A small device that supplies electrical impulses to the heart to maintain a regular heartbeat. A pacemaker is implanted when the sinoatrial node in the heart malfunctions, or when the passage of the electrical impulses that stimulate heart contractions is impaired (see heart block; sick sinus syndrome). Pacemakers can be fixed-rate (which discharge impulses at a steady rate) or demand (which discharge only when the heart rate slows or a beat is missed). They may be external (used as a temporary measure) or internal (implanted in the chest). Some types can increase the heart rate during exercise or change an abnormal rhythm into a normal one.


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