DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
Pacemaker. A heart pacemaker implanted under the skin below the collarbone of an elderly man. The large shape is the battery unit. Above this is the wire that runs to the heart. The two white marks on the skin are surgical scars made during implan- tation. Pacemakers supply electrical impulses to the heart to maintain a regular heartbeat. They are needed by patients with heart block, a condi- tion in which the heart's natural pacemaker (sino- atrial node) fails to work properly as a result of heart disease. The two main types of pacemakers are fixed rate pacemakers, which work constantly at a fixed rate, and demand pacemakers, which only discharge when the heart slows or misses a beat.
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