Cardiac arrest witnessed by prehospital personnel: Intersystem variation in initial rhythm as a basis for a proposed extension of the Utstein recommendations

Study objective: To test the hypothesis that intersystem variation in initial rhythm among EMS-witnessed arrests is of sufficient magnitude to warrant standardization of survival by creation of an Utstein-style denominator of EMS-witnessed ventricular fibrillation (VF). Methods: We conducted a planned subset analysis of a prospective observational cohort study of consecutive EMS-witnessed adult cardiac arrests occurring in New York City and meeting Utstein entry criteria. The primary outcome measure was intersystem variation in frequency of EMS-witnessed VF in New York City compared with that in other EMS systems. Secondary outcome measures were variations in survival after EMS-witnessed VF arrests and overall survival after all EMS-witnessed arrests. Results: Intersystem variation showed a threefold difference in the frequency of EMS-witnessed VF (24% in New York City versus 77% in Scotland; 99% confidence interval [CI] for 53% difference, 43% to 63%; P<10(-7)), a twofold difference in survival after EMS-witnessed VF (25% in NYC versus 48% in King County, WA; 99% CI for 23% difference, 6% to 39%; P<.002), and a fourfold difference in survival after all EMS-witnessed arrests (9% in New York City versus 35% in King County; 99% CI for 26% difference, 18% to 34%; P<10(-7)). Conclusion: The marked variation in frequency of initial rhythm in EMS-witnessed arrests suggests that a modified Utstein denominator of EMS-witnessed VF would facilitate more uniform intersystem comparison of survival in this unique cohort. However, even after adjustment for initial rhythm, large residual intersystem survival differences remain unexplained.