Incidence of cardiac arrest: a neglected factor in evaluating survival rates

To add to our understanding of survival rates in out-of-hospital cardiac arrest studies, we examined the incidence of cardiac arrest in the published literature. We specifically estimated if incidence rates are uniform between communities and if any relationship exists between incidence and the reported survival rates.

A retrospective study of nearly 100 cardiac arrest peer-reviewed articles from 1970 to 1989 was performed to identify reports that included rates for incidence and survival or provided sufficient data for the calculation of these rates.

We were able to obtain reported or calculated incidence and survival rates for 20 communities. Statistical analysis was performed to compare incidence rates between communities and examine the relationship across these 20 studies between incidence rates and reported survival rates. Incidence rates ranged significantly from 35.7 to 128.3 per 100,000, with a mean of 62. Survival rates ranged significantly from 1.6% to 20.7%. Incidence rates in these communities were negatively related to survival rates; that is, as the incidence rate increased, the survival rate decreased. We determined the regression curve that describes this inverse relationship. This nomogram can be used to identify survival/incidence rate combinations that are significantly above or below average.

The marked variations in incidence and inverse relationship between incidence and survival could be due to true variation in risk among the populations reported (ie, some populations may be older or sicker than others). Also, different research methodologies may create artifactual differences among studies as standards for designing studies, terminology, and reporting data have not been uniform. Therefore, these findings may reflect methodological differences and true epidemiological differences among communities. Future reports should include a method, such as an incidence/survival nomogram, to analyze survival rates while taking into account the community incidence rate of cardiac arrest. Further analysis of incidence and survival is necessary to improve intersystem comparisons, a prerequisite to sound decisions about cardiac arrest treatment, health policy, and allocation of resources.