Another small prehospital study published in 2012 enrolled 42 patients in cardiac arrest with any rhythm [3]. Among 32 patients with no cardiac activity on initial field echocardiogram, only one survived to hospital admission. In contrast, 4 of the 10 patients with cardiac activity survived to hospital admission. Only one of forty-two patients survived to hospital discharge (and did so with full neurology recovery). He had cardiac activity on his prehospital ultrasound.

While these results were interesting, both studies were underpowered to detect the key outcome of neurologically-intact survival without cardiac activity on ultrasound due to the overall low incidence of survival from OHCA.

However, an adequately-powered multi-center Emergency Department study of 993 pre-hospital and ED patients with cardiac arrest in PEA or asystole was recently published [4]. Lack of cardiac activity portended an extremely poor likelihood of survival to hospital discharge (0.6%, neurologic status not reported). In addition, POCUS was able to identify causes (Pulmonary embolism, cardiac tamponade) of cardiac arrest not amenable to traditional ACLS interventions.

Given high utilization of resources with prolonged resuscitation and the potential to identify reversible causes of cardiac arrest, these results suggest that cardiac ultrasound may be beneficial in prehospital management of OHCA.

The FAST Exam and Trauma Triage

In emergency department patients with torso trauma, performing a FAST exam decreases time to operative care and the number of CT examinations of the torso [5]. FAST and eFAST (FAST + lung ultrasound) have since become key clinical decision making tools in the triage and management of trauma patients in the ED. Extrapolating this to the field, could early identification of free fluid on abdominal exam better delineate which patients require one trauma center versus another? Could lung ultrasound be used to help identify who needs needle decompression versus who does not, thus avoiding unnecessary intervention?

The prehospital FAST exam may allow for more appropriate transport destination decisions by providing valuable information to be obtained[6,7,8]. One prospective, multicenter study carried out in Germany study sought to compare the accuracy of physical exam and prehospital FAST exam to detect hemoperitoneum and to determine whether it changed clinical management [9]. They enrolled 230 patients with blunt trauma. Among 202 patients who were fully scanned and were not lost to follow-up, 28 patients were found to have hemoperitoneum by ED ultrasound or CT imaging. 26 of these were identified prehospital, leading the authors to conclude that prehospital FAST has a sensitivity of 93 % (95 % CI 76 – 99 %) and specificity of 99 % (95% CI 97 -100 %). However, as the study excluded patients lost to follow-up or in whom ultrasound was too technically difficult, the sensitivity of prehospital FAST for accurate detection of hemoperitoneum could be falsely inflated. The study was interesting in that the there were several examples where prehospital detection of abdominal free fluid changed patient management, including minimizing prehospital interventions and alerting the receiving hospital to reduce time to surgical intervention. As this was not a randomized trial, it was unclear whether this actually changed to the time to surgical intervention, but based on results of ED-based studies it is likely to have done so.

Several systematic reviews have examined the current evidence regarding the potential usefulness of prehospital ultrasound to change diagnosis or treatment of trauma patients [10,11]. Their overwhelming conclusion? The evidence is promising, although the quality of evidence very low and more studies are needed.

Practical for prehospital use?

Development of handheld, battery-powered, low-weight US machines has created the possibility of bringing US to the prehospital setting. In addition, field ultrasound images can be transmitted en route to the emergency department (ED) similar to 12 lead EKGs [12,13,14].

A 2014 survey of medical directors using the NAEMSP mailing list demonstrated that 4.1% of EMS systems were already using ultrasound and that an additional 21.7% of systems were considering the implementation of pre-hospital ultrasound [15]. The vast majority cited equipment costs (89.4%), as well as training costs (73.7%), and challenges related to the training process (53.5%) as the major points of concern of why they the medical directors thought that it could not be implemented in their system.