Background
Defibrillation is essential for achieving return of spontaneous circulation (ROSC) following out‐of‐hospital cardiac arrest (OHCA) with shockable rhythms. This study aimed to investigate if the type of defibrillator used was associated with ROSC in OHCA.

Methods and Results
This study included adult patients with OHCA from the Danish Cardiac Arrest Registry from 2016 to 2021 with at least 1 defibrillation by the emergency medical services. We used multivariable logistic regression and a difference‐in‐difference analysis, including all patients with or without emergency medical services shock to assess the causal inference of using the different defibrillator models (LIFEPAK or ZOLL) for OHCA defibrillation. Among 6516 patients, 77% were male, the median age (quartile 1; quartile 3) was 70 (59; 79), and 57% achieved ROSC. In total, 5514 patients (85%) were defibrillated using LIFEPAK (ROSC: 56%) and 1002 patients (15%) were defibrillated using ZOLL (ROSC: 63%). Patients defibrillated using ZOLL had an increased adjusted odds ratio (aOR) for ROSC compared with LIFEPAK (aOR, 1.22 [95% CI, 1.04–1.43]). There was no significant difference in 30‐day mortality (aOR, 1.11 [95% CI, 0.95–1.30]). Patients without emergency medical services defibrillation, but treated by ZOLL‐equipped emergency medical services, had a nonsignificant aOR for ROSC compared with LIFEPAK (aOR, 1.10 [95% CI, 0.99–1.23]) and the difference‐in‐difference analysis was not statistically significant (OR, 1.10 [95% CI, 0.91–1.34]).

Conclusions
Defibrillation using ZOLL X Series was associated with increased odds for ROSC compared with defibrillation using LIFEPAK 15 for patients with OHCA. However, a difference‐in‐difference analysis suggested that other factors may be responsible for the observed association.