Biomedical Engineering Reference
In-Depth Information
Fig. 7.7
Type IS-1 connector
quite a voluminous part of the defibrillation lead at a place of
the individual connectors branching. For that reason a new
connector was developed, designated DF-4, according to the
international standard
ISO 27186:2010 Active Implantable
Medical Devices - Four-Pole Connector System for
Implantable Cardiac Rhythm Management Devices -
Dimensional and Test Requirements
. Usage of this connector
also reduces the volume of the device connector header
because up to three connectors were integrated into only one
(IS-1 + 2 DF-1).
electrically as a capacitor, with quite a high capacity that
is dependent on the material use for the pacing electrodes.
This stray capacity is charged by the current of the cardiac
pacing pulse, and it influences voltage behavior at the pacing
electrode. The voltage created at the electrode, which is
necessary for overcoming of the charged double layer, is
called the polarization voltage. Corrosion-resistant steels and
alloys dispose of the highest polarization voltage; gold has a
lower polarization voltage and platinum even less. Silver
compounds dispose of the lowest polarization voltage.
However, they cannot be used because of their toxicity, as
stated above.
Apart from suitable mechanical properties, another
requirement for manufacturing is the resistance to corrosion
in the aggressive environment of body fluids. Some alloy
components dissolve or are corroded by the influence of
polarization during the current flow. From this point of view,
platinum is considered to be the best. When iridium is added,
the allow also has suitable mechanical properties.
Nowadays, platinum and iridium alloys are used exclu-
sively for the electrodes of implantable leads because the
added iridium improves the chemical properties of the plati-
num. The alloy is even more chemically stable against the
influence of various environments. The distal pacing elec-
trode (tip) usually has higher platinum content in the alloy
(90 % Pt, 10 % Ir), whereas the content of platinum in the
proximal pacing electrode (ring) is lower (80 % Pt, 20 % Ir).
Some types of electrodes are coated with iridium oxide. The
size of the electrically active area of the pacing electrodes is
dependent on the type of electrode (passive/active fixation,
unipolar/bipolar, distal/proximal).
The distal shock electrode serves as the anode for sensing
and pacing (for the integrated defibrillation leads) and as the
cathode or anode for delivery of the electric shock during
cardioversion or defibrillation. The proximal shock electrode
serves as the anode or cathode for the delivery of electric
shock during cardioversion or defibrillation. The shock elec-
trodes also enable sensing of the electrograms that are used
to monitor and evaluate the rhythm's morphology. The shock
electrode is made of a wound wire that creates a coil around
the lead body. Requirements for functionality and safety are
similar to those of pacing leads. In addition, there is a neces-
sity to withstand a high current density and to avoid the inter-
growth of tissue around and among fibers of the shock
electrode. The tissue ingrowth might make later extraction
7.2
Lead Materials
An implantable lead is a heterogeneous corpus that is charac-
terized by every part has different requirements about which
materials are selected for use. When considering the materi-
als required, the basic parts of an implantable lead, regardless
of its purpose or positioning and apart from the fixing mecha-
nism, are the pacing, defibrillation shock electrodes, lead
conductor, insulation, and connectors for connection to the
implanted device. Although the materials in question already
have been well investigated, to provide a good understanding
of the biomedical and biophysical context, some details about
the selection of various materials must be mentioned here.
7.2.1
Materials for Pacing and De fi brillation
Shock Electrodes
The pacing electrodes are located at a position most distant
from the device and thus at the farthest position in the heart
chambers. Their function is to transfer electrical energy to
the tissue. As with the other parts, the pacing electrodes must
be innocuous, especially regarding biological and mechani-
cal properties, and they must not damage the paced tissue in
any way. Noble metals, such as platinum and gold, and cor-
rosion-resistant alloys comply with this condition. Silver has
toxic effects and cannot be used.
From the point of view of the cardiac pacing function, it
is important to be concerned with the polarization voltage
[4]. Between the metal pacing electrode and the tissue, a
double layer of electric charge carriers is created in which
electron conductivity of the metals is transferred to ion con-
ductivity of the tissue electrolytes. The double layer behaves
