IntroductionThe return of spontaneous circulation after cardiac arrest (RACA) score is a well-validated model for estimating the probability of return of spontaneous circulation (ROSC) in patients with out-of-hospital cardiac arrest (OHCA) by incorporating several variables, including gender, age, arrest aetiology, witness status, arrest location, initial cardiac rhythms, bystander cardiopulmonary resuscitation (CPR), and emergency medical services (EMS) arrival time. The RACA score was initially designed for comparisons between different EMS systems by standardising ROSC rates. End-tidal carbon dioxide (EtCO2) is a quality indicator of CPR. We aimed to improve the performance of the RACA score by adding minimum EtCO2measured during CPR to develop the EtCO2+ RACA score for OHCA patients transported to an emergency department (ED).MethodsThis was a retrospective analysis using prospectively collected data for OHCA patients resuscitated at an ED during 2015–2020. Adult patients with advanced airways inserted and available EtCO2measurements were included. We used the EtCO2values recorded in the ED for analysis. The primary outcome was ROSC. In the derivation cohort, we used multivariable logistic regression to develop the model. In the temporally split validation cohort, we assessed the discriminative performance of the EtCO2+ RACA score by the area under the receiver operating characteristic curve (AUC) and compared it with the RACA score using the DeLong test.ResultsThere were 530 and 228 patients in the derivation and validation cohorts, respectively. The median measurements of EtCO2were 8.0 times (interquartile range [IQR] 3.0–12.0 times), with the median minimum EtCO2of 15.5 millimeters of mercury (mm Hg) (IQR 8.0–26.0 mm Hg). The median RACA score was 36.4% (IQR 28.9–48.0%), and a total of 393 patients (51.8%) achieved ROSC. The EtCO2+ RACA score was validated with good discriminative performance (AUC, 0.82, 95% CI 0.77–0.88), outperforming the RACA score (AUC, 0.71, 95% CI 0.65–0.78) (DeLong test: P < 0.001).ConclusionThe EtCO2+ RACA score may facilitate the decision-making process regarding allocations of medical resources in EDs for OHCA resuscitation.