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. 2025 Mar 22:23:100934.
doi: 10.1016/j.resplu.2025.100934. eCollection 2025 May.

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

Niels Saaby Hald  1   2   3 , Harman Yonis  4   5 , Mathias Hindborg  4 , Helle Collatz Christensen  6 , Jannie Kristine Bang Gram  1 , Erika Frischknecht Christensen  2   3 , Fredrik Folke  5   7   8 , Gunnar Gislason  5   7 , Christian Torp-Pedersen  4   5 , Kristian Bundgaard Ringgren  9
Affiliations

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

Niels Saaby Hald et al. Resusc Plus. .

Abstract

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%, p = 0.414) and PAD shock (8.9%-9.8%, p = 0.266) remained consistent throughout the year. The proportion of residential OHCAs were higher during winter than in summer (79.4% vs 77.5%, respectively, p = 0.023). Crude 30-day survival rates showed significant seasonal variation with lower survival rates during winter (11.1%) compared to spring (12.4%), summer (13.4%) and fall (12.2%, p = 0.001). However, after adjusting for factors such as sex, comorbidities, and OHCA circumstances (witnessed status, bystander CPR, PAD shock), no significant seasonal variation in survival remained (all p > 0.05).

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.

Keywords: Bystander; Out-of-hospital cardiac arrest; Public defibrillator; Seasons; Survival; Weather.

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Conflict of interest statement

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Harman Yonis reports a relationship with TrygFonden that includes: funding grants. Harman Yonis reports a relationship with Laerdal Medical AS that includes: funding grants. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Flowchart illustrating the selection process of the study sample. OHCA; out-of-hospital cardiac arrest. DK; Denmark. EMS; emergency medical services.
Fig. 2
Fig. 2
Crude incidence of OHCA shown according to month, location and 30-day survival. The black line indicates total of all OHCA. Lines are coloured according to survival status and the shapes indicate location type.
Fig. 3
Fig. 3
Forest plot showing the adjusted monthly rate ratio of OHCA incidence using January as reference month. Adjusted for year of OHCA, sex and co-morbidities.
Fig. 4
Fig. 4
Forest plot showing the adjusted monthly rate ratio of 30-day survival following OHCA using January as reference month. Adjusted for sex, co-morbidities, witnessed status, bystander CPR, bystander defibrillation and location type.
See this image and copyright information in PMC

References

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