstitial syndrome such as cardiogenic pulmonary edema can be diagnosed and the exacerbation of COPD,
Fig. 6 Diffuse B-lines (arrow) detected in a dyspneic patient, indicating that pulmonary edema was the cause of dyspnea.
Prehospital Ultrasound another important cause of dyspnea, can be ruled out (Fig. 6) . This technique can be performed using an eight-zone protocol (2 anterior and 2 lateral zones on each side of the thorax) or a two-zone protocol described by Liteplo et al . A comparison of the eight- and two-zone US tests suggests that a faster and easier two-zone test may be sufficient for the evaluation of CHF . Prosen et al  compared lung US (comet-tail artifacts) and N-terminal pro-brain natriuretic peptide (pro-BNP) in differentiating acute CHF from COPD in a prehospital emergency setting. The US comet-tail sign has 100% sensitivity, 95% specificity, 100% NPV, and 96% positive predictive value for the diagnosis of CHF. Pro-BNP (cutoff point 1000 pg/mL) has 92% sensitivity, 89% specificity, 86% NPV, and 90% positive predictive value. Comparing the two methods, a significant difference was observed between US comet-tail sign and pro-BNP (p < 0.05). The combination of US sign and pro-BNP has 100% sensitivity, 100% specificity, 100% NPV, and 100% positive predictive value. Furthermore, using lung US, acute CHF can be excluded in patients with pulmonary-related dyspnea who have a positive pro-BNP (>1000 pg/mL) and a history of HF. The second method for differentiating acute CHF from COPD is to recognize pleural effusion by US. Neesse et al  examined 56 patients with acute dyspnea prospectively who underwent chest US in a prehospital setting. Pleural effusion was detected in 100% of CHF and 20% of COPD patients, constituting a highly significant parameter in the differential diagnosis (p < 0.01). For the diagnosis of CHF, pleural effusion had a specificity of 82% and sensitivity of 100% (Fig. 7).
High-altitude pulmonary edema High-altitude pulmonary edema (HAPE) is the most lethal of all the altitude illnesses. The cause of death is usually lack of early recognition or misdiagnosis. Portable US machines has been used successfully at expedition elevations to diagnose HAPE. Fagenholz et al  described the use of US to screen persons deployed at an altitude of 4240 m at the
Pleural effusion (arrow).
75 Himalayan Rescue Association Clinic in Nepal for HAPE. The presence of comet-tail artifacts in chest US correlated well with HAPE prediction. Furthermore, symptom resolution correlated well with the decrease in comet-tail artifacts when serial assessments were made in patients during treatment. The comet-tail technique effectively recognizes and monitors the degree of pulmonary edema in HAPE .
Pericardial effusion/tamponade Cardiac tamponade is a life-threatening disease, which should be recognized as soon as possible . In a prehospital study by Neesse et al , pericardial effusion was detected in 11 out of 56 patients (20%). Of these, only one was diagnosed to have severe pericardial effusion, which resulted in admission to the intensive care unit and was finally treated by US-guided pericardiocentesis. In contrast to pleural effusion, the distribution of pericardial effusion between the cardiac disease cluster and pulmonary disease cluster was rather balanced (22% and 20%, respectively), without significant difference. Therefore, the detection of minor pericardial effusions seems to have no relevant impact on the prehospital management; otherwise, it is important to keep in mind that epicardial fatty tissue can be a possible differential diagnosis (Fig. 8). In an air medicine study, nonphysician air medical crews performed cardiac US for pericardial effusion, and adequate examinations could be obtained in 86 out of 91 cases (94.5%). Although the sensitivity and specificity were both 100%, this study was limited in that only one patient had a positive examination .
Pulmonary embolism Pulmonary emboli occur when a proximal portion of a venous clot breaks off, travels through the veins, traverses the right ventricle, and lodges in the precapillary pulmonary arteries. Although minor pulmonary embolism may be asymptomatic and self-limited, massive pulmonary embolism may cause severe hypoxia and even death. Therefore, pulmonary embolism is an important differential diagnosis in acute dyspneic patients . In US, one of the most significant findings for pulmonary embolism is right heart distension with a flattened interventricular septum (Fig. 9) .
Pericardial effusion (arrow) detected on sonography.
J.-T. Sun et al.
Right heart distension with flattened interventricular septum in patients with pulmonary embolism.
The use of prehospital US to detect pulmonary embolism was described in a previous study. Two of 56 patients (3%) were suspected to have massive pulmonary embolism, as revealed by prehospital US. In both cases, pulmonary embolism was confirmed by echocardiography and CT angiography after admission .
Cardiac arrest Some factors were related with the prognosis of patients with cardiac arrest, such as bystander cardiopulmonary resuscitation, early defibrillation and capnography. US can provide another prognostic factor. Ultrasonographic cardiac standstill in cardiac arrest patients is associated with a poor outcome. Blaivas and Fox  reported that no patient with cardiac standstill on US can survive. In a prehospital setting, Aichinger et al  reported that, of 32 patients with cardiac standstill on sonography, only one survived to admission. In addition, cardiac US also can provide useful diagnostic information on PEA and certain shock states such as cardiac tamponade, hypovolemia, pneumothorax, and pulmonary embolism, which can alter the treatment plans .
Conclusion As technology improves, bringing small-size US devices to prehospital settings becomes feasible. Compared with traditional physical examination, prehospital use of US provides additional useful information. This valuable information may alter patients’ treatment plans or influence their choice of hospital. In conclusion, adequate use of prehospital US in critical patients may play a key role in improving patient outcome.
Financial support None.
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