Biomedical Engineering Reference
In-Depth Information
7
Leads
Cardiac pacing or defibrillation leads are the most critical
part of implantable systems. Because they ensure sensing of
heart activity and transfer cardiac pacing pulses or
defibrillation electric shocks from the implantable devices to
the tissue, we can say that from the point of view of their
efficiency, reliability, and long life, leads are more important
than the devices themselves. Furthermore, they are implanted
right in the heart's chambers either endocardially or epimyo-
cardially; thus, compared with the devices implanted just
under the skin or muscle, the technical or clinical failure of a
lead represents a much higher risk for the patient.
The leads must comply with conditions for biological
safety and materials used, and their construction must not
cause any undesirable allergic or inflammatory responses or
damage to tissue. Nowadays, the presumption is that the lon-
gevity of an implantable lead must be the patient's lifetime.
Extraction of leads, especially in the case of defibrillation
leads, is connected with high risk and might require cardiac
surgical intervention (thoracotomy).
In the course of development, various kinds of leads were
used. Historically, surface skin leads, esophageal leads for
indirect cardiac pacing, or needle leads for direct cardiac
pacing were developed. Later, epimyocardial leads were
used that, on a limited scale and in justified cases, are still
used today. For the time being, endocardial (intracardial)
leads are used nearly exclusively. They are inserted via a
venous route to the right ventricle or right atrium or via the
coronary sinus to the coronary veins, where they are fixed
using techniques described below. The advantage of endo-
cardial leads is a quite simple method of their implantation
that does not require any cardiac surgical intervention.
The lead and its accessories usually are sterilized using
gas ethylene oxide before their final packing. Thus, the leads
supplied by the manufacturer are already sterile and prepared
for usage. Required temperature for their storage ranges
from 0 °C to 50 °C [48].
7.1
Construction of Leads
Regarding the design of implantable leads, they consist of a
fixing mechanism, pacing and shock electrodes, a conductor,
insulation, and a connector (terminal). According to the clin-
ical designation, we distinguish between atrial, ventricular,
or left ventricular pacing leads, as well as defibrillation leads.
The lead length ranges from 40 cm for the shortest atrial
leads up to 100 cm for the longest left ventricular leads. The
following lengths are considered standard: from 45 to 55 cm
for the atrium, from 50 to 60 cm for the right ventricle, and
from 75 to 100 cm for the left ventricle. A suitable length for
a defibrillation lead ranges from 55 to 65 cm [49, 50]. The
medical staff has to consider the optimal length of a lead dur-
ing its selection. Certainly, it is undesirable to use a lead that
is too short or too tight. However, length that exceeds opti-
mal size by, for example, 40 cm, must be a superfluous strain
on the patient.
Permanent assurance of excellent electrical performance
of pacing is one of the most important requirements for car-
diac pacing leads. However, the behavior of the cardiac pac-
ing threshold in time shows two increases. First, it rises
immediately after implantation of the lead in connection
with local damage to the tissue; second, it rises gradually as
a consequence of the inflammatory response of cellules and
fibrosis created around the pacing electrode. Because of sta-
bilization of the pacing threshold over time, modern leads
usually are equipped with a steroid-eluting ring at the distal
end. It is a ring made of porous silicone rubber that is filled
by steroid. A nominal dose contains about 1.0 mg of dexam-
ethasone acetate. The steroid is eluted from the ring slowly
and gradually during its contact with body fluids. That way it
depresses the inflammatory response that is considered to be
a reason for the increase in pacing threshold that usually
occurs in connection with implantation of the pacing leads.
