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Heart pacemaker. Coloured frontal X-ray of the chest of a male patient with a heart pacemaker (upper right). The heart is the blue mass at centre right, in between the lungs (white). The pacemaker is a device that has leads (running from upper right to lower centre) to supply regular electrical impulses to the heart, so that it beats normally. Pacemakers are surgically 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 can either provide a regular impulse or discharge only when a heartbeat is missed. (KEYSTONE/SCIENCE PHOTO LIBRARY/GUSTOIMAGES/SCIENCE PHOTO LIBRARY)