Seasonal Variation in Out-of-Hospital Cardiac Arrest in Victoria 2008-2017: Winter Peak

Background: Out-of-Hospital Cardiac Arrest (OHCA) incidence and survival rates are known to vary between seasons in some locations. The winter of 2017 saw the highest ever incidence and lowest survival rate of OHCA recorded in Victoria at the time. Seasonal variation of OHCA has not previously been examined in Australia and there may be a significant effect. We aimed to describe the seasonal incidence of OHCA and examine seasonal differences in survival to discharge and 12-month quality of life outcomes. In addition, we investigated whether recent respiratory infection or pre existing respiratory disease influenced OHCA outcomes. Methods: The Victorian Ambulance Cardiac Arrest Registry (VACAR) was used to identify OHCA occurring in Victoria between 2008 and 2017. We examined OHCA characteristics and outcomes between seasons. We then used multivariate logistic regression, adjusting for the Utstein factors, to examine if season, recent respiratory infection or preexisting respiratory disease is associated with survival to discharge and 12-month quality of life. Results: There were 44 973 OHCA cases of which 22 209 received an attempted resuscitation (49.4%). The incidence of OHCA was highest during winter (22 per 100 000 vs. 18 per 100 000 in summer). In winter, survival to discharge was lower for OHCA patients (12.2% vs. 15.9% in summer). Both recent respiratory infection (AOR: 0.67, 95% CI: 0.52 - 0.87) and OHCA occurring during winter (AOR: 0.79 95% CI: 0.70 - 0.91) were associated with lower odds of survival to discharge. Preexisting respiratory disease was associated with lower odds of moderate to good recovery at 12 months (AOR:0.60 95% CI: 0.41 - 0.89). Conclusion: In winter the incidence of OHCA was at its highest and survival to discharge was at its lowest compared to other seasons. Recent respiratory infection was associated with lower odds of survival to discharge. Measures targeted to vulnerable groups, such as preventative public health measures for respiratory infections and the influenza vaccine may reduce the incidence of OHCA and improve survival rates.