Beyond the Presenting Rhythm: Estimating Shockable Rhythm at the Time of Cardiac Arrest in Out-of-Hospital Cardiac Arrest

Abstract Background Out of hospital cardiac arrest (OHCA) remains a leading cause of death worldwide with survival strongly dependent on the presence of a shockable rhythm at the time of first rhythm analysis. Thus, determinants of a shockable rhythm in cardiac arrest is important. Methods We analysed adult non-traumatic witnessed OHCA in New South Wales from 2017–2022 with bystander CPR. We used generalised additive modelling (GAM) to estimate the probability that an OHCA in this cohort was initially shockable and to characterise how this probability varied according to age and sex. Results NSW Ambulance attended 45,607 during the period January 2017 through March 2022 of which 5,277 met eligibility criteria. The GAM identified significant sex-specific relationships between age, elapsed time and the probability of a shockable rhythm. The relationship between age and shockable rhythm was non-linear in males (edf 4.14, χ² = 167.2, p < 0.001), whereas the relationship was approximately linear in females (edf 1.00, χ² = 51.8, p < 0.001). Elapsed time was significantly associated with shockable rhythm in both males (edf 1.00, χ² = 89.3, p < 0.001) and females (edf 1.88, χ² = 23.9, p < 0.001), with greater evidence of non-linearity among females. At the estimated time of collapse (t = 0), the GAM estimated a probability of a shockable rhythm of 41% (95% CI 27–55%) among females, representing a 17% increase compared with the observed presenting shockable rhythm rate of 24%. Among males, the corresponding estimated probability was 61% (95% CI 56–67%), representing an 18% increase compared with the observed presenting rate of 43%. Conclusions Our modelling suggests the proportion of arrests initially presenting as shockable may be substantially higher than that observed at first EMS rhythm analysis. We conclude that presenting rhythm in OHCA is a dynamic, time-dependent phenomenon and demonstrates the potential value of GAM-based modelling for estimating the probability of a shockable rhythm at or near the time of collapse. These findings provide a more nuanced framework for understanding age and sex differences in OHCA and highlight the importance of considering the timing of rhythm assessment when interpreting epidemiological differences in shockable rhythm.