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Heart pacemaker. Coloured X-ray of the chest of a patient, showing a fitted heart pacemaker. This electronic battery-run device is seen above the ribcage with a green lead which connects it to the heart. The heart lies at lower right, taking up part of one lung field. A pacemaker supplies electrical impulses to the heart to maintain the heartbeat at a regular rate. It may be external (worn on a belt) or internal (implanted in the chest). They are fitted in patients suffering a heart block disrupting heart rhythm. Heart pacemakers may provide a fixed rate impulse or discharge only when a heartbeat is missed. (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)