Characteristics of midsized urban EMS systems.

Emergency medical services (EMS) systems in 25 midsized cities (population, 400,000 to 900,000) are described. Information describing EMS system configuration and performance was collected by written and telephone surveys with follow-ups. Responding cities provide either one- or two-tier systems. In a one-tier system, an advanced life support (ALS) unit responds to and transports all patients who use 911 to activate the system. Three types of two-tier systems are identified. In system A, ALS units respond to all calls. Once on scene, an ALS unit can turn a patient over to a basic life support (BLS) unit for transport. In system B, ALS units do not respond to all calls; BLS units may be sent for noncritical calls. In system C, a nontransport ALS unit is dispatched with a transporting BLS unit. For ALS calls, ALS personnel join BLS personnel for transport. Overall, cities staff an average of one ambulance per 51,223 population. One-tier systems average one ambulance per 53,291 compared with two-tier systems, which average one ambulance per 47,546. In the two-tiered system B, the average ALS unit serves 118,956 population. In the 60% of cities that use a one-tier system, one ALS unit serves 58,336 (P less than .0005). Overall, the code 3 response time for all cities is an average of 6.6 minutes. The average response time of two-tier systems is 5.9 minutes versus 7.0 minutes for one-tier systems (.05 less than P less than .1). These data suggest that the two-tiered system B allows for a given number of ALS units to serve a much larger population while maintaining a rapid code 3 response time.