Typically, the majority of patients brought in during a MCE will fall into the nonurgent category with minor to moderate injuries not requiring immediate operation [9]. These patients are usually easy to identify in the triage assessment because they will be talking coherently, are not in cardiorespiratory distress, and are not bleeding significantly. They can be directed to a designated minimal care area of the emergency department and have their injuries partially treated or their care delayed entirely [14]. As resources need to be cautiously reserved, these patients will initially receive only the care required to keep them from deteriorating, allowing resources to be reserved for more seriously injured future patients. For example, a patient with penetrating chest injury may have an empiric thoracostomy tube placed without imaging. A patient with penetrating abdominal injury who is hemodynamically stable will be given analgesia and then triaged to the floor to await possible operation once more resources are available. The intent of this care is to temporize as is possible until casualties are no longer arriving and the full scope of the event is understood in the context of the available resources [9].

Urgent casualties are the most important patients to correctly identify. Typically they will have injuries that require immediate, life-saving intervention, usually in the OR. The most common presentation of an urgent casualty is a patient with active hemorrhage, the source of which is often surgically correctable. This frequently is the result of penetrating gunshot or knife injuries as has been observed in highly publicized domestic mass shootings and the 2014 China subway knifings. However, the Boston Marathon bombings demonstrated that explosive attacks producing hemorrhagic extremity injuries and traumatic amputations typical of the military setting are also an unfortunate reality in the civilian world.

Correct identification of urgent casualties is the cornerstone of successful MCE management because accurate triage of this cohort saves the most lives. The underlining principle of facility triage is to divert resources away from nonurgent and expectant patients so that medical care can be rendered to the urgent patients at or near the level that would be under normal conditions. When triage is performed correctly, this will be done without overtaxing available resources [9, 14].

An expectant casualty can be difficult to identify because what specifically constitutes expectant will not be clearly defined prior to the arrival of casualties. How the triage officer decides what meets criteria to categorize as expectant depends on the characteristics of the MCE itself. It will be based not solely on the condition of a particular patient, but also the number and condition of other casualties, and the resources available. In other words, expectant in a certain set of circumstances may be a patient whose injuries are so severe they will die regardless of the available resources, or a patient whose survival requires such a large share of the valuable limited resources that several other patients will die if those resources are used in an attempt to save this casualty.

An example of the former is a patient in extremis with a devastating open head injury and significant visibly damaged brain tissue. Though the patient has not yet succumbed to the injury, it is clear that chances of survival, much less meaningful recovery, are not realistic. An example of the latter is a victim of blast injury in extremis with multiple sources of hemorrhage. Under normal conditions, this patient might be salvageable if a mass transfusion protocol is undertaken while a trauma surgeon, vascular surgeon, and neurosurgeon operate simultaneously. However, as previously stated, in MCE’s the patients outnumber resources. And in this example, multiple resources that could be used to save several lives would be used inappropriately in attempt to save one life, thus violating the principle of doing the greatest good for the greatest number.

The definition of expectant in any MCE is dynamic and may change significantly as resources become more or less available and the inflow rate or complexity of incoming casualties change. Typically, when a patient is deemed expectant they are given comfort care in an area away from the main hospital, perhaps a chapel or cafeteria, so as not to utilize valuable resources and space that might otherwise be utilized for salvageable patients. At a later time if the characteristics of the MCE have changed so as to allow the opportunity, patients initially deemed expectant can be re-triaged and their status changed to be consistent with the current MCE conditions [2, 3].