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Heart pacemaker. Coloured chest X-ray showing a surgically-implanted heart pacemaker (yellow oval, upper left). The two lungs (large black areas) and the heart (red, centre) are also seen. The pacemaker supplies electrical impulses along a lead (white) to the heart to keep it beating regularly. Pacemakers are fitted in patients with a malfunctioning sinoatrial node, the part of the heart which initiates each heartbeat, or 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). (KEYSTONE/SCIENCE PHOTO LIBRARY/MEHAU KULYK)