AAI pacemaker    atrial demand inhibited p.

AAIR pacemaker  an atrial demand inhibited pacemaker that is responsive to the patient's respiratory rate and thus to exercise and metabolic needs.

AAT pacemaker    atrial demand triggered p.

antitachycardia pacemaker  an implanted pacemaker that terminates tachycardia by delivering one or more pacing stimuli; it may require arrhythmia detection and device activation by the patient or may detect arrhythmias and deliver stimuli automatically.

AOO pacemaker    atrial asynchronous p.

artificial pacemaker  , artificial cardiac pacemaker  a device that uses electrical impulses to reproduce or regulate the rhythms of the heart. Battery-driven and connected to the heart by leads and electrodes, it may be temporary or permanent and is inserted transvenously, transcutaneously, epicardially, or via the esophagus or coronary artery. Most pacemakers are either triggered or inhibited to modify output by sensing the intracardiac potential of one or more cardiac chambers; they have some degree of programmability and may also have antitachycardia functions. A five letter code is used to categorize pacemakers by their combinations of these features; see table. Popularly called pacemaker.

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Pacemaker lead (arrow) terminating in the right ventricle

asynchronous pacemaker  an artificial cardiac pacemaker that delivers stimuli at a fixed rate, independent of any atrial or ventricular activity.

asynchronous atrial pacemaker    atrial asynchronous p.

asynchronous ventricular pacemaker    ventricular asynchronous p.

atrial asynchronous pacemaker  an artificial cardiac pacemaker that stimulates the atrium at a constant rate, without sensing atrial or ventricular activity; now rarely used except to initiate or terminate some tachycardias. Called also   AOO p.

atrial demand inhibited pacemaker  an artificial cardiac pacemaker that delivers stimuli to the atrium at a fixed rate in the absence of sensed atrial activity; spontaneous cardiac activity causes inhibition of pacemaker output, termination of the current stimulation cycle, and initiation of a new cycle. Called also   AAI p.

atrial demand triggered pacemaker  an artificial cardiac pacemaker that delivers stimuli to the atrium at a fixed rate in the absence of sensed atrial activity; spontaneous cardiac activity triggers pacemaker output, which falls ineffectively in the myocardial refractory period and initiates a new pacemaker stimulation cycle. Called also   AAT p.

atrial synchronous ventricular pacemaker  a dual chamber cardiac pacemaker that senses atrial activity and delivers a ventricular stimulus after a preset interval; used in patients with impaired atrioventricular conduction but normal sinus node function. Called also   VAT p. See also   atrial synchronous ventricular inhibited p.

atrial synchronous ventricular inhibited pacemaker  a dual chamber cardiac pacemaker similar to an atrial synchronous ventricular pacemaker but able to sense ventricular as well as atrial activity. Called also   VDD p.

atrioventricular junctional pacemaker  an ectopic pacemaker occurring in the atrioventricular junction.

atrioventricular (AV) sequential pacemaker  an artificial cardiac pacemaker used in patients with abnormal sinus node function and impaired AV conduction; it senses ventricular, and sometimes atrial, activity and in addition to stimulating the atrium also stimulates the ventricle after an appropriate delay. Committed versions invariably deliver the ventricular stimulus while noncommitted versions deliver it only in the absence of interim ventricular activity. See also   DVI p.,   DDD p., and   DDI p.

automatic pacemaker    universal p.

bipolar pacemaker  an implanted cardiac pacemaker in which the lead contains both electrodes, anode and cathode, and is thus a complete circuit.

cardiac pacemaker  the group of cells rhythmically initiating the heartbeat, characterized physiologically by a slow loss of membrane potential during diastole. Usually the pacemaker site is the sinoatrial node. See also artificial cardiac p.

cilium pacemaker  the biological regulator which controls the frequency of the beat of the cilia of cells by determining the rate of contraction and excitation.

DDD pacemaker  an artificial cardiac pacemaker that can sense and pace both the atria and ventricles; it is capable of operating in both triggered and inhibited modes as necessary.

DDDR pacemaker  a universal pacemaker that is responsive to the patient's respiratory rate and thus to exercise and metabolic needs.

DDI pacemaker  a type of atrioventricular sequential pacemaker that delivers impulses to the atrium and ventricles on the basis of sensed ventricular and atrial activity, although the latter can only inhibit atrial impulses and cannot trigger ventricular stimulation. Cf.   DVI p. and   DDD p.

demand pacemaker  an implanted cardiac pacemaker in which the generator stimulus is inhibited for a set interval (refractory period) by a signal derived from depolarization (normal or ectopic), thus minimizing the risk of pacemaker-induced ventricular fibrillation.

diaphragmatic pacemaker    electrophrenic p.

dual chamber pacemaker  a pacemaker having two leads, one in the atrium and one in the ventricle, so that electromechanical synchrony between the chambers can be approximated.

DVI pacemaker  a type of atrioventricular sequential pacemaker that delivers impulses to the atrium and ventricle on the basis of sensed ventricular activity only. Cf.   DDI p. and   DDD p.

ectopic pacemaker  any biological cardiac pacemaker other than the   sinoatrial node; under normal conditions it is not active.

electronic pacemaker    artificial p.

electrophrenic pacemaker  the device used in   electrophrenic respiration; it converts radiofrequency signals into electrical impulses that stimulate the phrenic nerve, resulting in descent and flattening of the diaphragm and improved inhalation of air. Called also   diaphragmatic or   phrenic p.

escape pacemaker  an   ectopic pacemaker that assumes control of cardiac impulse propagation because of failure of the sinoatrial node to generate one or more normal impulses.

external pacemaker  an artificial cardiac pacemaker located outside the body with output wires connected to circular chest electrodes, with a wire sewn directly into the heart, or with an electrode inserted through an intravenous catheter.

fixed-rate pacemaker  an artificial cardiac pacemaker set to pace at only a single rate.

fully automatic pacemaker    universal p.

gastric pacemaker  a saddle-shaped area of the greater curvature of the stomach at the junction of its proximal and middle thirds, where electric potentials originate that regulate the frequency of gastric contractions.

pacemaker of heart    cardiac p.

implantable pacemaker  , implanted pacemaker  an artificial cardiac pacemaker completely implanted into the subcutaneous tissue.

junctional pacemaker    atrioventricular junctional p.

latent pacemaker    ectopic p.

phrenic pacemaker    electrophrenic p.

radiofrequency pacemaker  a cardiac pacemaker consisting of an antenna coil on the skin and a subcutaneously implanted receiving coil with an electrode inserted into the ventricular myocardium. Pulses from a lightweight radio transmitter carried by the patient are transmitted to the pacemaker.

rate responsive pacemaker  an artificial cardiac pacemaker that can deliver stimuli at a rate adjustable to some parameter independent of atrial activity, such as respiratory rate, physical activity level, blood temperature, or mixed venous oxygen saturation level.

runaway pacemaker  a malfunctioning artificial cardiac pacemaker that abruptly accelerates its pacing rate, resulting in pacemaker-induced ventricular tachycardia.

secondary pacemaker    ectopic p.

single chamber pacemaker  an implanted cardiac pacemaker having only one lead, which is placed in either the atrium or the ventricle.

synchronous pacemaker  an implanted cardiac pacemaker that synchronizes the electromechanical events in the atrium with those of the ventricle by delivering stimuli in response to sensed activity in the atrium, ventricle, or both.

transthoracic pacemaker  an external cardiac pacemaker delivering stimuli through the chest wall, such as one connected to the heart by percutaneous pacing wires introduced through a transthoracic needle or one in which large electrodes are placed on the skin over the heart.

transvenous pacemaker  an artificial cardiac pacemaker, either external or implantable, that is connected to the heart by pacing leads passed through the venous circulation.

unipolar pacemaker  an implanted cardiac pacemaker in which the lead has a single stimulating electrode, the cathode, with the anode connected to an indifferent electrode, usually the outer surface of the pulse generator.

universal pacemaker  a term sometimes used to describe a   DDD pacemaker, emphasizing that because it can be programmed to operate in one of numerous possible pacemaker modes under specific circumstances, it can be made to most closely approximate normal electrophysiologic functioning under a variety of conditions.

VAT pacemaker    atrial synchronous ventricular p.

VDD pacemaker    atrial synchronous ventricular inhibited p.

ventricular pacemaker  an ectopic pacemaker occurring in a ventricle.

ventricular asynchronous pacemaker  an artificial cardiac pacemaker that stimulates the ventricle at a constant rate, without sensing atrial or ventricular activity; now rarely used except to initiate or terminate some tachycardias. Called also   VOO p.

ventricular demand inhibited pacemaker  an artificial cardiac pacemaker that delivers stimuli to the ventricle at a fixed rate in the absence of sensed ventricular activity; spontaneous cardiac activity causes inhibition of pacemaker output, termination of the current stimulation cycle, and initiation of a new cycle. Called also   VVI p.

ventricular demand triggered pacemaker  an artificial cardiac pacemaker that delivers stimuli to the ventricle at a fixed rate in the absence of sensed ventricular activity; spontaneous cardiac activity triggers pacemaker output, which falls ineffectively in the myocardial refractory period and initiates a new cycle of pacemaker stimulation. Called also   VVT p.

VOO pacemaker    ventricular asynchronous p.

VVI pacemaker    ventricular demand inhibited p.

VVIR pacemaker  a ventricular demand inhibited pacemaker that is responsive to the patient's respiratory rate and thus to exercise and metabolic needs.

VVT pacemaker    ventricular demand triggered p.

wandering atrial pacemaker  a condition in which the site of origin of the impulses controlling the heart rate shifts from one point to another within the atria, including the sinus node, changing with almost every beat. P waves and PR intervals vary, and the rate of impulse formation is somewhat irregular. It occurs when the rate of sinus impulses falls below a critical level or fails.