A

A-V Interval — In a dual chamber pacemaker, the programmed period of time between an atrial event (sensed or paced) and a paced ventricular event.

A wave. (1) An atrial paced event. (2) The atrial stimulus or the point in the intrinsic atrial depolarization (P wave) at which atrial sensing occurs.

AA interval. The interval between two consecutive atrial stimuli, with or without an interceding ventricular event. See also demand interval.

AAI. The NBG code for single-chamber atrial inhibited pacing.

AAIR. The NBG code for single chamber atrial inhibited pacing with rate modulation.

AAT. The NBG code for single-chamber atrial synchronous pacing.

AATR. The NBG code for single chamber atrial synchronous pacing with rate modulation.

Absolute refractory period. (1) Physiology. Within cardiac tissue, the period of time directly following depolarization, during which a stimulus, regardless of strength, cannot initiate a new depolarization. This period of time constitutes the period for tissue repolarization. The refractory period of ventricular tissue starts at the beginning of the QRS complex and ends at approximately the peak of the T wave. The refractory period for atrial tissue starts at the beginning of the P wave and ends approximately 200 ms later. (2) Devices. For a pacemaker, the time period directly following a sensed or paced event during which all activity is ignored by the sense amplifier. This period is immediately followed by the relative refractory period. The total refractory period (absolute plus relative) is typically an independently programmable value for the atrial and ventricular channels (in dual-chamber pacemakers).

Accessory connection (or pathway). A rapid conduction pathway connecting the atrium and ventricle which totally or partially bypasses the AV node. Conduction through accessory pathways can interfere with normal conduction and cause pre-excitation of myocardial tissue. Accessory pathways may provide anterograde or retrograde conduction of impulses and frequently are the anatomical basis for reentrant arrhythmias.

Action potential. The changes in electrical potential generated by the muscle cell membrane or nerve cell tissue in response to intrinsic or extrinsic stimulation. There are five phases: phase O is the period of rapid depolarization (polarity changes from negative to positive) and phases 1 through 4 return the cell to resting membrane potential .

Active fixation lead. A pacing lead with an attachment mechanism, such as a screw-in tip, to actively secure it in the cardiac tissue at implant. See also passive fixation lead.acute. Of short duration. In pacing, the acute phase occurs immediately post-implant. Acute thresholds are thresholds taken during the first weeks after implantation of a new lead.

Afterpotential. The residual charge present at the electrode/myocardial interface immediately upon completion of a pacing stimulus. It is usually due to polarization effects.

Alert period. The portion of the pulse generator's timing cycle during which the pulse generator can sense and respond to intrinsic cardiac activity. The alert period follows the refractory period and ends with the next sensed or paced event. If the device is programmed to an inhibited or triggered mode, intrinsic activity sensed during the alert period will inhibit or trigger the pulse generator, respectively. Note that in dual-chamber devices, the atrial and ventricular sense amplifiers have separate alert periods and the ventricular alert period in such devices includes a portion of the AV delay.

Ampere. The basic unit of electric current, abbreviated A. In pacing, the main unit of current is expressed in milliamperes or thousandths of an ampere, abbreviated mA.

Ampere-hour. The basic unit of battery capacity. One ampere-hour indicates that one ampere of electrical current can be drawn from a battery for one hour before depletion. Pulse generator batteries generally hold between 0.8 to 2.5 ampere-hours. Abbreviated A-h.

Amplitude. (1)The height of the output pulse as expressed in volts or milliamperes. Also known as pulse amplitude. (2) The height of the R wave or P wave as expressed in millivolts.

Anode. (1)The positive pole. (2) In unipolar pacing systems, the anode is the pulse generator case; in bipolar systems, the anode is the proximal ring electrode on the pacing lead. It may also refer to the positive terminal on the pulse generator connector block or the positive terminal of a battery.

Anterograde conduction. Forward conveyance of the electrical impulse. This is normal, healthy conduction proceeding from the SA node, to the atria, through the AV junction, and finally to the ventricles. Sometimes called antegrade conduction. See also retrograde conduction. AGO. The NBG code for single chamber atrial asynchronous pacing.

AOOR. The NBG code for single chamber atrial asynchronous pacing with rate modulation.

Apex. The pointed tip of the heart formed by the right and left ventricles.

AR. One of the four pacing states in DDD and DDDR pacing, starting with an atrial paced event and ending with a ventricular sensed event. See also pacing state.

AR interval. The time period, stated in milliseconds, beginning with an atrial pacing stimulus and ending with the onset of the QRS complex.

Arrhythmia. In pacing, any rhythm disturbance. Examples include bradycardia, tachycardia, any markedly irregular rhythm, block or the presence of premature contractions. Also known as dysrhythmia.

Artifact. (1)An ECG recording malfunction that resembles an arrhythmia or pacing system problem. (2) In a pacemaker EGG, the spike or small recorded mark representing the pulse generator's electrical output. Sometimes spelled artefact. See also pacing stimulus.

Asynchronous. A type of pacing in which output pulses are delivered at a fixed rate with no modification due to sensing. Examples of asynchronous modes are AOO, AOOR, VOO, VOOR, DOO, and DOOR. Also known as fixed rate pacing.

Asystole. The complete absence of ventricular contractions in the heart. See also cardiac standstill.

Atrial asystole. The complete absence of atrial contractions. Sometimes called atrial standstill.

Atrial contribution. The augmented filling of the ventricles caused by the contraction of the atria during the final phase of ventricular diastole and immediately prior to ventricular contraction. The atrial contribution may contribute 20 percent or more to cardiac output. Sometimes called atrial kick.

Atrial escape interval. The period in a dual-chamber pacemaker's timing cycle initiated by a ventricular sensed or paced event and ending with the next atrial paced event. Sometimes called atrial output escape interval or VA interval. Note that in single chamber atrial pacemakers, the atrial escape interval is the interval from a sensed atrial event to the next atrial output pulse. For single-chamber pacemakers, see escape interval.

Atrial J lead. A J-shaped lead intended for placement in the right atrial appendage. The lead is kept straight by a straight guide wire until it reaches the right atrium. When the guide wire is removed, the lead regains its J shape and the electrode can be positioned in the right atrial appendage. Atrial J leads are available with either active or passive fixation mechanisms.

Atrial lead. Any pacing lead designed for use in the atrium; it may be endocardial or epicardial.

Atrial refractory period. The atrial timing cycle during which the atrial sense amplifier is unresponsive to signals. In some pacemakers, the atrial refractory period is divided into an absolute atrial refractory period, during which all input signals are ignored, and the relative (or noise-sampling) atrial refractory period, during which there is some monitoring for extraneous signals (noise). In single-chamber atrial applications, the atrial refractory period begins with an atrial sensed or paced event. In dual-chamber modes, the total atrial refractory period is composed of two segments, namely the AV interval and the post-ventricular atrial refractory period.

Atrial tracking. A pacemaker response in which an atrial sensed event is followed by a properly timed ventricular output pulse. Atrial tracking maintains AV synchrony for optimal cardiac output. Examples of modes with atrial tracking are DDD, DDDR, VDD and VDDR.

Atrial Tracking. A pacing mode in which the ventricles are paced in synchrony with sensed atrial events.

Auto Sensor Threshold. An automatic threshold feature for rate-modulated Pacesetter pulse generators to select automatically the appropriate activity sensor threshold for each patient, based on the running average of the sensor signal over the previous 18 hours. The automatic activity threshold value may be reset using the programmer and PDXTM software When the [Reset Auto Threshold] button is pressed on the programmer, the pulse generator accumulates the average sensor signal over the next 30 pacing cycles. This then becomes the new automatic activity threshold value which is then continuously updated as a running average. Sometimes called automatic activity sensor threshold or auto threshold old.

Auto Set. A feature of Pacesetter rate modulated pulse generators which facilitates programming the slope parameter of the sensor-based system. See also slope. automatic interval. The time period between two consecutive paced events without an intervening sensed event. It is expressed in milliseconds (AA interval, VV interval). Also known as basic interval, demand interval or pacing interval.

Automaticity. (1) Physiology. The ability of tissue to spontaneously generate an electrical impulse and propagate an action potential. Automaticity is an inherent property of the normal SA node. (2) Devices. The ability of a device to recognize the need for a change in values and to make these corresponding adjustments automatically. An example of device automaticity is an increase in pacing rate based on sensor input.

Autonomic nervous system. The nervous system regulating tissues and functions not normally under conscious control, such as heartbeat and blood pressure. It is divided into the sympathetic and parasympathetic nervous systems which have opposite effects on the cardiovascular system. When stimulated, the sympathetic nerves increase heart rate and myocardial contractility, constrict blood vessels and raise blood pressure, whereas the parasympathetic nerves decrease heart rate, relax blood vessels and lower blood pressure. See also sympathetic nervous system and parasympathetic nervous system.

AV. (1) Abbreviation for atrioventricular or atrial-ventricular. (2) One of four pacing states in DDD and DDDR pacing, starting with an atrial paced event and ending with a ventricular paced event. See also pacing state.

AV block. A partial or total interruption of the conduction of electrical impulses from the atria to the ventricles. AV block may delay or completely prevent normal conduction. AV block can occur in the atrium, the AV node, the bundle of His or the Purkinje system. AV delay. In dual-chamber pacing, the length of time (usually programmable) between an atrial sensed or atrial paced event and the delivery of a ventricular output pulse. An AV delay may be terminated if an intrinsic ventricular event is sensed before the AV delay times out. The AV delay is measured in milliseconds. Also known as AV interval.

AVdissociation. A condition in which the atria and ventricles beat independently of each other and without any synchronization of rhythms. Sometimes called atrioventricular dissociation. See also AV synchrony.

AV interval. In dual-chamber pacing, the length of time (usually programmable) between an atrial sensed or atrial paced event and the delivery of a ventricular output pulse. An AV interval may be shortened if an intrinsic ventricular event is sensed before the AV intervals times out. The AV interval is measured in milliseconds. Also known as AV delay.

AV interval extension. A prolongation of the AV interval caused by delaying the ventricular output pulse until the maximum tracking rate times out. This occurs only in dual-chamber systems in which there is a ventricular maximum tracking rate and the intrinsic atrial rate exceeds the programmed value of the maximum tracking rate.

AV nodal reentrant tachycardia. A type of rapid atrial arrhythmia characterized by periods of sudden-onset supraventricular tachycardia disrupting the normal (sinus) rhythm.. The sudden onset of the tachycardia is caused by reentry within the AV node

AV node. A collection of specialized cardiac cells located in the right atrium between the coronary sinus and the tricuspid valve's septal cusp which form a portion of the heart's electrical conduction system. In conducting the electrical impulse from the atria to the ventricles, the AV node introduces a delay that allows the atrial contraction to occur first, thus augmenting the filling of the relaxed ventricles. It also acts as a physiologic gate to prevent the ventricles from contracting too rapidly in response to pathologic atrial tachyarrhythmias.

AV synchrony. The condition of the healthy heart in which the atria and ventricles beat in such a way that their individual depolarizations are coordinated to allow atrial contraction to augment the filling of the ventricles. Sometimes called atrioventricular synchrony. See also AV dissociation.