The time dependence of a multi-tier response for out-of-hospital cardiac arrest (OHCA) is unclear. The aim of this study was to evaluate the time-dependent effect of EMS response type in a multi-tiered system on the clinical outcomes of OHCA.

Adult EMS-treated OHCA of presumed cardiac etiology who were not witnessed by EMS between January 2015 and December 2017 were included. The main exposure was EMS response type: single-tier response, early multi-tier response (0–18 min from call to second EMS arrival), and late multi-tier response (19 min from call to second EMS arrival). The primary outcome was good neurologic recovery at the time of discharge from the hospital. Multivariate logistic regression analysis was performed, adjusting for patient–community and prehospital variables.

Among 54,436 patients, 29,995 patients (55.1%), 21,552 patients (39.6%), and 2889 patients (5.3%) were treated by single-tiered EMS, early multi-tiered EMS, and late multi-tiered EMS, respectively. Good neurological recovery and survival to discharge were more frequent in the early multi-tiered response group (6.4% and 9.7%) than in the single-tiered response group (4.8% and 7.5%) or late multi-tiered response group (3.1% and 5.8%). Compared to the single-tiered response group, the early multi-tiered response group was more likely to have good neurological recovery (adjusted OR, 95% CI: 1.15 [1.06–1.26]), but the late multi-tiered response group was less likely to have good neurological recovery (adjusted OR, 95% CI: 0.76 [0.61–0.96]).

In our basic to intermediate–tiered EMS system, early multi-tier response was associated with improved survival and good neurological recovery.