On-Scene Time-Dependent Probabilities of Return of Spontaneous Circulation and Favorable Neurological Outcome in Cases of Out-of-Hospital Cardiac Arrest with an Initial Shockable Rhythm: A Retrospective Study Using the National Utstein Data in Japan

Background/Objectives: In out-of-hospital cardiac arrest (OHCA) with an initial shockable rhythm, the likelihood of achieving on-scene return of spontaneous circulation (ROSC) and favorable neurological outcomes changes with elapsed resuscitation time. We aimed to describe these time-dependent patterns under existing Japanese emergency medical services (EMS) practice. Methods: We conducted a nationwide retrospective descriptive cohort study using linked Japanese Utstein and emergency transport data from 2017–2022. Adults with cardiogenic OHCA and an initial shockable rhythm were included. For each time point after EMS contact, we calculated the observed probability of subsequent on-scene ROSC and, among patients achieving on-scene ROSC, the probability of a favorable neurological outcome (CPC 1–2 at 1 month) according to ROSC timing. Results: Among 22,383 eligible patients, 5095 achieved on-scene ROSC and 3182 had favorable neurological outcomes. The probability of subsequent on-scene ROSC was 6.97% (1295/18,583; 95% CI, 6.61–7.34%) at 10 min and 0.50% (87/17,375; 95% CI, 0.41–0.62%) at 20 min. At 10 min, favorable neurological outcomes occurred in 50.0% of patients aged <75 years and 20.5% of those aged ≥75 years who achieved on-scene ROSC at or after that time point. A sensitivity analysis restricted to 3128 patients receiving first EMS defibrillation within 1 min showed broadly similar temporal patterns. Conclusions: Under existing Japanese EMS practice, subsequent on-scene ROSC and favorable neurological outcomes became less frequent with later ROSC timing. Younger patients showed more favorable neurological outcomes at later ROSC times, but these findings do not establish age-based criteria for prolonged resuscitation or transport decisions.