For the Osaka Utstein Registry database, 3414 OHCA patients with an initial shockable rhythm were included in the analysis [the derivation cohort (n = 3,088) and the validation cohort (n = 326)]. Of the SG-PAROS database, 2905 patients with initial shockable rhythm were included in the analysis. The study flowchart is described in the Additional file 1 (S-Results 1). The median [IQR] age was 62 [51, 68] in the derivation cohort in Osaka, and 58 [50, 65] in the SG-PAROS. Good neurological outcomes were 32% (987/3,088) in Osaka and 17% (500/2,905) in Singapore. The prehospital ROSC were 36% (1,125/3,088) in Osaka and 27% (773/2,905) in Singapore, and among them, one-month good neurological outcomes were 70% (791/1,125) in Osaka and 57% (440/773), respectively. In comparison, among the patients without prehospital ROSC, good neurological outcomes were 10.0% (196/1,963) and 2.8% (60/2,132), respectively. The other details of characteristics, in-hospital information, and outcomes are described in Table 1 and the Additional file 1: (S-Results 2–5). ECMO was rarely performed in both populations of Singapore (with prehospital ROSC, 0.9% and without 0.5%). The prediction model derived from the Osaka derivation data performed well in the validation data. The details of the prediction model are described in the Additional file 1 (S-Results 6–7).

The one-month good neurological outcome in patients with prehospital ROSC in Singapore was the same or slightly better than expected compared to Osaka (OE ratio, 1.067 [95%CI 1.012 to 1.125] and OE difference, 0.036 [95%CI 0.006 to 0.066]). Conversely, the neurological outcome in patients without prehospital ROSC in Singapore was worse than expected compared to Osaka (OE ratio, 0.238 [95%CI 0.173 to 0.294] and OE difference, − 0.09 [95%CI − 0.096 to -0.083]), (Fig. 1). Similarly, for one-month survival, the result was consistent as follows: with prehospital ROSC, OE ratio, 0.97 [95%CI 0.925 to 1.012] and OE difference, − 0.021 [95%CI − 0.051 to 0.008], and without prehospital ROSC, OE ratio 0.297 [95%CI 0.242 to 0.345] and OE difference, -0.128 [95%CI -0.138 to -0.119], (Fig. 1).

OE difference and the incremental number of patients with or without Prehospital ROSC. (Left) The Observed-Expected ratio and 95% confidence interval (CI). (Right) The Observed-Expected difference and 95% confidence interval (CI). Good Neuro One-month good neurological outcome, Survival One-month survival, ROSC, Return of spontaneous circulation

The incremental one-month good neurological outcome in patients with prehospital ROSC was better than expected (27.8 [95% CI 4.9 to 50.7]), but was fewer than expected (− 191.9 [95% CI −205.6 to − 177.6]) in patients without prehospital ROSC) between 2010 and 2019. The incremental number of one-month survival cases had a similar trend to neurological outcomes (patients with pre-hospital ROSC: − 16.3 [95%CI − 39.1 to 5.9], patients without prehospital ROSC: − 272.7 [95%CI − 293.2 to − 253.8]). The incremental one-month good neurological outcome in each year is described in the Additional file 1 (S-Results 8).