Heart pacemaker. Frontal X-ray of the chest of a patient with a heart pacemaker (upper right). The heart, which is enlarged, is at centre to lower right. 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 supplies electrical impulses, along a lead, to the heart to keep it beating regularly. 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.