Biomedical Engineering Reference
In-Depth Information
7.1.2
Construction of Pacing and De fi brillation
Shock Electrodes
The pacing threshold is dependent on the current density cre-
ated by the electrodes. The lesser the electrode diameter is,
the higher the current density and the lower the pacing
threshold are. In addition, the small surface of the electrode
increases the contact resistance between the electrode and
the tissue, which decreases the consumption of current from
the source. On the other hand, per good sensing properties, a
bigger electrode surface is better because it decreases sens-
ing impedance and polarization. Although the issue of sens-
ing impedance might be solved by appropriately setting the
impedance of the input sensing amplifier, it is necessary to
use electrodes with bigger surface area because of the polar-
ization at the point of contact between the electrode and tis-
sue. It was proved that it is possible to construct electrodes
with a small diameter but a quite large surface. It can be
achieved by creation of a porous texture (spatial microscopi-
cal ridging) at the pacing electrode, which is called fractal in
some of the literature. In this way the electrode surface is
enlarged while the diameter of the pacing electrode is kept
the same. The textured surface minimizes polarization phe-
nomena and increases pacing and sensing efficiency.
Technologically it is achieved by sintering basic electrode
material with platinum and iridium oxide or by constructing
the electrode of microscopical metal fibers. A porous elec-
trode made of platinum and iridium increases the active area
and permanent stability of the electrode tine. For the time
being, the most often used bipolar electrodes with active
fixation have the area of the distal pacing electrode (the
helix) - about 2 mm
2
- and of the proximal electrode (the
ring), an area of about 40 mm
2
. The relative distance of the
pacing electrodes is dependent on the manufacturer, but it
usually ranges from 10 to 20 mm.
Defibrillation shock electrodes are constructed as wound,
small coils with minimal ascending, which ensures good
mechanical elasticity. The winding is regarded as an integral
area. Defibrillation leads can have the only defibrillation
electrode (single-coil) or possibly two electrodes (dual-coil).
The area of the distal shock electrode is about 310-450 mm
2
;
the area of the proximal shock electrode is about 480-
660 mm
2
(the proximal electrode is always larger) [ 51, 52 ] .
Because of the occasional need to extract cardiac pacing
or defibrillation leads, it is necessary to minimize the possi-
ble ingrowth of fibrous tissue to the pacing and especially the
defibrillation shock electrodes. Two approaches are used
here. Either the area between individual fibers of the
defibrillation electrode coil is injected with additional silicon
insulation that restricts ingrowth of the fibrous tissue or the
surface of the defibrillation electrode is coated with a
polytetrafluoroethylene layer. It creates a porous structure
with microscopic orifices that are too small for the fibrous
Fig. 7.4
Single-wound and triple-wound helix coil conductors
tissue cells to penetrate, but they do not represent any
difficulties for good electrical contact and, by that, good
defibrillation. The leads must be constructed isodiametri-
cally, which means with the same diameter along the entire
length of the lead or with a decreasing diameter toward the
distal end. It guarantees better patency through respective
stenoses and makes eventual extraction simpler.
7.1.3
Construction of the Lead Conductor
To a large extent, the conductor designates mechanical prop-
erties of the whole lead and its good manipulability [4]. The
conductor, together with its insulation, is the most mechani-
cally stressed part of the entire implanted system. It is
stressed by both the cardiac chamber afterloads and the
patient's movements. At a pacing rate of 70 beats/min, it
must withstand stress volume of 36 million cardiac after-
loads per year. Mechanical properties of the leads depend on
how it is wound internally. In terms of the arrangement of a
bundle, one can distinguish between winding with a single-
wound helix or multiwound helices (see Fig.
7.4
). Winding
with multiwound helices gives several advantages. First, a
higher pitch of every helix, which the conductor consists of,
ensures lower mechanical stress. Furthermore, the total elec-
trical resistance is decreased by the parallel connection, and
the eventual opening of one helix in the winding does not cut
off the entire bundle.
Bipolar leads, which are used nearly exclusively nowa-
days, are distinguished by a coaxial or co-radial design of
two bundles in the conductor (see Fig.
7.5
). The bundles are
insulated reciprocally by, for example, tetrafluoroethylene.
There is an inner tubular channel winding through the inside
of the conductor, which a stylet or a guidewire introduced
while handling the lead.
The body of defibrillation leads generally contains the
conductor for pacing and sensing with a co-radial structure
of winding because the coaxial structure would further
increase its diameter. It also contains one or two conductors
