The changing incidence of ventricular fibrillation in Milwaukee, Wisconsin (1992-2002).

OBJECTIVES: To investigate the changes in annual incidence and survival of out-of-hospital cardiac-etiology arrests of different initial rhythms, particularly ventricular fibrillation (VF) and ventricular tachycardia (VT), among adults (> 21 years old) in Milwaukee County between 1992 and 2002 and establish correlations with patient and emergency medical services (EMS) system-dependent factors.

METHODS: The study was a retrospective, observational study of all adult (> 21-year-old) patients with out-of-hospital cardiac-etiology arrests with identifiable rhythm and resuscitation attempted by the Milwaukee County EMS system from 1992 to 2002. Nine thousand one hundred seventy cases were enrolled. Primary outcome measures were changes in annual incidence of initial cardiac arrest rhythm, with a focus on VF/VT. Secondary outcome measures were changes in survival to hospital admission and hospital discharge for VF and VT. Patient and EMS system factors potentially affecting the outcome measures were identified and modeled using multivariate logistic regression.

RESULTS: The incidence of out-of-hospital VF/VT arrests decreased steadily from 37.1 per 100,000 in 1992 to 19.4 per 100,000 in 2002. While the incidences of pulseless electrical activity and overall cardiac arrest remained unchanged, the incidence of asystole during the study period increased from 27.3/100,000 to 44.9/100,000. Multivariate regression analyses revealed that age < 80 years, male gender, white race, previous cardiac surgery, and cardiac history were patient-dependent factors predictive of VF/VT. Witnessed arrest, public location, and shorter response time were EMS system-dependent factors predictive of VF/VT. Based on observed trends, none of these correlated factors could explain the decrease in the incidence of VF/VT arrests. Rates of patient survival to hospital admission and discharge were not significantly changed over time. EMS system factors predictive of survival to admission and discharge were witnessed arrest, public location, and decreased number of defibrillations. Prior cardiac surgery and absence of chronic problems were the only patient factors predictive of survival to hospital admission but were not significantly related to survival to hospital discharge.

CONCLUSIONS: The incidence of out-of-hospital cardiac arrests in adult patients with presenting rhythm of VF/VT declined, while an increase in asystole occurred. This was not explained by any patient or EMS system-dependent factor. Rate of survival for VF/VT arrest did not significantly change over time. Survival was primarily influenced by EMS system factors and unrelated to patient-dependent factors.