A total of 11 698 out-of-hospital cardiac arrests were identified in the seven study sites (). Arrests occurring in public or private nonresidential locations were excluded (range 9%–22% across sites), as were arrests with an obvious noncardiac cause (range 3%–16% across sites). After these exclusions, 71% to 88% of the out-of-hospital cardiac arrests in the study sites were retained for analysis, for a total of 9235 cardiac arrests likely of cardiac origin occurring in residential locations (n = 638 to 2586 across sites).
Selection of incidents of sudden cardiac arrest for analysis. *The arrest was not classified as having an obvious (noncardiac) cause.
The incidence of sudden cardiac arrest varied across the sites, with the US sites generally reporting a higher incidence than the Canadian sites (). The mean age at which sudden cardiac arrest occurred was lower in the US sites (range 64–69 years) than in the Canadian sites (range 69–70 years). The proportion of patients who were male ranged from 58% to 65% across sites ().
Characteristics of out-of-hospital sudden cardiac arrests in the registry of the Resuscitation Outcomes Consortium in seven metropolitan areas in the United States and Canada, from Apr. 1, 2006, to Mar. 31, 2007
The median household income was similar across sites (range US$50 000–US$63 000), except in Pittsburgh, where it was substantially lower (). Income inequality, as measured by the ratio of highest to lowest quartile of median income, ranged from 2.2 to 3.2 across sites ().
At six of the seven sites, the incidence of sudden cardiac arrest was significantly higher in the lowest than in the highest socioeconomic quartile. Site-specific IRRs in the US sites ranged from 1.8 (Seattle–King County) to 2.2 (Dallas); in the Canadian sites, they ranged from 1.1 (Ottawa) to 2.3 (Toronto) (). The trend across quartiles was significant at each site, with lower socioeconomic status associated with higher incidence of sudden cardiac arrest (p < 0.001).
Incidence rate ratios for out-of-hospital sudden cardiac arrest in lowest versus highest socioeconomic quartiles, by age
At all seven sites, disparities in the incidence of sudden cardiac arrest across socioeconomic quartiles were greatest among people less than 65 years old. Compared with the incidence in the highest quartile, the incidence in the lowest quartile was two- to fourfold greater in the US sites, and up to threefold greater in the Canadian sites (, ). As with all ages combined, disparities across socioeconomic quartiles among people younger than 65 were significant at all sites except Ottawa. Among people 65 years and older, the disparities were attenuated in the US and Toronto compared with those among younger individuals, and were reversed in Ottawa (, ).
Figure 2: Incidence of sudden cardiac arrest in seven metropolitan areas in the United States and Canada among people less than 65 years old (top panel) and those 65 years and older (bottom panel) from Apr. 1, 2006, to Mar. 31, 2007, by quartile of median household (more ...)
For all sites combined, the incidence of sudden cardiac arrest in the lowest socioeconomic quartile was nearly double that in the highest quartile (, ). After adjusting for site and for population age structure of each census tract, we found that the disparity across socioeconomic quartiles was greater in the United States (IRR 2.0 v. 1.8) (), with a significant interaction term for country by lowest socioeconomic quartile (p < 0.001). In the model with socioeconomic status quartile modelled from 1 to 4, the slope of the linear trend was significantly steeper in the United States than in Canada (p < 0.001) ().
Incidence of sudden cardiac arrest among people of all ages in all sites combined from Apr. 1, 2006, to Mar. 31, 2007, by quartile of median household income.
In this large, multicentre observational study, the incidence of sudden cardiac arrest was significantly higher in the neighbourhoods of lowest versus highest socioeconomic status in six of the seven metropolitan areas studied. We observed a significant linear trend of increasing incidence of cardiac arrest associated with decreasing quartile of median household income across all sites. The association between socioeconomic status and incidence of sudden cardiac arrest was most apparent among people less than 65 years old and was stronger in the United States than in Canada. Among people 65 years and older who experienced sudden cardiac arrest, significant disparities in incidence across socioeconomic quartiles were observed in only three of the seven sites.
Findings from our analysis are consistent with results from the Oregon Sudden Unexpected Death Study, which reported that the incidence of sudden cardiac arrest in the lowest socioeconomic quartile was about double that in the highest quartile, with a stronger association among patients less than 65 years old.11
Although the overall incidence of sudden cardiac arrest was higher among older patients, the disparities across socioeconomic quartiles that we observed among patients less than 65 translate to substantial socioeconomic disparities in premature death.
The persistent association between low socioeconomic status and poor health outcomes has been observed in other studies, regardless of access to care. Despite universal health care in Canada, lower socioeconomic status has been associated with an increased burden of risk factors for cardiovascular disease,13
a decreased likelihood of cardiac catheterization following acute myocardial infarction22
and a decreased likelihood of receiving an implantable defibrillator among patients admitted to hospital because of heart failure.23
In the United States, among older individuals with Medicare, low socioeconomic status has been associated with late presentation with acute myocardial infarction.24
It is unclear why the disparity between socioeconomic status and incidence of sudden cardiac arrest in our study was somewhat more pronounced in the US sites combined than in the Canadian sites combined, although the difference was relatively small in magnitude, and the 95% CIs overlapped. Income inequality was similar across US and Canadian sites. The population prevalence of established heart disease and history of myocardial infarction appear to be similar in the two countries.25,26
Measures of socioeconomic status at the census-tract level may reflect individual or neighbourhood socioeconomic status less accurately in Canada than in the United States; this may have resulted in an underestimation of true associations between socioeconomic status and sudden cardiac arrest in Canada. However, census tracts were similarly defined in each country, and median household income was computed in a similar manner.
Another possible explanation for the weaker association between socioeconomic status and incidence of sudden cardiac arrest in Canada may be the access to universal health care. In the United States, uninsured people less than 65 may have more undiagnosed cardiovascular disease or they may not seek care to manage risk factors for cardiovascular disease or to manage established heart disease. Such underdiagnosis or undertreatment could increase risk for sudden cardiac arrest. Our results showing a younger mean age of sudden cardiac arrest in the United States may be consistent with more poorly controlled cardiovascular disease. Previously published US data have shown socioeconomic disparities in the control of risk factors for cardiovascular disease and a decrease in differences after eligibility for Medicare at age 65.14
Patients of lower socioeconomic status in the United States have also been shown to delay seeking hospital care for acute myocardial infarction.15
A substantial proportion of patients who have sudden cardiac arrest have symptoms such as angina, dyspnea, nausea and syncope for minutes to hours before the arrest.27
A delay in calling for emergency medical services could result in a higher incidence of sudden cardiac arrest in the poorer, uninsured population.
Other data have shown Canadians to be more likely than their US counterparts to seek preventive care.28
It is difficult from our current study and others to tease out the potential effects of health care systems, access to care and components of socioeconomic status on individual risk behaviour. Studies in other countries with near-universal access to health care have reported socioeconomic disparities in the incidence of sudden cardiac arrest in the United Kingdom12
and in cardiovascular-related mortality in Israel.29
Finally, we observed higher absolute rates of sudden cardiac arrest among people less than 65 years old in Pittsburgh than in the other sites. Although the registry data do not include health status information at the census tract level, data from the 2009 Behavioral Risk Factor Surveillance Study indicated that the burden of cardiovascular disease may be unusually high in Pittsburgh: the age-adjusted rate of death from coronary artery disease per 100 000 was 153.5 in Allegheny County (Pittsburgh), compared with 141.6 in Tarrant County in Texas (near Dallas), 110.6 in Seattle–King County and 99.3 in Oregon.30
Strengths and limitations
Among the strengths of our study are its large size, and the availability of data from seven diverse sites in the United States and Canada collected using consistent methodology. A standard definition of out-of-hospital sudden cardiac arrest was used across the study sites, as was an unbiased estimate of socioeconomic status based on high-quality census data. We restricted our analysis to sites with high levels of ascertainment of out-of-hospital cardiac arrests and to census tracts with likely complete reporting of arrests, which limited bias related to incomplete data.
Our study has several limitations. First, we may have introduced bias by including only arrests that occurred at home or at a residential institution. If people of lower socioeconomic status were more or less likely to have an arrest at home, this could affect the study’s generalizability. However, because more than 80% of sudden cardiac arrests occur in a private residence,3
we were able to include most arrests.
Second, although the vast majority of sudden cardiac arrests (> 90%) are due to a primary cardiac cause,31
a small minority of the arrests included in our study may have been due to noncardiac causes such as pulmonary embolism or aortic rupture.
Third, median household income at the census-tract level may be a more accurate measure of socioeconomic status for younger than for older individuals. Some of the observed differences in the association between socioeconomic status and incidence of sudden cardiac arrest by age group may have been due to differences in how well socioeconomic status was measured for each age group. The use of only census-level measures of socioeconomic status is a potential limitation. Direct inference regarding the effects of individual- or neighbourhood-level socioeconomic status on individual risk of cardiac arrest may not be possible. However, measures of individual and census-tract socioeconomic status have produced similar estimates of association with various health outcomes in other studies.32
The incidence of sudden cardiac arrest at home or at a residential institution was greater in lower- than in higher-income neighbourhoods in the US and Canadian sites studied, although the association was attenuated in Canada. The disparity across socioeconomic quartiles was greatest among people younger than 65. Population-based interventions to reduce cardiovascular risk factors and treat unrecognized cardiovascular disease in lower-income communities may prove to be effective in reducing disparities in the incidence of sudden cardiac arrest. Also, effective targeting of training for cardiopulmonary resuscitation in lower-income communities with the highest incidence of sudden cardiac arrest may improve survival. Placement of automated external defibrillators in lower-income communities may also help, although only for arrests that occur outside the home.