MODEL RELEASED. Ambulanceman using a portable defibrillator machine in an attempt to revive a man who has suffered a heart attack. A defibrillator delivers a controlled electric shock to restore normal heart rhythm in cases of cardiac arrest due to ventricular fibrillation (a rapid and chaotic beating of the many individual muscle fibres of the heart). The transparent sheets visible under the defibrillator electrodes are saline pads, used to ensure good conductivity. The two circular electrodes are connected to an electrocardiograph which is monitoring the electrical activity of the heart.