Heart pacemaker. Coloured frontal X-ray of the chest of a 64-year-old patient with a heart pacemaker (upper right). The heart is at lower centre. Cardiac diseases can make the heart need to pump harder. As it is a muscle, the heart grows if it has an increased workload. The pacemaker has leads to supply regular electrical impulses to the heart so that it beats normally. 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. Pacemakers may provide a regular impulse or discharge only when a heartbeat is missed.