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Background: Rates of out-of-hospital cardiac arrest (OHCA) and bystander cardiopulmonary resuscitation (CPR) have been shown to vary considerably in Victoria. We examined the extent to which this variation could be explained by the sociodemographic and population health characteristics of the region., Methods: Using the Victorian Ambulance Cardiac Arrest Registry, we extracted OHCA cases occurring between 2011 and 2013. We restricted the calculation of bystander CPR rates to those arrests that were witnessed by a bystander. To estimate the level of variation between Victorian local government areas (LGAs), we used a two-stage modelling approach using random-effects modelling., Results: Between 2011 and 2013, there were 15 830 adult OHCA in Victoria. Incidence rates varied across the state between 41.9 to 104.0 cases/100 000 population. The proportion of the population over 65, socioeconomic status, smoking prevalence and education level were significant predictors of incidence in the multivariable model, explaining 93.9% of the variation in incidence among LGAs. Estimates of bystander CPR rates for bystander witnessed arrests varied from 62.7% to 73.2%. Only population density was a significant predictor of rates in a multivariable model, explaining 73% of the variation in the odds of receiving bystander CPR among LGAs., Conclusions: Our results show that the regional characteristics which underlie the variation seen in rates of bystander CPR may be region specific and may require study in smaller areas. However, characteristics associated with high incidence and low bystander CPR rates can be identified and will help to target regions and inform local interventions to increase bystander CPR rates., Competing Interests: Conflicts of Interest: None declared., (Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/.)

Background: Out-of-hospital cardiac arrest (OHCA) remains a major public health issue and research has shown that large regional variation in outcomes exists. Of the interventions associated with survival, the provision of bystander CPR is one of the most important modifiable factors. The aim of this study is to identify census areas with high incidence of OHCA and low rates of bystander CPR in Victoria, Australia., Methods: We conducted an observational study using prospectively collected population-based OHCA data from the state of Victoria in Australia. Using ArcGIS (ArcMap 10.0), we linked the location of the arrest using the dispatch coordinates (longitude and latitude) to Victorian Local Government Areas (LGAs). We used Bayesian hierarchical models with random effects on each LGA to provide shrunken estimates of the rates of bystander CPR and the incidence rates., Results: Over the study period there were 31,019 adult OHCA attended, of which 21,436 (69.1%) cases were of presumed cardiac etiology. Significant variation in the incidence of OHCA among LGAs was observed. There was a 3 fold difference in the incidence rate between the lowest and highest LGAs, ranging from 38.5 to 115.1 cases per 100,000 person-years. The overall rate of bystander CPR for bystander witnessed OHCAs was 62.4%, with the rate increasing from 56.4% in 2008-2010 to 68.6% in 2010-2013. There was a 25.1% absolute difference in bystander CPR rates between the highest and lowest LGAs., Conclusion: Significant regional variation in OHCA incidence and bystander CPR rates exists throughout Victoria. Regions with high incidence and low bystander CPR participation can be identified and would make suitable targets for interventions to improve CPR participation rates.