Case definition in survival studies of out-of-hospital cardiac arrest.

The purpose of this investigation was to determine problems with case definition and selection biases in studies of survival from out-of-hospital cardiac arrest, by comparing characteristics of subjects with cardiac arrest who entered the emergency medical services (EMS) system and those who did not enter the system. Data for 143 prehospital cardiac arrest patients in Johnson County, Iowa, were obtained from death certificates and EMS reports. Approximately one half of cardiac arrest patients entered the EMS system. Mean total number of causes of death listed on death certificates was significantly higher in subjects who did not enter the EMS system. Several factors, including age, sex, and number of causes of death listed on death certificates were significant univariate factors in whether a cardiac arrest victim entered the EMS system, but multivariate logistic regression indicated age by itself was the most significant factor. These results indicate there are possible initial biases determining who will enter the EMS system, which affects the generalizability of previous studies.