Abstract Body: Introduction: AEDs are essential in diverse out-of-hospital cardiac arrest (OHCA) scenarios. After a successful defibrillation, refibrillation occurs frequently and poses challenges to sustained resuscitation success. Analyzing refibrillation characteristics among groups defined by specific AED use scenarios may guide improvements in resuscitation strategies. This study investigates the characteristics of refibrillation across different AED user groups.

Methods: This retrospective study examined ECG rhythms from 2,358 OHCA patient cases in the US and Europe, between 2007 and 2022, treated with three AED models (Philips HS1, FRx, and FR3), each optimized for specific scenarios. Cases were divided into three groups based on the AED used: residential and public access lay-responder (HS1); trained first-responder (FRx); and EMS (FR3). Each ECG rhythm, before and after initial shocks, was reviewed independently by three experts, with consensus on final annotations. Shock success was determined by the presence of a non-shockable rhythm for at least 5 seconds (s) post-shock. Time to refibrillation was annotated for the rhythm following each successful initial shock with a shockable pre-shock rhythm. Refibrillation rate, defined as the cumulative proportion of successful shocks where refibrillation occurred by a specified post-shock time point, was calculated for the successful initial shocks in each group using the Turnbull estimator. Refibrillation rates across the groups were compared using the weighted log-rank test.

Results: Refibrillation was analyzed for rhythms following 487 successful initial shocks (lay-responder: 157, first-responder: 104, EMS: 226). Figure 1 shows the refibrillation rates within two minutes post-shock, with the lay-responder group consistently showing the lowest refibrillation rate. Figure 2 lists the estimated refibrillation rates at 30 s, 60 s, 90 s, and 120 s after successful initial shocks. Refibrillation rates varied significantly across the three groups (p = 0.016). Among refibrillations within two minutes post-shock, the median time to refibrillation was 38.7 s (lay-responder), 31.0 s (first-responder), and 27.4 s (EMS).

Conclusion: After successful initial shocks, OHCA patients in residential or public access lay-responder settings experienced lower refibrillation rates and longer times to refibrillation, compared to first-responder/EMS scenarios. The variations in refibrillation may be accounted for by response times in OHCA.