How pacemakers work How pacemakers work. Animated explanation of the mechanics of the human heart, and the devices that can assist it Implantable pacemakers and defibrillators are devices that apply electric shocks to maintain the rhythm of the heart and, if necessary, restart it. As the technology improves and the list of treatable conditions grows, the number of devices being implanted is increasing steadily and now exceeds half a million a year. The heart is made up of four chambers; two atri