Evidence has suggested that the pathophysiology of ventricular fibrillation cardiac arrest may consist of 3 time-sensitive phases: electrical, circulatory, and metabolic. We performed a retrospective cohort study of adults in a metropolitan county who had had witnessed ventricular fibrillation arrest before emergency medical services were undertaken to investigate this 3-phase model with regard to bystander cardiopulmonary resuscitation (CPR). We hypothesized that the survival benefit from bystander CPR depends on the collapse-to-shock interval, with the highest benefit occurring during the circulatory phase. The collapse-to-shock interval was a priori grouped into 4 categories: 1 to 5, 6 to 7, 8 to 10, and > or = 11 minutes. We used logistic regression analysis to assess whether the association between CPR and survival to hospital discharge depended on the collapse-to-shock interval category. Of the 2,193 events meeting the inclusion criteria, 67.0% had received bystander CPR. The average collapse-to-shock interval was 8.2 +/- 2.8 minutes. The survival rate was 33.4%. A higher likelihood of survival was associated with bystander CPR (odds ratio [OR] 1.41, 95% confidence interval [CI] 1.15 to 1.73) and a shorter collapse-to-shock interval (OR -1.84, 95% CI 1.62 to 2.10, for each additional SD of 2.8 minutes less) after adjustment. The beneficial association of CPR increased as the collapse-to-shock interval increased (p = 0.05 for interaction). The bystander CPR was associated with an OR of survival of 0.96 (95% CI 0.64 to 1.46) for a 1- to 5-minute collapse-to shock interval, OR of 1.25 (95% CI 1.00 to 1.58) for a 6- to 7-minute interval, OR of 1.62 (95% CI 1.25 to 2.11) for an 8- to 10-minute interval, and OR of 2.11 (95% CI 1.32 to 3.37) for an > or = 11-minute interval. The results of this investigation support a phased model of ventricular fibrillation arrest. The findings suggest that the transition from the electrical to circulatory phase may occur at about 5 minutes, and the circulatory phase may extend to 15 minutes.