Biomedical Engineering Reference
In-Depth Information
impossible and might influence the electric parameters of the
system. Some manufacturers use the platinum and iridium
alloy (80 % Pt, 20 % Ir) for the shock electrodes. However,
others apply drawn brass wire instead. To avoid ingrowth of
tissue, the coiled electrodes might be injected by a suitable
material, for example, expanded polytetrafluoroethylene.
tion made of 80A polyurethane was shown to be insufficient
because of microscopic cracks in the surface. Polyurethane is
also subject to degradation by metal ion oxidation, especially
in the case of silver chlorides contained in drawn-brazed-strand
conductors.
7.2.4
Lead Connector Materials
7.2.2
Lead Conductor Materials
The point of the lead connection to the implanted device
is under a high strain with regard to mechanical as well as
electric stress. Connectors and the device are connected by
means of setscrews with defined pressure. The materials
used for the setscrews (device contacts) and lead connectors
must be the same, otherwise electrolytic corrosion might
occur during penetration of body fluids into the system. The
pacing connectors use corrosion-resistant steel, for example,
surgical steel designated 316L. For insulation of individual
elements of the contact system, which should create water-
proof interconnection, polyurethane is used.
The lead conductor transmits sensing and transfers the pac-
ing pulses and defibrillation shocks from the device to the
pacing or defibrillation electrodes. Triple or quadruple wound
helices made of MP35N material are used most often. MP35N
is the registered trademark for a special alloy of cobalt, chro-
mium, and nickel. Other nickel alloys including a tantalum
or silver core, called drawn-brazed-strand, also are used. The
specific resistance of the materials tends to be in a degree of
tens of an ohm per centimeter. The resistance of the entire
lead conductor amounts to several tens of an ohms and rarely
more than a hundred ohms. Bipolar leads use, for example,
ethylene-tetra fl uoroethylene [ 40 ] for insulation of individual
bundles of the co-radial or coaxial winding.
7.3
Cardiac Pacing Leads
Cardiac pacing leads serve for pacing and sensing of the
right atrium, ventricle, or both (see Fig.
7.8
), and they must
be compatible with the device used. The usage of endocar-
dial leads is contraindicated for patients who are hypersensi-
tive to the steroid contained in the lead tine, those with severe
tricuspid valvular disease, and patients with mechanical tri-
cuspid heart valves. Basic requirements for the endocardial
leads are a small diameter, good mechanical flexibility, reli-
able fixation, and long-term electric stability.
The leads are supplied with stylets, fixation tools, a stylet
introducer, a vein pick, a lead end-cap, and documents in the
external and internal sterile bowl, both of which can be
opened by tearing the packaging. Various types of stylets are
supplied. They might be soft or firm, and straight or pre-
formed to a
J-
shape for implantation to the atrium. A straight
stylet might be introduced to the supplied lead in advance.
The stylet introducer serves to simplify insertion of the stylet
into the lead. The suture sleeve is an adjustable reinforce-
ment in the shape of a small tube that is positioned around
the external insulation of the lead. It serves for the secure
fixation of the lead and its protection where suturing occurs.
The usage of the suture sleeve decreases risk of damage
caused by suturing directly across the lead body.
7.2.3
Insulation Materials
The leads are located in the aggressive environment of body
fluids, so the insulation materials are required to meet high
demands with regard to their imperviousness, elongation at
rupture, reaction to blood clotting, and long-term mechanical
and chemical stability. Some materials might also require
retentivity. In the past, different materials were tested, for
example polyamide (nylon), polyurethane, polyethylene, or
silicon rubber. Nowadays, silicon rubber and polyurethane
predominantly are used for lead conductors' insulation.
Silicon rubber is chemically stable in a body and it does not
cause any inflammatory or rejection responses. However,
this material is sticky when touched, and handling two leads
made of this material in a vein is quite difficult. But this dis-
comfort can be reduced by various kinds of surface coatings
on the silicon insulation. Polyurethane has a good resistance
against tearing and it is not so sticky. Polyurethane 55D as
well as 75D or 90A are used most often, especially for the
parts that are under higher mechanical stress. It deals with the
connectors for interconnection with the device, suture
sleeves, and mechanical fixation elements. The lead insula-
Fig. 7.8
Endocardial pacing lead
