Biomedical Engineering Reference
In-Depth Information
1 0
Implantable Cardioverter-De fi brillators
An implantable cardioverter defibrillator (ICD) is an active
implantable medical device primarily used to deliver a high-
energy electric shock to the heart via intracardial leads. The
electric shock causes concurrent depolarization of all cell
membranes in myocardial volume followed by an absolute
refractory period. As a result, a tachycardia impulse spread-
ing in the myocardium is interrupted. This implantable
device also works as a pacemaker and applies antitachycar-
dia pacing (ATP) to terminate ventricular tachycardia (VT)
when high-energy shocks are not required.
Like in pacemakers, a literal code is used in ICDs to iden-
tify basic device modes (Table
10.1
); the code contains only
four characters. VVEV is the most common mode for single-
chamber ICDs; VVED is used for dual-chamber ICDs.
High-voltage shocks in ICDs may be set up using a pro-
grammer and turned off by either using a programmer or
applying a magnet above the ICD. Tachycardia functions
may either be turned off completely, or detection may
be turned on without a therapy, or both the detection and
therapy may be turned on (the common long-term opera-
tion). The tachycardia function is switched off for safety
reasons while handling the device and during implantation,
end-of-life device replacement, or electrocauterization
because handling the device, attaching leads using set-
screws, etc., may cause artifacts that possibly could be
interpreted as tachycardia. Following surgical interventions,
in particular when the use of electrocautery is required, the
ICD shock function must be switched off, and monitoring
and an external defibrillator must be provided. In addition,
electrocauterization may inhibit pacing in all types of
implantable devices. If not equipped with the electrocauter-
izing (asynchronous) mode function, the device must be
switched over to asynchronous pacing using a programmer
or a magnet.
Requirements for ICDs are defined in
ISO 14708-6:2010:
Implants for surgery - Active implantable medical devices -
Part 6: Particular requirements for active implantable medi-
cal devices intended to treat tachycardia (including
implantable de fi brillators)
[ 72 ] . The standard provides
technical information on measuring parameters, susceptibil-
ity to electromagnetic interference, etc.
10.1
The Design of Implantable
Cardioverter-De fi brillator Systems
An ICD is similar in design to a pacemaker. Mechanically, it
comprises a titanium can and a polymer header. In certain
types, even an antenna used for radio communication with a
programmer or a remote monitoring unit may be visible. The
metal can contains, in addition to a battery and electronic
circuits, comparatively large high-voltage electrolytic capac-
itors with the capacity range of 60-140
F (according to the
manufacturer) necessary for energy accumulation before
delivery of a defibrillation shock. Maximum accumulated
energy is currently around 40 J. Capacitors, together with the
battery, make up most of the ICD's volume (Fig.
10.1
). In
terms of electronics, the ICD contains only a monolithic inte-
grated circuit and several external components [ 73 ] .
If a longer period of time passes between ICD charging
cycles, dielectric material re-formation in high-voltage
capacitors may dissipate. This may prolong the charging
period. For that reason, high-voltage capacitors should be
charged on a regular basis, either automatically or at regular
follow-ups by re-formation using a programmer. High-
voltage capacitors thus are charged to maximum voltage;
after charging, the voltage gradually decreases, and, depend-
ing on the value, dissipates within several minutes.
m
