Emergency Medical Services/Original ResearchOptimal defibrillation response intervals for maximum out-of-hospital cardiac arrest survival rates*,**,★,*
Introduction
Forty years ago, in the original description of closed-chest cardiopulmonary resuscitation (CPR), Kouwenhoven et al1 emphasized the importance of timely defibrillation: “External defibrillation is not likely to be followed by the return of spontaneous heart action, unless the counter-shock is applied in less than three minutes after the onset of ventricular fibrillation.” Since then, the role of rapid defibrillation in the survival of victims of out-of-hospital cardiac arrest has endured the test of a battery of retrospective analyses,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and recently, it has been validated as an independent predictor of survival in a large, prospective clinical trial.16, 17 Many systems have adopted an 8-minute standard as a targeted defibrillation response interval of “call received by dispatch” to “arrival at scene by responder with defibrillator” in at least 90% of cardiac arrest cases. However, there is little evidence to support 8 minutes as a suitable, let alone optimal, criterion.
The objective of the current investigation is to analyze survival as a function of time to test the evidence for the 8-minute standard. This investigation is a substudy of the ongoing Ontario Prehospital Advanced Life Support (OPALS) study.18 The OPALS study is the largest out-of-hospital investigation yet conducted. With 21 communities and more than 10,000 patients with cardiac arrest, this study provides a unique opportunity to further elucidate the cardiac arrest picture and to generate hypotheses for future study. The study uses a before-and-after clinical trial design to test the incremental benefit of the addition of rapid defibrillation and advanced life support to an existing basic life support system. Within this framework, we conducted an observational cohort study to examine the relationship between response intervals and cardiac arrest survival. The data cover the time before the implementation of advanced life support in the OPALS study.
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Materials and methods
The OPALS study involves 21 Ontario urban-suburban study communities that have been identified within the response areas of 11 emergency medical services (EMS) base hospital programs. According to 1991 census data, the combined population served in these 21 centers is 2,815,000. Study community base populations range from 16,000 to 750,000. Ambulance services in these centers respond to more than 120,000 code 4 (highest priority) calls each year. In the study sites, EMS responds to
Results
From January 1, 1991, to December 31, 1997, there were a total of 9,969 cardiac arrest cases in the 21 OPALS study communities. The 696 (7.0%) cases that were EMS witnessed were excluded from this analysis.
Table 1 identifies the patient and EMS characteristics of the remaining 9,273 patients that are the subject of this study.Characteristic Patients (N=9,273) Mean age, y (SD) (n=9,134) 68.3 (13.8) Male sex, No. ( %) (n=9,267) 6,269 (67.7) Bystander-witnessed
Discussion
Pursuing a reduction in the out-of-hospital defibrillation response interval is a key optimization strategy for improving cardiac arrest survival within a community. However, our data indicate that the 8-minute target currently adopted by many EMS systems does not represent sufficient optimization. Rather, it is clear that significant survival gains can be attributed to decreasing the defibrillation response interval well below 8 minutes. The 8-minute mark is not accompanied by any dramatic
Acknowledgements
We thank the OPALS Study Group investigators from the following base hospital programs: Burlington, Ontario, Canada: Matthew W. Stempien, MD, CCFP(EM), Carrie I. Parkinson, RN, BScN; Cambridge, Ontario, Canada: David Waldbillig, MD, CCFP(EM), Kieran W. Ballah, EMCA; Kingston, Ontario, Canada: Gordon J. Jones, MD, FRCPC, Mark R. Halliday, EMCA; London, Ontario, Canada: Jonathan F. Dreyer, MDCM, FRCPC, Kenneth A. Boyle, EMCA, EMT; Niagara, Ontario, Canada: Douglas P. Munkley, MD, MCFP(EM),
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Dr. Stiell holds a Distinguished Investigator Award from the Canadian Institutes of Health Research.
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Supported by peer-reviewed grants from the Emergency Health Services Branch of the Ontario Ministry of Health and Long-Term Care and the Canadian Health Services Research Foundation.
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Reprints not available from the authors.
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Author contributions: VDM, IS, and GW conceived the study and designed the study. IS and GW obtained research funding. VDM, IS, and GW supervised the conduct of the study and data collection. VDM and IS managed the data. VDM, GW, and IS provided statistical advice on study design and analyzed the data. VDM drafted the manuscript, and all authors contributed substantially to its revision. VDM takes responsibility for the paper as a whole.