Introduction
Clinician-performed ultrasound has proven crucial for the evaluation of critical disease. Improvements in size, weight, cost, user-friendliness, and communications have allowed the enthusiasm for hospital ultrasound to migrate into the out-of-hospital arena. With increasing evidence that ultrasound can play a role in out-of-hospital emergency care, this diagnostic modality has been used in international explorations on all continents, in challenging high-altitude expeditions, on cruise ships, in hyperbaric chambers, and even in outer space on the International Space Station [1].
Ultrasound-guided diagnosis of critical conditions in the field has the potential for improving triage decisions, hastening therapy prior to hospital arrival, avoiding unnecessary or harmful treatments, and expediting transport to correct facilities. Prehospital ultrasound has been described in advanced ground and flight EMS systems, in military medicine for both service personnel and civilians, in austere or underdeveloped environments, and in mass casualty situations. Non-physicians with limited medical backgrounds have demonstrated the ability to perform and interpret ultrasounds with adequate training. Despite the recognition of a need for field use of point-of-care ultrasound, its routine incorporation into prehospital algorithms has not yet been established.
Indications
The 2008 American College of Emergency Physicians policy statement regarding emergency ultrasound lists the following examinations as core emergency ultrasound applications: trauma, intrauterine pregnancy, abdominal aortic aneurysm (AAA), cardiac and volume status, biliary, urinary tract, deep venous thrombosis (DVT), soft tissue and musculoskeletal, thoracic, ocular, and procedural guidance [1]. Prehospital use in many of these areas is described in the following sections.
Trauma
The area of most extensive study regarding prehospital ultrasound is the Focused Assessment with Sonography in Trauma (FAST) examination to detect traumatic cardiac tamponade and intraperitoneal bleeding [6,9–13]. The current standard is for the FAST exam to be performed immediately upon arrival to the trauma center during advanced trauma life support physical examination surveys. However, Walcher et al. demonstrated that performing prehospital FAST (PFAST) ultrasounds at the trauma scene changed management in 30% of patients with a 93% sensitivity and 99% specificity for detecting intraperitoneal free fluid [6]. Identification of free fluid enabled providers to reduce patient blood loss by providing permissive hypotension, and non-essential therapies were avoided to shorten time to surgery. Advance notification of PFAST results was provided to receiving hospitals, which then activated surgical teams when needed. In 22% of patients, the choice of receiving hospital was changed based on the ultrasound findings. Due to the results of this study, one major German air rescue provider incorporated PFAST into its algorithm for trauma management [6]. Other studies have demonstrated successful paramedic performance of the PFAST exam while en route, on ground or in air, without prolonging time to transport [4,10,14].
Of note, ultrasound cannot distinguish blood from ascitic fluid or pinpoint exact areas of bleeding. It is not sensitive in the detection of retroperitoneal fluid, organ injury, or hollow viscus injury. These limitations of ultrasound may cause delayed or missed fluid detection on out-of-hospital or triage FAST exam [15–17]. It remains to be seen whether positive findings on a PFAST exam in the United States would alter management as illustrated in the Walcher study [6], since many trauma centers in the US have immediate response by trauma teams and protocols in place to mobilize operating theaters quickly.
The FAST detection of pericardial fluid may have more potential for prehospital intervention. A dramatic case report details the course of a 17-year-old 26-week pregnant female suffering from a stab injury. Despite field chest tube placement with evacuation of air and blood, the patient’s vital signs declined. Ultrasound revealed a significant amount of pericardial fluid, which was immediately drained in the field and again in the emergency department. The patient ultimately survived, largely due to prehospital intervention [18]. Similarly, another report describes how in-ambulance paramedic detection of traumatic pericardial effusion and subsequent alerting of the receiving team facilitated direct operative intervention [11]. These cases highlight the potential for the PFAST exam to change prehospital practice and guide on-scene resuscitative therapies.
Pulmonary
While the Extended FAST exam (eFAST), including evaluation of pleural sliding, has been imprinted into emergency department and trauma protocols, it has not become standard in the prehospital environment. Adoption of sonographic pneumothorax evaluation may be invaluable in the trauma setting, as physical exam findings and ancillary monitoring have proven insensitive or difficult to discern in a noisy ambulance or helicopter [5,7,8,19]. Detection may facilitate prehospital needle thoracostomy and prevent development of tension pneumothorax. Additionally, ruling out pneumothorax avoids unnecessary procedures and their sequelae, allowing focus on other resuscitative efforts [5].
Equally, assessments of lung sliding and pleural effusion have become useful adjuncts in the management of acute dyspnea. Zechner et al. report a common scenario encountered by prehospital personnel: a patient with a history of both chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF) presenting in severe respiratory distress with wheezing. When pulmonary edema was discovered via on-ambulance sonographic B-lines, treatment was immediately altered to discontinue terbutaline and proceed with urapidil (an alpha1-antagonist), enabling rapid improvement in the patient’s clinical status [20]. Subsequently, a German group developed a prehospital chest protocol to evaluate undifferentiated dyspnea. Using the subxiphoid cardiac view, bilateral coronal views, and bilateral anterior intercostal views, this protocol investigates pericardial or pleural effusion, pneumothorax, and right heart distension for pulmonary embolus. Providing supportive information in 68% of their patients and most useful for finding pleural effusion in decompensated CHF, prehospital ultrasound guided emergency physician management at the hospital [8].
Another area of rising interest is sonographic confirmation of endotracheal tube placement as an adjunct to capnometry. Brun et al. describe verification of tube position during cardiopulmonary resuscitation (CPR) by viewing bilateral pleural sliding when there was sudden absence of end-tidal CO2 detection and limited ability to perform auscultation due to noise in the vehicle [21].
Prehospital lung ultrasound appears useful in revealing the presence or absence of pneumothorax, detecting pulmonary edema, narrowing diagnosis among differing respiratory disease processes, identifying pleural effusion, and as a supplement to current respiratory monitoring techniques.
Cardiac
Dedicated prehospital cardiac examination is very amenable to ultrasound. Brun et al. illustrate prehospital use of transthoracic echo for evaluation of shock in a patient with prior cardiac surgery presenting with dyspnea, tachypnea, crackles on exam, and hypotension. Ultrasound revealed pericardial effusion with thrombus in contact with the right ventricular free wall causing diastolic collapse of the right heart from a vitamin K antagonist overdose. The prehospital team notified the receiving hospital to prepare prothrombin complex concentrates in advance of arrival, and shortened time to drainage by the cardiac surgeons [22].
Out-of-hospital groups have also diagnosed pulmonary embolus from acute right heart strain [8] and examined cardiac output using non-physicians with tele-ultrasonography [23]. The same challenges that affect interpretation of in-hospital echocardiography exist, such as differentiating between acute versus chronic right heart strain, epicardial fatty tissue versus small pericardial effusion, and stable versus unstable pericardial effusion. These physiological processes may require a more in-depth level of training [7,8].
