GLOSSARY
- Atrioventricular Delay
- The time that a dual chamber system waits after detecting (or initiating)
an atrial event before pacing the ventricle. Some generators shorten this time as
heart rate increases (termed "rate adaptive AV delay" or "dynamic AV delay"). Some
generators can be programmed to extend the AV delay to search for intrinsic conduction
("search AV delay"). Some generators will prolong an AV delay after any atrial event
where the last ventricular event was intrinsic ("AV delay hysteresis"). In a patient
with a conducting AV node, the sensed AV delay will be slightly longer than the PR
interval on the surface electrocardiogram (see "Fusion Beat" and "Pseudofusion Beat"),
since the ventricular sensing element is attached to the apex of the right ventricle
and detects the depolarization only after right ventricular activation (typically
greater than 60 msec after activation of the AV node).
- Automatic Rate
- See Lower Rate Limit.
- Bipolar Lead
- An electrode with two conductors. Bipolar pacing typically uses less energy
than unipolar pacing, and it will produce smaller artifacts on analogue monitors.
Bipolar sensing is more resistant to oversensing from muscle artifact or stray electromagnetic
fields. Most pacing generators can be programmed to unipolar mode (separate settings
for pacing and sensing) even in the presence of bipolar electrodes.
- Dynamic Atrial Overdrive (DAO)
- A programmable rate enhancement for pacemakers that increases the pacing
rate in response to the presence of intrinsic atrial activity. DAO is designed to
pace the atrium at a rate just above the intrinsic rate in order to prevent atrial
fibrillation.[89]
It should be disabled prior to
an anesthetic to prevent high pacing rates. It is also called AF (for atrial fibrillation)
suppression.
- Electrogram Storage Mode (EGM Mode)
- Passive acquisition and internal storage of electrocardiographic data for
diagnostic purposes while pacing (or monitoring) with programmed parameters. A pacemaker
programmed to enter the EGM mode upon application of a magnet will not demonstrate
asynchronous pacing behavior.
- Fusion Beat (FB)
- A pacemaker impulse delivered shortly before a native depolarization of
the ventricle which alters the morphology of the QRS, often misdiagnosed as undersensing.
In a FB, the pacing stimulus is delivered after the activation of the AV node, but
before the sensing element detects the pacemaker-induced depolarization. It is similar
to a pre-excitation of the ventricle seen in Lown-Ganong-Levine syndrome or Wolff-Parkinson-White
syndrome. Confirmation of appropriate sensing behavior can be made by lengthening
the sensing interval (i.e., lengthening the AV delay). Fusion beats suggest, but
should not be used to confirm, ventricular capture. See also Pseudofusion Beat.
- Generator
- The device with a power source and circuitry to produce an electrical impulse
designed to be conducted to the heart. Typically, pacing generators are placed in
a pectoral pocket, and leads are inserted into the right atrium, right ventricle,
or both. Since 1995, implantable cardioverter-defibrillators (ICDs) have also been
approved for pectoral pocket placement.
- Hysteresis
- If present, the amount by which the patient's intrinsic rate must fall below
the programmed rate before the generator begins pacing. Some pacers periodically
decrease the pacing rate in order to search for resumption of intrinsic activity
(called "Search Hysteresis"). These functions, when present, can mimic pacemaker
malfunction.
- Implantable Cardioverter-Defibrillator (ICD) Mode
- The designation of chambers shocked, chambers paced for antitachycardia
pacing, method of tachycardia detection, and chambers paced for antibradycardia therapy.
Table 35-7
shows the NASPE/BPEG
generic ICD code.
- Lower Rate Limit (LRL, also Automatic Rate or Programmed Rate)
- The lowest sustained regular rate at which the generator will pace. Typically,
the device begins pacing when the patient's intrinsic rate falls below this value.
Hysteresis settings can permit an intrinsic heart rate lower than the LRL.
- Oversensing
- Detection of undesired signals that are interpreted as cardiac activity.
Oversensing can lead to pacemaker driven tachycardia (pacing device, DDD mode with
atrial oversensing and ventricular tracking); ventricular pause (pacing device with
electrosurgically induced ventricular oversensing, leading the pacer to "detect"
ventricular activity), or inappropriate shock (defibrillator, event oversensing).
- Pacing Mode
- The designation of chambers paced, chambers sensed, sensing response, rate
responsiveness, and multisite function for a pacemaker system. Table
35-1
shows the NASPE/BPEG generic pacemaker code.
- Programmed Rate
- See Lower Rate Limit.
- Pseudofusion Beat (PFB)
- A pacemaker impulse delivered shortly after a native depolarization without
alteration of the QRS morphology. PFBs are often misdiagnosed as undersensing, and
they result from the position of the sensing electrode relative to the depolarizing
wavefront (see Fusion Beat). Confirmation of appropriate sensing behavior can be
made by lengthening the sensing interval (i.e., decreasing the program rate [atrial
FB] or lengthening the AV delay [ventricular PFB]). PFB cannot be used to confirm
electronic capture.
- Rate Enhancements
- The features such as rate adaptive AV delay (shortens the AV delay with
increasing heart rate); AV search hysteresis (lengthens/shortens the AV delay to
produce intrinsic AV conduction); AF supression (also called Dynamic Atrial Overdrive—increases
the lower rate upon appearance of native atrial depolarization so as to create nearly
constant atrial pacing but at a rate only slightly higher than the patient's intrinsic
rate); rate smoothing (limits changes in paced rates due to changes in intrinsic
activity, rising and falling rate limits can be programmed); Sleep Rate (see below);
ventricular rate regulation (similar to rate smoothing but used to prevent atrial
fibrillation); and hysteresis (see above). Each of these enhancements can produce
pacing/nonpacing that can mimic pacemaker dysfunction, and these enhancements should
be programmed "OFF" prior to any anesthetic.
- Rate Modulation
- The ability of the generator to sense the need to increase heart rate in
response to patient activity. Mechanisms include a mechanical sensor in the generator
to detect motion or vibration; electronic detection of QT interval (shortens during
exercise); transthoracic impedance to measure changes in respiration; and sensors
for central venous blood temperature or oxygen saturation (see Table
35-2
). Some generators now incorporate multiple sensors. The pacing rate
determined by the rate modulation algorithm is called "Sensor Indicated Rate."
- Sensor Indicated Rate
- The pacing rate determined by the sensor in a pacemaker programmed to a
rate-responsive mode (fourth character of NBG = "R").
- Sleep Rate (also Circadian Rate)
- The rate (lower than the Programmed Rate) at which the pacing generator
will pace during programmed "nighttime" hours.
- Undersensing
- Failure to detect a desired event.
- Unipolar Lead
- An electrode with only one conductor. Some devices with bipolar leads are
programmed to the unipolar lead mode. Systems with unipolar leads produce larger
spikes on the electrocardiogram than bipolar leads. Systems with unipolar leads
utilize the generator case as the second conductor.
- Upper Sensor Rate (USR, also called Upper Activity Rate [UAR])
- The maximum rate to which a rate modulated pacemaker can drive the heart.
USR in a dual chamber mode is not affected by UTR, since, when USR becomes active,
the pacemaker is pacing the atrium.
- Upper Tracking Rate (UTR, also called Upper Rate Limit [URL])
- Pacemakers programmed to VDD or DDD mode cause the ventricles to track atrial
activity. Should a patient develop an atrial tachyarrhythmia, such as a supraventricular
tachycardia, atrial fibrillation, or atrial flutter, the generator acts to limit
ventricular pacing. When the atrial rate exceeds the UTR, the generator can change
mode (i.e., switch to DDI) or introduce second degree A-V block. Second degree blocks
can be Mobitz type I (Wenckebach) or Mobitz type II, depending on a variety of programmed
settings within the pacemaker.
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