Page 683 - Cardiac Nursing

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C HAPTER 2 8 / Pacemakers and Implantable Defibrillators 659
DISPLAY 28-2 Single Chamber Pacing Terminology (continued)
Refractory period: (1) In the heart, the period of time during which the myocardium is incapable of responding to a stimu-
lus. (2) In the pacemaker, an interval or timing cycle following a sensed or paced event during which the pacemaker will
not respond to incoming signals. A single-chamber pacemaker has one refractory period, a dual-chamber pacemaker has
an atrial refractory and a ventricular refractory period.
Sensing: The ability of the pacemaker to recognize and respond to intrinsic cardiac depolarization.
Sensing threshold: The smallest intrinsic atrial or ventricular signal (measured in mV) that can be consistently sensed by
the pacemaker.
Stimulation threshold: The minimum amount of voltage necessary to capture the heart consistently. It is also known as
capture threshold or pacing threshold.
Undersensing: Failure of a pacemaker to sense intrinsic cardiac depolarizations. This can result in competition between the
pacemaker and the intrinsic rhythm.
Unipolar: Having one pole. (1) A unipolar lead has only one pole, located at the distal tip. (2) A pacing system with one pole
in or on the heart and the second pole located remote from the heart to complete the circuit. Permanent unipolar
systems utilize the back of the pulse generator as the second pole. Temporary epicardial pacing systems utilize a ground
wire in subcutaneous tissue as the second pole.
DISPLAY 28-3 Dual Chamber Pacemaker Terminology

Adaptive AV delay (or rate adaptive AV delay): See AV interval.
Alert period: The portion of the pulse generator’s timing cycle during which it can sense and respond to intrinsic cardiac
activity. The alert period follows the refractory period.
Atrial escape interval: Period of time form a sensed or paced ventricular event to the next paced atrial event. Also known
as the V–A interval.
Atrial refractory period: Period of time during which the atrial channel is unable to respond to sensed signals. In dual-
chamber pacemakers, the total atrial refractory period is divided into two parts: the AV interval and the postventricular
atrial refractory period (PVARP).
Atrial tracking: A state of pacing in which sensed atrial activity triggers a ventricular pacing output at the end of the
programmed AV delay. Also known simply as tracking.
AV interval (or AV delay): The “electronic PR interval,” or the length of time between a sensed or paced atrial event and the
delivery of the ventricular pacing output. The AV interval is programmable and is measured in milliseconds (e.g., an AV in-
terval of 120 ms a PR interval of 0.12 second). Many pacemakers have an “adaptive AV delay,” meaning that the AV de-
lay can be programmed to shorten when the intrinsic atrial rate increases, thus mimicking the heart’s own physiological
increase in AV conduction as heart rate increases. Many devices also have a “differential AV delay,” meaning that the AV
interval can be programmed to be longer on an atrial paced beat than on an atrial sensed beat (e.g., 200 ms when the
atrium is paced and 150 ms when P waves are sensed).
Blanking period: A very short ventricular refractory period that occurs simultaneously with every atrial pacing output to
prevent the ventricle from sensing the atrial stimulus. It is intended to prevent inhibition of ventricular output due to
crosstalk (see definition below). Many pacemakers allow the blanking period to be programmed longer to prevent
crosstalk.
Crosstalk: The sensing of a signal in one chamber by the sensing circuit in the other chamber, usually used in reference to
the sensing of the atrial output pulse by the ventricular channel. Crosstalk due to sensing of atrial signals by the ventricu-
lar channel causes inhibition of ventricular pacing output because the ventricular channel thinks that the atrial output is
a ventricular event.
Differential AV delay: See AV interval.
Endless loop tachycardia: See Pacemaker mediated tachycardia.
Maximum tracking rate (MTR): The programmable upper rate limit of a dual-chamber pacemaker that determines the
fastest rate at which 1:1 tracking of atrial-sensed events will occur. The MTR prevents the ventricular channel from pacing
faster than the upper rate limit when the intrinsic atrial rate exceeds the programmed MTR. When the intrinsic atrial rate
is faster than the upper rate limit, the pacemaker reverts to its “upper rate response” to prevent the ventricular rate from
exceeding the MTR. Also known as the ventricular tracking limit or upper rate limit.
Mode switching: Ability of a dual-chamber pacemaker to switch from an atrial tracking mode (e.g., DDD) to a nontracking
mode (e.g., DDI or VVI) when rapid atrial impulses are sensed by the atrial channel. This prevents the pacemaker from
pacing the ventricle rapidly and erratically when atrial fibrillation or flutter occur.
Noncompetitive atrial pace (NCAP): A feature in some dual-chamber pacemakers that delays the delivery of the next atrial
output when an atrial signal is sensed in the atrial channel’s refractory period (e.g., a PAC that occurs in PVARP delays the
delivery of the next atrial pacing output). This prevents the delivery of an atrial output during the atrial refractory period
in an attempt to prevent induction of atrial fibrillation.
Pacemaker mediated tachycardia: A tachycardia induced by competition between the pacemaker and the intrinsic rhythm
and sustained by the continued participation of the pacemaker. Most commonly used to describe the endless loop tachy-

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