Heart pacemaker. Coloured chest X-ray showing a surgically-implanted heart pacemaker (red, centre right). The pacemaker supplies electrical impulses along leads (running to lower centre) to the heart to keep it beating regularly. It is fitted in patients with a malfunctioning sinoatrial node, the part of the heart that initiates each heartbeat, or those with a heart block impairing the nerve impulses generated by the node. Heart pacemakers may provide a regular impulse (fixed rate pacemaker) or discharge only when a heartbeat is missed (demand pacemaker).