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pacemaker (pas´ma-ker)

1. Biologically, any rhythmic center that establishes a pace of activity; 2. An artificial regulator of rate activity. 3. In chemistry, the substance whose 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 malfunction of the respiratory control center on 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.electronic p;
electronic p. electric cardiac p
external p. an artificial cardiac p. whose electrodes for delivering rhythmical electrical stimuli to the heart are placed on the chest wall.
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. wandering p
subsidiary atrial p. secondary source for rhythmic control of the heart, available for controlling cardiac activity if the sinoatrial pacemaker fails; located within the crista terminalis and atrial free wall near the inferior vena cava.
transthoracic p. artificial p. delivering stimuli through the chest wall usually applied as a temporizing measure in patients with atrioventricular block.
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 A-V nodes, often with gradual sequential changes in P waves between upright and inverted in a given ECG lead.shifting p;


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