Timing is vital. According to the American Heart Association, reducing the interval between cardiac arrest and delivery of a shock by even a couple of seconds will increase an individual's chance of survival. for each minute that passes prior to defibrillation, the probabilities for survival reduce 7-10%. After ten minutes, survival is unlikely.
CPR alone cannot totally resuscitate an individual in SCA. Most SCA victims are in a ventricular fibrillation arrhythmia (VF), an electrical malfunction of the heart that causes it to twitch without coordinated beats. Defibrillation, the delivery of a shock to the heart, will restore traditional heart operation if it happens within minutes of SCA onset.