Apples to apples: can differences in out-of-hospital cardiac arrest incidence and outcomes between Sweden and Ireland be explained by core Utstein variables?

The significant potential for the presence of unmeasured differences between countries should be acknowledged, whether for research or for the development of national outcome targets.

There is international variation in the reported incidence and outcome from out-of-hospital cardiac arrest (OHCA). Chamberlain and Eisenberg stated that in order to compare outcomes between different systems of care, it is necessary to have “a comparator that enables areas of weakness to be defined and addressed whether it be at local, national and international level”. The Utstein criteria were developed for this reason, and identify patients based on a number of indicators [1]. International benchmarking is a highly desirable aspiration, and many notable studies and reviews have been carried out that compare the outcomes from OHCA across multiple countries and jurisdictions [2‐5]. To ensure that comparison is informative, it is essential that data is collected for the same purpose, data definitions and collection methodologies are similar, and that the population covered is equally representative [6]. Assuring uniformity in OHCA data collection and reporting systems is essential, as differences in outcomes may well be attributable to differences in data availability and processing methodologies [7]. This aim of this study was therefore to investigate the extent to which patient-level analysis using core ‘Utstein’ variables explains variation in OHCA incidence and outcome between two countries, namely Sweden and the Republic of Ireland.

This patient-level analysis of 3 years of data from two well established national registries shows that the incidence of attempted resuscitation is similar for the Utstein subgroup in both countries, but that percentage survival is greater in Sweden than in the Republic of Ireland overall, for all age categories and both subgroups. Even when data from two countries has been collected using similar methods and rationale, the reasons for inter-country differences in outcome are not fully explained by the core Utstein variables used in this study.

Our study highlights that differences in OHCA outcomes between countries are not solely down to differences in patient age and gender profile or pre-hospital interventions. By using patient level data, this analysis serves to quantify the degree of variation that can be explained by inter-country comparison in a way that cannot be achieved with aggregate outcome data. The critical value of OHCA data collection is that it can focus national efforts on improving national outcomes [19, 20]. In the latest revision of the Utstein dataset, Perkins et al. advised on a range of core and supplemental OHCA elements that are likely to help explain a larger proportion of inter-country outcome variation, including pre-existing co-morbidities and in-hospital treatments and interventions [21]. Implementation or improved systematic collection of these data elements is likely to explain substantial variation in outcome within and between countries.

There are clearly differences in the patient and intervention characteristics in both countries. On average, Swedish patients are older in all cases and in both subgroups, and the higher overall incidence of OHCA resuscitation is largely accounted for by the greater proportion of older OHCA patients in Sweden (Table 2). The explanation for this difference in age profile may lie in cultural attitudes and expectations surrounding death and morbidity in both countries. A survey of public attitudes to resuscitation in older people has not been previously carried out, but may help explain the significant difference in resuscitation incidence in this age category.

As also shown in Table 2, the incidence of Utstein subgroup cases is similar in both countries. This is likely to be largely driven by the fact that similar proportions of patients had an initial recorded shockable rhythm (23.7%). The ‘three phase model’ of cardiac arrest suggests that most patients will deteriorate into an asystole within 5 min without intervention [22]. Considering the significantly shorter median EMS call response-interval in Sweden, it may have been expected that the proportion of Swedish patients with an initial recorded shockable rhythm would be greater than in the Republic of Ireland. One explanation may be the higher proportion of older people in the Swedish OHCA resuscitation population, as older people have been found to have a lower incidence of initial shockable rhythm [23]. Additionally, a decline in the proportion of patients with an initial shockable rhythm has previously been observed in Sweden, despite efforts to improve call-to-shock times [24]. It has been proposed that this decline may be due to a reduction in untreated ischaemic heart disease (IHD) in the Swedish population and that the proportion of cases with cardiac aetiology is less than presumed [25]. Diagnosis of IHD continues to increase the Republic of Ireland and what proportion of this increase is due to increasing prevalence or improved detection is unclear [26]. Both registries primarily rely on the clinical impression formed by the attending ambulance crew to determine the aetiology of arrest. Previous work on validation of aetiology in paediatric OHCA has shown the potential value of adding coronial data to an OHCA registry [27]. It is suggested that inclusion of coronial data in the Swedish and Irish registries may assure the validity of data on aetiology and ensure realistic expectations for the proportion of cases with an initial shockable rhythm.

The proportion of CPR provided by those who were ‘trained, may be dispatched by ambulance control’ in the Republic of Ireland is encouraging (Table 1). The Republic of Ireland already has an active and growing Community First Responder (CFR) network. While there is evidence that trained first responders can contribute to survival, the best model of CFR is not yet determined [28]. The fire service plays a greater role in the provision of CPR in Sweden, suggesting there is potential for Irish Fire Services to participate more often in the OHCA response. It should be noted that – despite the fact that that dual dispatch of ambulance and fire services in Sweden has been shown to have the greatest effect on response intervals in rural Swedish areas – survival benefit was most significant in densely populated areas [29]. This suggests that there is a response interval beyond which any form of dual dispatch may not be of additional benefit to ambulance dispatch only.

Proportionate survival from OHCA is greater in Sweden for all patients, both subgroups and all age categories (Table 3). Patients who collapse in the presence of EMS are likely to receive good quality CPR and rapid defibrillation, which in turn is more likely to be immediately effective if performed soon after collapse [30]. This is borne out in the relatively high proportion of survival in this subgroup in both countries, and partially explains the higher overall percentage survival in Sweden.

The multivariable logistic regression model of survival explains – at best – 17% of variation between countries, and includes a large ‘country effect’ in favour or Sweden that is not explained by the predictor variables (Table 4). Rather than suggesting that the chances of patients in the Utstein group surviving an OHCA are over four times greater in Sweden than in the Republic of Ireland, this result points to the large proportion of variation which is not explained by our Utstein predictor variables. The implication is that while improving the availability of important outcome predictors such as bystander CPR and defibrillation, and reducing EMS call response intervals is likely to increase survival in the Republic of Ireland, these measures alone are unlikely to achieve parity of outcomes with Sweden.

The ability to compare OHCA incidence and outcomes across countries and systems is essential to establishing international benchmarking. The use of patient-level data have highlighted the proportion of variation outside of the well-known predictors of OHCA outcome, something that is not possible in comparative studies that rely on aggregated data. We believe the approach used in this study is transferable to other comparative studies of OHCA national incidence and outcome, and that such an approach will improve the validity and value of inter-country comparison, whether for research or for the development of national outcome targets.