Among the 1941 eligible patients, approximately 70% had arrests with a cardiac cause, less than half the arrests were witnessed, and nearly a third had a shockable rhythm. The average EMS response time from dispatch to arrival at the scene was 6.5 minutes. Patients, circumstance, EMS response, and presenting rhythm characteristics were similar in the two groups (Table 1Table 1Characteristics of the Patients According to Dispatcher's CPR Instructions.). Patients randomly assigned to instructions for the bystander to perform chest compression alone were more likely to undergo bystander-performed chest compression (80.5% vs. 72.7%, P