Abstract Body: Background
The optimal timing of epinephrine dosing in out-of-hospital cardiac arrest (OHCA) remains uncertain, and practice patterns may vary across emergency medical services (EMS) agencies. Understanding how epinephrine dosing intervals align with published guidelines as well as patient and arrest characteristics may provide insight into system-level influences on cardiac arrest care.

Methods
We conducted a retrospective analysis of non-traumatic adult OHCAs in the United States from 2019–2023 using the National EMS Information System (NEMSIS) database. Cases were included if they received at least two doses of epinephrine with associated timestamps. Agencies that treated fewer than five OHCAs during the study period were excluded. The most frequent (modal) epinephrine interval was calculated for each agency, and agencies were stratified into four groups: <3 minutes, 3-5 minutes (guideline-reference), >5 & <10 minutes, and ≥10 minutes. Agency, patient, and cardiac arrest characteristics were compared across groups, including agency volume, urbanicity, age, sex, race/ethnicity, location, witnessed status, CPR prior to EMS arrival, and initial rhythm.

Results
Of 2,170,129 OHCAs, 811,959 treated by 6,728 EMS agencies met inclusion criteria. Most OHCAs (85.9%) were treated by agencies in the guideline-reference group (3-5 minute), 9.5% ≤3 minute group, 3.5% >5 & <10 minutes, and 1.2% ≥10 minutes. Agencies with longer modal intervals were primarily urban (>5 & <10 - 94.9%, ≥10 minutes - 94.9%, 3-5 minutes - 83.7%), and had higher median cardiac arrest volumes (>5 & <10 – 2,576, ≥10 minutes - 2,125, 3-5 minutes - 432). Urbanicity and OHCA volume were similar between the 3-5 minutes and <3 minutes groups. Patient and arrest characteristics were generally similar across groups, though agencies in the ≥10 minute group had a higher proportion of public location arrests (13.0% vs 10.9% for 3-5 minutes). CPR prior to EMS arrival was also less frequent in the ≥10 minute group (21.5 vs. 26.0% for 3-5 minutes).

Conclusion
The majority of agencies demonstrated epinephrine dosing intervals consistent with guidelines. A small proportion exhibited longer intervals, particularly among higher-volume urban agencies, which suggests this is likely an intentional practice. These findings highlight variability in epinephrine dose-interval practices across US EMS agencies with the potential to explore resulting differences in OHCA outcomes.