Siddiqui, Fahad JavaidFahad JavaidSiddiquiFook-Chong, StephanieStephanieFook-ChongShahidah, NurNurShahidahPflug, DavidDavidPflugLeong, Benjamin Sieu-HonBenjamin Sieu-HonLeongShin, Sang DoSang DoShinRyoo, Hyun WookHyun WookRyooRyu, Hyun HoHyun HoRyuLin, Chih-HaoChih-HaoLinKuo, Chan-WeiChan-WeiKuoOng, Marcus Eng HockMarcus Eng HockOngWEN-CHU CHIANGet al.2026-03-062026-03-062026-01https://scholars.lib.ntu.edu.tw/handle/123456789/736110Objectives The study aimed to examine the influence of contextual settings on the complex OHCA intervention outcomes thereby providing evidence to guide managers and policymakers in optimizing implementation strategies. Our secondary analysis of an international multi-site trial examined site-level effectiveness of comprehensive versus basic Dispatcher-Assisted CPR (DACPR) interventions on bystander CPR rates, hypothesizing site-specific variation and declining variability as an early sign. Methods Using 2009–2018 data from the Pan-Asian Resuscitation Outcomes Study (PAROS), we compared monthly and quarterly trends in BCPR rates at selected PAROS sites before and after the intervention. Interrupted time series analysis was performed using regression of monthly BCPR rates for the pre- and post-intervention periods. Results Six PAROS sites contributed 37,872 out-of-hospital cardiac arrest (OHCA) cases from January 2009 to June 2018. Comprehensive package sites showed consistent improvement in average monthly BCPR rates, with absolute increases ranging from 4.2 to 30.8 percentage points. Sharper post-intervention increases in DACPR rates were observed, indicated by positive site-wise regression coefficient differences (0.07–0.38). However, some sites experienced an initial post-intervention slowdown of the pre-existing trend. The two Basic package sites also improved their BCPR rates by 9.3 and 25.1 percentage points, though the shorter pre-intervention periods limited meaningful interpretation of the rate of change (0.22 & −0.33). Conclusion Both intervention packages improved BCPR rates, with most sites showing steeper improvements post-intervention. However, variations in timing and magnitude between sites highlight differing levels of system readiness and implementation rigor. Reduced variability has been observed post-intervention.entrueBystander CPRDispatcher-assisted cardiopulmonary resuscitationPAROSTelephone instructionsjournal article10.1016/j.resplu.2025.101193415839352-s2.0-105026906802