Determining the variables that influence survival in the treatment of cardiac arrest is important as both a research and a quality assurance tool. Out-of-hospital cardiac arrest remains the only medical condition in which it has been determined rigorously that prehospital care affects survival. Thus, it uniquely is suited to outcome-based research and quality assurance studies. After approximately 20 years of study, the determinants of survival after cardiac arrest are well-known; however, a plethora of fascinating research questions remain to be addressed.

The prevalence of out-of-hospital cardiac arrest seems to be decreasing. In the Seattle area, there are 30% fewer cases than there were ten years ago. This parallels the overall decline in mortality from ischemic heart disease in the United States in the last thirty years. There also has been a change in the profile of cardiac arrest victims, with today's arrestee several years older than the victim of fifteen years ago. Thus, when one performs comparative clinical trials, the control group of today is very different from the historical control of ten or fifteen years ago.