Temporal trends in initial rhythms of out-of-hospital cardiac arrest in an aging society: a nationwide analysis from Japan.

BACKGROUND: Temporal changes in the distribution of initial rhythms in out-of-hospital cardiac arrest (OHCA) remain incompletely characterized at a national level in Japan. We evaluated long-term trends in shockable and non-shockable rhythms and their relationships with population aging and age-dependent temporal trends. METHODS: We analyzed 2,342,720 OHCA cases recorded in the All-Japan Utstein Registry between January 2005 and December 2023. Temporal trends in initial rhythms were evaluated overall and by age group. Multivariable logistic regression, including a calendar year × age group interaction, was used to assess age-dependent temporal trends in shockable rhythm presentation. Population-based OHCA incidence rates were calculated using national population estimates. RESULTS: The annual number of OHCA cases increased over time, mainly owing to increases among adults aged ≥80 years and those with non-shockable rhythms, whereas the number of shockable cases remained relatively stable. Despite the increasing OHCA burden, age-specific population-based incidence remained largely unchanged across all age groups. Older age was strongly associated with lower odds of presenting with a shockable rhythm. Temporal trends differed significantly by age group, with progressive declines in shockable rhythm presentation among patients aged 60-79 and ≥80 years, whereas those aged <60 years showed stable or slightly increasing trends. CONCLUSIONS: Temporal changes in the distribution of initial OHCA rhythms were strongly age-dependent. The increasing OHCA burden largely reflected population aging, while age-specific incidence remained stable. These findings highlight the importance of considering population aging when interpreting temporal changes in initial OHCA rhythms.