Cardiac arrest (CA) is defined as the sudden cessation of effective cardiac pumping function as a result of either ventricular asystole (inactivity of the heart) or pulseless ventricular tachycardia/ventricular fibrillation (VT/VF). Pulseless ventricular tachycardia/ventricular fibrillation is an abnormal electrical activity of the ventricles of the heart. Rapid diagnosis and treatment are essential because first, more than a few minutes of total CA results in permanent damage to the brain due to lack of oxygen, and second, the success of resuscitative measures is related to the rapidity with which they are instituted following arrest. For a person in VT/VF the probability of successful defibrillation and subsequent survival to hospital discharge is directly and negatively related to the interval between onset of VT/VF and delivery of first shock.