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A cardiac pacemaker is a medical device that generates electrical impulses delivered by electrodes to cause the heart muscle chambers to contract and therefore pump blood. By doing so, this device replaces and/or regulates the function of the electrical conduction system of the heart.
The primary purpose of a pacemaker is to maintain an adequate heart rate, either because the heart's natural pacemaker is not fast enough, or because there is a block in the heart's electrical conduction system. Modern pacemakers are externally programmable and allow a cardiologist, particularly a cardiac electrophysiologist, to select the optimal pacing modes for individual patients. Modern devices are demand pacemakers, in which the stimulation of the heart is based on the dynamic demand of the circulatory system.
A specific type of pacemaker called an implantable cardioverter-defibrillator combines pacemaker and defibrillator functions in a single implantable device, which should be called a defibrillator, for clarity. Others, called biventricular pacemakers, have multiple electrodes stimulating differing positions within the lower heart chambers to improve synchronization of the ventricles, the lower chambers of the heart.