Pacemaker comparison. At left is a Chardack-Greatbatch pacemaker, which was invented in 1960. It is one of the earliest implantable pacemakers. It included a mercury battery and an epoxy resin case. At right is a 21st Century REPLY DR pacemaker. Pacemakers are fitted in patients with a malfunctioning sinoatrial node, the part of the heart that initiates the heartbeat, or in patients with a heart block, which impairs the nerve impulses generated by the node. A pacemaker supplies electrical impulses along wires, to the heart to keep it beating regularly. The main body of the pacemaker is implanted in the chest just underneath the skin and the ends of the wires are implanted in the heart muscle.