Seasonal variation in bystander efforts and survival after out-of-hospital cardiac arrest.

Background: This study investigated the hypothesis that out-of-hospital cardiac arrest (OHCA) incidence, public automated external defibrillator (PAD) utilization and outcome vary by season, with increased incidence and lower survival rates expected in winter. The aim was to provide insights that might optimize resuscitation efforts throughout the year. Methods: Cases of OHCA from 2016 to 2021 were included from the Danish Cardiac Arrest Registry. Cases were stratified by season and month and analysed for frequency, witnessed status, location, bystander cardiopulmonary resuscitation (CPR), bystander PAD defibrillation and 30-day survival. The primary outcome was incidence of OHCA and variability in utilization of PADs by season. Secondary outcome was 30-day survival rates. Categorical variables were compared using Chi-square and multivariable analyses were conducted using Poisson regression. Results: A total of 25,248 OHCA cases were included with a median age of 74 years [IQR 63-82] and 64% were male. Multivariable analyses revealed a lower incident rate ratio (IRR) of OHCA for most months (IRR 0.78 to 0.92 with 95%CI 0.72-0.98, all p-values < 0.05), except March and December, using January as reference. Seasonal rates of bystander CPR (78.4%-79.4%, Conclusion: Incidence of OHCA was higher in winter, but rates of bystander CPR and PAD shock remained consistent across seasons. Crude mortality rates were significantly lower during winter. However, multivariable regression analysis revealed no significant variation in survival rates by month.