title: Cardiac arrest witnessed by emergency medical services personnel: descriptive epidemiology, prodromal symptoms, and predictors of survival. OPALS study group.
abstract: OBJECTIVE
  The Utstein guidelines recommend that emergency medical services (EMS)-witnessed cardiac arrests be considered separately from other out-of-hospital cardiac arrest cases. The objective of this study was to assess EMS-witnessed cardiac arrest and to determine predictors of survival in this group.
  
  
  METHODS
  This prospective cohort included all adults with an EMS-witnessed cardiac arrest in the 21 communities of the Ontario Prehospital Advanced Life Support (OPALS) study. Systems provided a basic life support with defibrillation (BLS-D) level of care. Case and survival definitions followed the Utstein style. Descriptive and univariate methods (chi(2) and t test) were used to characterize EMS-witnessed cardiac arrest. Multivariate logistic regression was undertaken to assess predictors of survival to hospital discharge.
  
  
  RESULTS
  From January 1, 1991, to December 31, 1996, there were 9,072 cardiac arrest cases in the study communities. Of these, 610 (6.7%) were EMS-witnessed. The majority had preexisting cardiac or respiratory disease (81.5%) and experienced prodromal symptoms before EMS personnel arrived (91.4%). An initial rhythm of pulseless electrical activity was present in 50.1% of the patients, ventricular fibrillation/ventricular tachycardia in 34.2%, and asystole in 15.7%. Survival to discharge was 12.6%. Multivariate analysis identified the following as independent predictors of survival (odds ratio with 95% confidence intervals [CIs]): nitroglycerin use before EMS arrival: 2.3 (95% CI 1.2 to 4.5), prodromal symptoms of chest pain: 2.5 (95% CI 1.4 to 4.5) or dyspnea: 0.5 (95% CI 0.3 to 1.0), and unconsciousness on EMS arrival: 0.5 (95% CI 0.2 to 0.9). Patients with chest pain were more likely than dyspneic patients to experience ventricular fibrillation/ventricular tachycardia (62% versus 17%, P<.0001), and were 5 times more likely to survive (30.6% versus 6.3%, P<.0001).
  
  
  CONCLUSIONS
  EMS-witnessed cases are clearly an important subset of out-of-hospital cardiac arrest and are characterized by 2 distinct symptom groups: chest pain and dyspnea. These symptoms are important predictors of survival and may also help determine underlying mechanisms before patient collapse. A later phase of the OPALS study will prospectively evaluate the impact of out-of-hospital advanced life support on the survival of victims of EMS-witnessed cardiac arrest.
authors.affiliation: Department of Epidemiology and Community Medicine, Division of Emergency Medicine, Department of Medicine, and Ottawa Hospital Loeb Health Research Institute, University of Ottawa, Ottawa, Ontario, Canada.
authors.forename: V J
authors.initials: VJ
authors.lastname: De Maio
authors.forename: I G
authors.initials: IG
authors.lastname: Stiell
authors.forename: G A
authors.initials: GA
authors.lastname: Wells
authors.forename: D W
authors.initials: DW
authors.lastname: Spaite
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000552:Ambulances; D015331:Cohort Studies; D016001:Confidence Intervals; D004554:Electric Countershock; D004634:Emergency Medical Technicians; D005260:Female; D006323:Heart Arrest / Q000453:epidemiology* / Q000401:mortality / Q000628:therapy; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D005996:Nitroglycerin / Q000627:therapeutic use; D016017:Odds Ratio; D011379:Prognosis; D011446:Prospective Studies; D012151:Resuscitation; D012307:Risk Factors; D016019:Survival Analysis; D017180:Tachycardia, Ventricular / Q000401:mortality; D013997:Time Factors; D014474:Unconsciousness; D014665:Vasodilator Agents / Q000627:therapeutic use; D014693:Ventricular Fibrillation / Q000401:mortality
chemical_list: D014665:Vasodilator Agents; D005996:Nitroglycerin