Pacemaker, artificial: A device that uses electrical impulses to regulate the heart rhythm or to reproduce that rhythm. An internal pacemaker is one in which the electrodes into the heart, the electronic circuitry and the power supply are implanted (internally) within the body.

Although there are different types of pacemakers, all are designed to treat bradycardia, a heart rate that is too slow). Pacemakers may function continuously and stimulate the heart at a fixed rate or at an increased rate during exercise. A pacemaker can also be programmed to detect too long a pause between heartbeats and then stimulate the heart.

History: The implantable pacemaker was invented by Wilson Greatbatch in 1958. While building an oscillator to record heart sounds, he installed a resistor with the wrong resistance in the unit. It began to give off a steady electrical pulse. Greatbatch realized that the device could be used to regulate the heart and hand-crafted the world's first implantable pacemaker. Greatbatch later invented the corrosion-free lithium battery to power the pacemaker.

The first person to receive an internal pacemaker was Arne H. W. Larsson, an engineer in Sweden. A viral infection damaged the normal electrical circuitry in the heart, causing bradycardia, decreased blood flow to the brain, and recurrent Stokes-Adams attacks fainting spells). Dr. Ake Senning, a heart surgeon, and Dr. Rune Elmquist, an electronics engineer, put a device together about the size of "a thin hockey puck." Dr. Senning implanted it in Mr. Larsson's chest on Oct. 8, 1958. It and succeeding pacemakers permitted Mr. Larsson "to fly around the world supervising the repair of electrical systems aboard ships." He died in late 2001 at age 86 of melanoma, according to his obituary in The New York Times.