Severity-Driven Trends in Mortality in a Large Regionalized Critical Care Transport Service

•A 10-year descriptive analysis of the largest standardized nonprofit helicopter emergency medical service in the United States reveals a high conditional and demographic diversity.•The longitudinal evolution of regionalized care with the affiliated regional hospital system drove trends in the illness severity of patients.•These trends were reflected in mortality, hospital length of stay, and the proportion of patients with complex medical conditions versus scene-run trauma.