Biomedical Engineering Reference
In-Depth Information
8
Pacing Systems
A pacing system is defined as a medical device used for
treatment of bradycardia or heart failure; it comprises the
device itself and implanted leads. The pacemaker function is
also incorporated into implantable defibrillators and biven-
tricular pacemakers or defibrillators, which are, in a sense,
considered to be superior devices. Most of the special func-
tions of these devices will be dealt with in individual sec-
tions. Figure 8.1 shows a general block diagram of an
implantable dual-chamber pacemaker. Up-to-date systems
are controlled by a microprocessor, often with automatic
gain control of input amplifiers.
biosignals (myopotentials). In this sensing configuration, the
pacemaker may generally discern intrinsic cardiac signals
with lower amplitudes, but it is also more susceptible to
external electromagnetic interference.
In a bipolar configuration, the pacing pulse is applied
between proximal and distal electrodes. Thus the anode is
also placed in a heart chamber. In a bipolar sensing
configuration, the sensitivity to signals occurring close to the
electrodes is higher because of the relatively shorter distance
between the lead electrodes. As a consequence, the pace-
maker will less probably sense myopotentials and other sig-
nals unrelated to heart depolarization. A bipolar configuration
provides higher pacing circuit impedance (because of the use
of two lead conductors) and a slightly higher pacing
threshold.
Because of the automatic setting of the sensitivity of the
defibrillator sensing circuit, a large unipolar pacing pulse
may avert sensing of weak intracardial fibrillation signals.
For this reason, in implantable cardioverter-defibrillators or
pacing systems with an additionally implanted defibrillator,
bipolar configuration is used solely for the right atrium and
ventricle. As far as left ventricular leads for cardiac resyn-
chronization therapy are concerned, not only common bipo-
lar or unipolar configurations are available. Based on the
type of the lead and device, extended bipolar combinations
of pacing electrodes can be opted for because the right ven-
tricular proximal electrode can be utilized.
8.1
Pacing Con fi guration
Fundamentally, two types of pacing or sensing configurations
are available, according to the types of pacing leads: unipolar
and bipolar (Fig. 8.2 ). This designation is rather misleading
because both types contain an anode and a cathode, and both
make use of one pacing electrode (cathode) in contact with
tissue. The difference is in the placement of the second pac-
ing electrode (anode) [ 4 ] . Pacing and sensing con fi gurations
are usually separately programmable for the atrium and the
ventricle, and secure algorithms are included in the systems
to prevent unintentional programming of a bipolar
configuration when a unipolar lead is used. Up-to-date sys-
tems are capable of detecting the connection and diagnosing
the lead type automatically.
If a unipolar pacing configuration is programmed, the
pacing pulse will be applied between the cathode on the lead
distal pacing electrode and the anode placed extracardially
on the pacemaker can. The entire device or a certain limited
area can be used as an indifferent pacing electrode. In a uni-
polar pacing configuration, pacing artifacts will be clearly
visible on the surface electrocardiogram (ECG), which may
help with interpretation. In a unipolar configuration, how-
ever, the anode is placed close to large muscles, which more
often causes muscle stimulation or sensing of noncardiac
8.2
Pacing Impedance
Pacing impedance refers to the total value of pacing circuit
impedance measured on output contacts of an implanted
device. This value is important partly for long-term monitor-
ing of the lead condition and partly for energy consumption.
A rapid increase in the impedance indicates possible fracture
of the lead conductor or another conduction pathway defect;
a sudden drop, on the other hand, may signal damage to the
 
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