Defining a mission-based method to determine a HEMS unit’s actual service area

BACKGROUND: Geographical service areas are used as descriptive system indicators in Emergency Medical Service (EMS) related studies and reporting templates. The actual service area may differ significantly from administrative areas; this may lead to inaccuracy in determining indicator values, such as population or mission density, thus making it biased when comparing results between different areas and organizations. The aim of this study was to introduce a univocal, repeatable and easily adaptable method to determine the actual service area of a helicopter emergency medical service (HEMS) unit for statistical, quality measurement and research purposes using widely available geographical information (GIS) and statistical analysis tools. METHODS: The method was first tested with Tampere HEMS unit. All accepted missions in 2017 were extracted from FinnHEMS database (FHDB). We calculated distance from HEMS base to each accepted mission location. Missions were reordered based on the distance and 99th and 95th percentiles were calculated for mission distances. Convex hulls including 100, 99 and 95% of the missions, and the population and area covered by these missions, were then calculated. The method was repeated for all Finnish HEMS bases. RESULTS: Approximately 90% of Tampere HEMS unit’s accepted missions took place within 100 km from the base. 10.9% of the missions occurred outside of the administrative service area. 95% convex hull areas are most in line with the everyday experience of where the units actually operate. In Tampere, the 95% convex hull area corresponds to 76,5% of the administrative area’s population and to 89,8% of its area. Calculating the 95% convex hull areas for all Finnish HEMS units results in service areas that overlap at some points, and some areas of the country fall outside of all HEMS service areas. CONCLUSIONS: Administrative areas do not correspond to the actual service areas of HEMS units. The service area of a HEMS unit defined by administrative boundaries may differ significantly from actual operations. Using historical mission data to create a convex hull that incorporates mission locations could offer a standardized and comparable solution for determining actual HEMS unit service areas, which can be used for statistical comparison, quality measurement and system development.