MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Heart pacemaker. Coloured chest X-ray showing a surgically-implanted heart pacemaker (yellow, centre left). The two lungs (multicoloured, centre left and right) and the heart (light green, lower right) are also seen. The pacemaker supplies electrical impulses along a lead (not clearly seen) to the heart to keep it beating regularly. Pacemakers are fitted in patients with a malfunct- ioning 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).
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