Biomedical Engineering Reference
In-Depth Information
(pacing/sensing, cardioversion/de fi brillation), con fi guration
(unipolar/bipolar), and connector geometry (length and
diameters in millimeters or a reference to issued standards).
Each lead, and possibly each adaptor, must be perma-
nently and visibly marked with the manufacturer and type
designation and a serial number or a lot number. Naturally, it
is required that any implantable system component is sterile
upon unpacking and contains no excessively loosened solid
particles - sterile dust. The amount of particles depends on
the surface area, not the lead content. This particularly
applies to leads typically having a large surface area but
small contents. Reference values are based on a standardized
particulate contamination test. An implantable device must
also be designed to resist minor mechanical impacts caused
by manual handling during implantation.
Sustained direct currents leaking from implanted leads may
result in damage to tissues or material corrosion. Unless on
purpose, an implantable device must always be electrically
neutral upon application. Direct leakage current exceeding
1
determine a minimum number of maximum-energy shocks
an ICD must be capable of applying after the recommended
replacement time is identified. Considering the period of
time between routine follow-ups, a minimum of six maxi-
mum-energy shocks can occur after the electric replacement
indicator is activated.
14.2.2 Requirements for Implantable Leads
Implantable leads are subject to tests of both electrical and
mechanical parameters. For example, defibrillation lead con-
ductors must pass an axial load test, and, at the same time,
the capability of delivering maximum defibrillation shocks
must not be impaired. A damaged lead conductor may have
a lesser ability to carry high currents, even though its imped-
ance is within the appropriate limits. In the axial load test, a
defibrillation shock of 1,000 V is simulated from a capacitor
with a capacity of 200
F at a system impedance between 20
and 25 Ω. Hence, all test values are overrated if compared
with values in practice. Ten shocks are applied for the pur-
pose of testing, and signs of damage by the test current then
are searched for visually [ 72 ] .
Minimum requirements for implantable lead resistance to
bending should be determined by means of mechanical test-
ing. A lead conductor or connector must withstand a mini-
mum of 47,000 - or possibly 82,000 - load cycles flawlessly.
The manufacturer determines the specimen size, data analy-
sis, and safety reserve to meet the requirements for the mini-
mum number of cycles. Furthermore, the manufacturer is
responsible for defining a comprehensive set of requirements
concerning the reliability of a specific lead conductor solu-
tion. The value of 5 N has been agreed upon as the endurance
limit for axial tensile force exerted on an implanted lead.
In particular, resistance to bending (fracture) and lead insula-
tion resistance are tested. A larger bend radius naturally
causes less mechanical stress and a smaller bend radius
causes more stress. Thus the minimum number of bending
cycles decreases with higher stress. According to standards,
the tests usually are conducted on an accelerated basis; a
higher load is exerted on leads on purpose, which results in
shorter longevity of test specimens.
The lead characteristics, as specified in documentation,
must provide the following information:
General description of materials used in individual com-
m
A must not be induced in any current pathway between
defibrillation lead connectors and the can, and direct leakage
current exceeding 0.1
m
A must not be induced in the current
pathway of any pacing connector. A test for direct leakage cur-
rent is carried out with inactivated tachycardia therapies. Load
resistances are applied, which substitute impedance in an
implanted device. Limits for pacing/sensing connectors com-
ply with ISO 14708-2. In shock connectors, the limits are ten
times higher as a result of the much larger area of the shock
lead. In pacing/sensing leads with an area of pacing electrodes
less than 10 mm
2
, leakage current of approximately 10
m
A is
acceptable. In shock leads with the area exceeding 300 mm
2
,
maximum direct leakage current of approximately 100
m
A is
acceptable. Alternating leakage currents may occur while
high-voltage ICD capacitors are being charged. Low-frequency
currents may induce ventricular fibrillation, whereas high-
frequency currents may cause heating and damage to tissues.
During charge and discharge cycles, a considerable
amount of energy may dissipate inside an ICD. Today's
ICDs, however, have sufficient weight, making them capable
of dissipating the heat; thus the temperature will not increase
by more than 4 °C at any point on the ICD's external surface.
Possible tissue damage is a function of exposure time.
Measured surface temperatures during testing shall be iden-
tical to temperatures measured in an implanted device. At
present, 43 °C is considered the temperature threshold for
tissue damage. Most damage to muscular and adipose tissue
by temperatures ranging between 43 °C and 45 °C are revers-
ible; under higher temperatures, necrosis occurs. Local
chronic heating of tissue caused by an AIMD should be
restricted to a maximum temperature of 41 °C.
The manufacturer must specify the time of recommended
replacement of an implantable device. Because the ICD-
provided treatment is of vital importance, it was necessary to
m
•
ponents (a connector, insulation, a conductor, pacing and
de fi brillation electrodes)
Notification of the drug content and the drug identification
•
•
Nominal values of mechanical dimensions (length, geo-
metric surface area of all electrodes, insertion diameter,
distance between lead pacing electrodes)
Electrical characteristics (conductor resistance, pacing
•
impedance, sensing impedance)
