Introduction

Public safety access points (PSAP) are tasked with the responsibility of taking all emergency calls (911 in the US), triaging and prioritizing each call using the provided information, notifying and dispatching the appropriate responders, and offering pre-arrival first aid instructions to callers.  Standardized dispatch systems that are available for querying callers, categorizing calls, and matching the call type to the appropriate response units and priority have been shown to improve metrics such as time to dispatch.[1] In EMS systems with multiple simultaneous requests for service or inadequate response resources to meet the call demand at any given time, decisions must be made which call is believed to be most emergent, and which call is less urgent or can be delayed.[2] Ideally, the call prioritization, as well as the response profile, follows consistent guidelines from the local EMS agency, a regional EMS regulatory body, a national accrediting organization, or even a governmental agency.

Issues of Concern

Trends have shown steady increases in annual EMS calls and subsequent EMS dispatches. As a result, appropriate prioritization of calls is necessary both to use limited resources effectively as well as assure that patients' needs are met in a timely fashion.  Also, prioritization can address public safety concerns by reducing the number of calls responded to emergently (lights and sirens) to those which are predicted to be genuinely urgent or emergent. This approach then reduces the risk of injury to both EMS providers and the general public from lights and sirens-related accidents and multiple responding public safety units.   

Several common frameworks are widely used by PSAPs to categorize and prioritize calls in the United States.[3]  Common elements include having dispatchers ask specific, scripted triage questions to determine the chief complaint and patient's clinical status.[4] Answers to these questions allow dispatchers to link the call type to the anticipated resource needs based on the information provided.  Some studies have shown significant improvement in the detection of cardiac arrest with standardization of the triage process and avoiding sole reliance on a dispatcher's judgment.[5]  However, dispatcher consistency in following the standardized scripts remains a valid concern.[3]  Another part of this process is determining which emergency resources should respond to each emergency call, rather than asking the dispatcher to make these decisions on a case-by-case basis. These predetermined response profiles or configurations link to specific call types (i.e., MVC vs. sick case) and which type of EMS unit (Basic versus Advanced Life Support unit) and other emergency apparatus (fire, law enforcement) with the urgency of the response (e.g., routine versus emergent).  While these are suggestions, in practice, they represent a "floor" that the dispatcher can add resources too based on information received, rather than subtracting from the response profile. Once the patient's needs are identified, the pre-established units within the response profile correlating to that complaint can be dispatched.  

Widely used models in the US rely on variations on a number-letter-number system, with many categories and subclassifications of anticipated disease processes. Several European countries, and some large American cities, use a simplified model with fewer categories focused on determining the urgency and timeframe in which response should occur. This second type of system is more feasible in more homogeneous systems without different levels of medical response capability, i.e., only ALS units. As such, there is a higher emphasis placed on the response time that is allowed for each call. In the United Kingdom, for example, the National Health Service Clinical Response Model sorts calls into one of five categories. Category 1 calls are the most severe/critically ill calls and categorize as "Life-Threatening Illnesses or Injuries," which warrant an immediate response, with the expectation that in-person response will arrive within 10 minutes. The next, less serious, category of calls are the category 2 calls, in which response is deemed emergent but not critical with ninety percent of response times within 40 minutes. Urgent calls are considered category 3 and are responded to within two hours ninety percent of the time. Category 4 calls are considered less urgent and generally receive a response within 3 hours. Category 5 is reserved for what are considered "non-urgent calls," deemed not to warrant any in-person EMS response but which instead are transferred to a telephone triage line or referral service to help resolve their issue.[6]

There is a misperception that the existing dispatch systems and categories have an evidence-based correlation with patient acuity and outcomes, yet the evidence does not support this.[4][7] For example, the lowest acuity levels in two common US systems merely identify that based on the information received from the caller, either no emergent response or a basic life support response is expected to be appropriate.  Some systems use these categories to determine eligibility for prolonged response times or nonresponse, as noted in the NHS system. Since the categorization is based entirely on the information from the caller, it relies on the quality of the information received, the proximity of the caller to the patient (with the patient, or a third party "drive-by" caller), the caller's ability to communicate (i.e., language barriers, etc.), and the pertinence and clarity of the questions asked by the dispatcher.  Some EMS systems choose to respond to all categories in the same manner, and with the same types of units, others use the categories stratify their response profiles, as noted above. Thoughtfully created response profiles decide whether the complaint warrants a first responder, requires additional personnel (i.e., engine/pumper for cardiac arrest), a BLS ambulance only, an ALS ambulance only, an EMS supervisor or chief, specialized responders (such as police, SWAT, a rescue truck for extrication, a helicopter). In this way, PSAPs can limit over-triage (too many resources) as well as under-triage (too few or too slow) responses.[6] It also specifies the mode of response for each responding unit, either "quiet" vs. red lights and sirens. Well-designed response profiles that are created by the collaboration between the leadership of all the relevant responding and dispatching agencies can help assure that the patient's apparent needs are timely met, that EMS and related public safety resources are utilized appropriately, that the general public's safety is considered in the response mode and that inherently more dangerous red lights and sirens responses are limited. 

New Innovations            

In the United States, the National Highway Traffic Safety Administration Office of EMS "EMS Agenda 2050" is a forward-looking guidance document that creates a vision for the advancement of EMS systems.[8] The goal of this document is to use data-driven, evidence-based principles to provide the most sustainable, innovative, socially equitable, and reliable patient care possible. 

One recent innovation in the US is the redirection of low acuity 911 calls to a secondary nurse triage line, as noted in the NHS example. This strategy allows EMS agencies to diverge from the traditional practice that every 911 call gets a response, regardless of how minor the caller's complaint is.  Rather than perpetuating this mismatch of emergency resources with non-emergent calls, the nurse triage line provides additional medical advice and connects the caller to non-emergent resources to meet their needs.[9] If the acuity of the call is deemed too high or otherwise inappropriate for the on-call nurse, the call is then routed back to EMS dispatch for an in-person response.[5] To date, this strategy has yielded mixed results, with one study showing that in a call center with an annual volume of 90,000 calls only averaged of two calls per day appropriate for nurse-triage with an average of less than one patient per day successfully completing this protocol and thus avoiding in-person EMS response. Of those patients who did complete the protocol for a nurse-triage response only, there was a high rate of satisfaction with their experience, with approximately 92% of patients reporting that the non-transport option met their healthcare needs.[10] While it appears that there is more research needed to investigate how to optimize the nurse-triage system best, logic would suggest that its use could be expanded to allow more patients to meet criteria, and thus reduce EMS response volume.

Another innovation informs EMS call prioritization and response, the use of advanced automatic crash notification (AACN) from car accidents, using sophisticated sensors and crash detection monitors, which automatically provide PSAP with information including the location of the crash, the speed at the time of impact and the forces involved. This data is analyzed automatically by the vehicle to determine the likelihood of serious injury, and the vehicle transmits that data directly to the nearest PSAP. If the AACN indicates that there is a high likelihood of serious injury, the call can be prioritized differently, and a more appropriate response profile than the generic motor vehicle collision response can be dispatched immediately, rather than waiting for responders to arrive on the scene, assess the patient, and request additional resources, including potentially having helicopter EMS on standby.[11]

Another focus both in the United States and abroad is on frequent callers or EMS system users.  Such callers are identified and flagged, and a multidisciplinary approach is applied to determine the root cause of the frequent emergency system activation and address the underlying need, whether that be access to food, healthcare, in-home support, or other needs.[12] However, as yet there is no definitive study demonstrating a statistically significant benefit to these types of programs.[13]

Clinical Significance

PSAP EMS response prioritization systems have been in place for decades and match patient needs to the most appropriate response units and urgency of response. While models vary, the common goal of PSAPs is to provide the most suitable, safe, and timely response to requests for service.  The expectation is that for a variety of reasons, the demand for EMS will continue to increase, so the ability to better match anticipated patient needs to resources is critical, since the continued expansion of the EMS to keep pace with demand is not expected. The EMS Agenda 2050 document highlights opportunities to ease this burden by leveraging technology to meet the patient’s needs while remaining patient-focused. Research on various strategies to show which are most effective at meeting patient needs while appropriately using resources is critical to tailor prioritization of response going forward.