a resuscitation system that includes at least two defibrillation electrodes configured to be applied to the exterior of the chest of a patient for delivering a defibrillation shock , a source of one or more ecg signals from the patient , a defibrillation circuit for delivering a defibrillation shock to the defibrillation electrodes , a control box that receives and processes the ecg signals to determine whether a defibrillation shock should be delivered or whether cpr should be performed , and that issues instructions to the user either to deliver a defibrillation shock or to perform cpr , wherein the determination of whether cpr should be performed and the instructions to perform cpr can occur at substantially any point during a rescue .