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Pacemaker

A system that sends electrical impulses to the heart in order to set the heart rhythm. The pacemaker can be the normal "natural" pacemaker of the heart or it can be an electronic device. The natural pacemaker of the heart is the sinus node, one of the major elements in the cardiac conduction system, the system that controls the heart rate. This stunningly designed system generates electrical impulses and conducts them throughout the muscle of the heart, stimulating the heart to contract and pump blood. The sinus node consists of a cluster of cells that are situated in the upper part of the wall of the right atrium (the right upper chamber of the heart). The electrical impulses are generated there. The sinus node is also called the sinoatrial node or, for short, the SA node. The electrical signal generated by the sinus node moves from cell to cell down through the heart until it reaches the atrioventricular node (AV node), a cluster of cells situated in the center of the heart between the atria and ventricles. The AV node serves as a gate that slows the electrical current before the signal is permitted to pass down through to the ventricles. This delay ensures that the atria have a chance to fully contract before the ventricles are stimulated. After passing the AV node, the electrical current travels to the ventricles along special fibers embedded in the walls of the lower part of the heart. Although there are different types of artificial pacemakers, all are designed to treat bradycardia, a heart rate that is too slow. Some pacemakers 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 an overly long pause between

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1. Biologically, any rhythmic center that establishes a pace of activity. 2. An artificial regulator of rate activity. 3. In chemistry, the substance of which the rate of reaction sets the pace for a series of chain reactions; the rate-limiting reaction itself; e.g., in a metabolic pathway, the enzyme catalyzing the slowest or rate-limiting reaction in that pathway. [L. passus, step, pace]

- artificial p. any device that substitutes for the normal p. and controls the rhythm of the organ; especially an electronic cardiac p., which may be implanted in the chest, with electrodes attached to the external cardiac surface, or passed through the venous circulation into the right side of the heart (pervenous p.).

- demand p. a form of artificial p. usually implanted into cardiac tissue because its output of electrical stimuli can be inhibited by endogenous cardiac electrical activity.

- diaphragmatic p. a device that paces the diaphragm, used in patients with chronic ventilatory insufficiency resulting from quadriplegia or certain types of phrenic nerve malfunction.

- ectopic p. any p. other than the sinus node.

- electric cardiac p. an electric device that can substitute for the normal cardiac p., controlling the heart's rhythm by artificial electric discharges. SYN: electronic p..

- electronic p. SYN: electric cardiac p..

- external p. an artificial cardiac p. of which the electrodes for delivering rhythmic electric stimuli to the heart are placed on the chest wall. SYN: transthoracic p..

- fixed-rate p. an artificial p. that emits electrical stimuli at a constant frequency.

- nuclear p. a nuclear-powered unit used to generate the electrical current for artificially pacing the heart; replaced by units using long-life nickel-cadmium and other power sources.

- pervenous p. an artificial p. passed through the venous circulation into the right side of the heart.

- runaway p. rapid heart rates over 140/min caused by electronic circuit instability in an implanted pulse generator.

- shifting p. SYN: wandering p..

- subsidiary atrial p. secondary source for rhythmic control of the heart, available for controlling cardiac activity if the sinoatrial p. fails; usually located within the crista terminalis and atrial free wall near the inferior vena cava.

- transthoracic p. SYN: external p..

- wandering p. a disturbance of the normal cardiac rhythm in which the site of the controlling p. shifts from beat to beat, usually between the sinus and AV nodes, often with gradual sequential changes in P waves between upright and inverted in a given ECG lead. SYN: shifting p..

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pace·mak·er 'pā-.smā-kər n

1) a group of cells or a body part (as the sinoatrial node of the heart) that serves to establish and maintain a rhythmic activity

2) an electrical device for stimulating or steadying the heartbeat or reestablishing the rhythm of an arrested heart called also pacer

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1. a device used to produce and maintain a normal heart rate in patients who have heart block. The unit consists of a battery that stimulates the heart through an insulated electrode wire attached to the surface of the ventricle (epicardial pacemaker) or lying in contact with the lining of the heart (endocardial pacemaker). A pacemaker may be used as a temporary measure with an external battery or it may be permanent, when the whole apparatus is surgically implanted under the skin. Some pacemakers stimulate the heart at a fixed rate; others sense when the natural heart rate falls below a predetermined value and then stimulate the heart (demand pacemaker).

2. the part of the heart that regulates the rate at which it beats: the sinoatrial node.

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pace·mak·er (pāsґma-kər) 1. an object or substance that influences the rate at which a certain phenomenon occurs. 2. the natural cardiac pacemaker or an artificial cardiac pacemaker. 3. in biochemistry, a substance whose rate of reaction sets the pace for a series of interrelated reactions.

Medical dictionary. 2011.