Page 682 - Cardiac Nursing

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DISPLAY 28-2 Single Chamber Pacing Terminology
Asynchronous (fixed rate) pacing: The pacemaker releases a pacing stimulus at the programmed rate regardless of the
heart’s intrinsic activity. No sensing occurs so the pacemaker fires in competition with the heart’s natural rhythm. Exam-
ples of asynchronous modes are AOO, VOO, and DOO.
Automatic interval: The time period between two consecutive paced events without an intervening sensed event. Also
l
known as the basic interval or pacing interval.
l
Base rate: The rate at which the pacemaker paces when no intrinsic cardiac activity is present. Also known as the minimum
rate or lower rate.
Bipolar: Having two poles. (1) A pacing lead with two electrical poles. The negative pole is the distal tip of the lead and the
positive pole is a metal ring located a few millimeters proximal to the distal tip. The stimulating pulse is delivered through
the distal tip electrode. (2) A pacing system with both electrical poles in or on the heart.
Capture: Ability of the pacing stimulus to depolarize the chamber being paced. Capture is recognized on the ECG whenever
the pacing spike is followed immediately by the appropriate waveform: an atrial spike followed by a P wave or a ventricu-
lar spike followed by a wide QRS.
Demand Pacing: The pacemaker only paces when the heart’s intrinsic rate is below the pacemakers programmed rate (only
when necessary or on demand). This mode means that the pacemaker senses intrinsic cardiac activity and inhibits its
output when intrinsic activity is present.
Electrode: The exposed metal tip of a pacing lead that contacts myocardium and directly transmits the pacing stimulus to
cardiac tissue.
Electromagnetic interference: Electrical signals from the environment (i.e., radiofrequency waves) which can be sensed by
the pacemaker and interfere with pacer function.
Escape interval: The period of time between a sensed cardiac event and the next pacemaker output. The escape interval is
usually equal to the basic pacing rate but it can be programmed longer in some pacemakers (hysteresis).
Fusion beat: A cardiac depolarization (either atrial or ventricular) that results from two foci both contributing to depolariza-
tion of the chamber. In pacing, a fusion beat results when an intrinsic depolarization and a pacing stimulus occur simulta-
neously and both contribute to depolarization (usually seen in the ventricle).
Hysteresis: A programmable feature in some pacemakers that allows the escape interval to be programmed longer than the
basic pacing interval (the pacing interval following a sensed beat is longer than the basic pacing interval). This allows
more time for the heart’s intrinsic activity to occur.
Inhibited response: A type of response to sensing that inhibits pacemaker output when an intrinsic beat is sensed. This re-
sults in demand pacing, or pacing only when the heart’s intrinsic activity is slower than the basic pacing rate.
Lead: The insulated wire and its electrode that transmits the pacing stimulus from the pulse generator to the heart and re-
lays sensed intrinsic activity back to the pulse generator. A single-chamber pacemaker uses one lead and a dual-chamber
pacemaker usually uses two leads, one in the atrium and one in the ventricle.
Magnet mode: A term used for the pacemaker’s response when a magnet is placed over the pulse generator. A magnet inac-
tivates the sensing circuitry and causes a pacemaker to function asynchronously at a predetermined rate and in a preset
manner. The magnet mode differs among manufacturers in pacing rate and number of impulses delivered with the magnet
in place. A change is magnet-induced pacing rate is often an indicator of battery depletion and warrants pulse generator
replacement.
Myopotential: An electrical signal generated by muscle movement. Myopotentials are sometimes sensed by the pacemaker
and cause inhibition of pacemaker output.
Output: The electrical stimulus delivered by the pulse generator, usually defined in terms of pulse amplitude (V volts) and
pulse width (ms milliseconds).
Oversensing: Detection of inappropriate electrical signals by the pacemaker’s sensing circuit, resulting in inappropriate inhi-
bition of pacer output. Sources of oversensing can include electromagnetic interference, myopotentials, T waves, or
crosstalk between atrial and ventricular channels in dual-chamber pacemakers.
Pacemaker syndrome: Adverse clinical signs and symptoms due to inadequate timing of atrial and ventricular contraction.
The syndrome can be due to loss of AV synchrony in VVI pacing, inappropriate AV interval in dual-chamber pacing, or
inappropriate rate modulation. Symptoms include fatigue, confusion, unpleasant pulsations in neck or chest, limited
exercise capacity, CHF, hypotension, syncope, or near syncope.
Pacing interval: The time between two consecutive paced events without an intervening sensed event. Measured in
milliseconds (ms). AA interval, atrial pacing interval; VV interval, ventricular pacing interval.
Pacer spike: Term used to describe the small vertical “blip” recorded on the ECG with every pacemaker output pulse. The
presence of a pacer spike indicates that a stimulus was released by the pacemaker.
Pseudofusion beat: An electrocardiographic phenomenon resulting from delivery of a pacemaker spike into an intrinsic
event. In the ventricle, it appears as a pacer spike in an intrinsic QRS complex, but since the ventricle is already depolar-
ized the spike is ineffective but may distort the QRS complex on the ECG.
Pulse generator: The device that contains the power source (battery) and the electronic circuits that control pacemaker
function. The term “pacemaker” is commonly used for the pulse generator.
Rate modulation: The ability of a pacemaker to increase the pacing rate in response to physical activity or metabolic
demand. The pacemaker uses some type of physiologic sensor to determine the need for increased pacing rate. The most
commonly used sensors at the present time are motion sensors and minute ventilation sensors. It is also known as rate
adaptation or rate response.
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