Frequency of Mass Casualty Incidents (MCIs) Responded to by Helicopter Emergency Medical Services (HEMS)

Mass casualty incidents (MCIs) challenge out-of-hospital and in-hospital personnel as well as impose significant pressure on available resources. Helicopter emergency medical services (HEMS) crews may be involved in the response to an MCI. Thus, there is epidemiological interest to understand the frequency of MCIs responded to by HEMS.We used a nationally representative database of emergency medical services activations in the United States (National Emergency Medical Services Information System). We queried 911-initiated (scene) activations during the years 2021 and 2022 within the database and paid special attention to activations defined by crews as MCIs. From this, we were able to calculate the frequency at which HEMS crews responded to MCIs relative to the total number of HEMS scene activations.Of a total of 177,509 HEMS scene activations, less than 1% involved MCIs (0.27%, 486/177,509). Nationally, HEMS crews responded to roughly 2.74 MCIs per 1,000 activations. Variation in MCI frequency was noted across the day of the week, the month of the year, and the time of the day.HEMS crews do respond to MCIs although infrequently. Certain time periods are associated with a greater frequency of MCIs. These data will hopefully be able to inform preparedness and training.