title: A comprehensive investigation of cardiac arrest before and after arrival of emergency medical services.
abstract: OBJECTIVE
  Many of the factors that affect survival from out-of-hospital cardiac arrest are not relevant in patients who arrest after arrival of emergency medical services (EMS). Because all arrests that occur after arrival of EMS are witnessed and care is immediate, one might expect survival to be very high. Several studies have described communities' experiences of arrest after arrival but few have compared survival rates stratified by rhythm and witness status. The purpose of this paper was to describe the characteristics of patients who arrested after arrival of EMS and to compare survival in this population to those who had witnessed and unwitnessed arrests before EMS arrival.
  
  
  METHODS
  We conducted a retrospective cohort study in King County, WA, USA. Descriptive statistics were calculated in patients whose arrests were not witnessed, in patients whose arrests were witnessed by citizens, and in those whose arrests were witnessed by EMS personnel.
  
  
  RESULTS
  The majority of bystander- and EMS-witnessed arrests were initially in ventricular fibrillation (VF), but EMS-witnessed arrests were more likely to initially have been in pulseless electrical activity (PEA) than bystander-witnessed events. Patients whose arrests were witnessed by EMS had the greatest likelihood of survival compared to patients whose arrests were not witnessed or were witnessed by bystanders. Patients whose arrests were witnessed by EMS and were initially in VF had the highest rates of survival (59%).
  
  
  CONCLUSIONS
  Patients whose arrests were witnessed by EMS were more likely to have survived their cardiac arrests than those who arrested before EMS arrived. We suggest that survival rates from VF arrests that occur after EMS arrival should be widely reported in order to measure overall EMS performance since many factors such as response times, bystander actions, and witness status are equalized in this subset of patients.
authors.affiliation: University of Washington School of Public Health and Community Medicine, Department of Epidemiology, Seattle, WA, USA. goldl@u.washington.edu
authors.forename: Laura S
authors.initials: LS
authors.lastname: Gold
authors.forename: Mickey S
authors.initials: MS
authors.lastname: Eisenberg
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D004632:Emergency Medical Services; D004634:Emergency Medical Technicians; D005260:Female; D006323:Heart Arrest / Q000401:mortality / Q000503:physiopathology*; D006801:Humans; D016013:Likelihood Functions; D008297:Male; D008875:Middle Aged; D011674:Pulse; D012189:Retrospective Studies; D015996:Survival Rate; D014693:Ventricular Fibrillation / Q000503:physiopathology