Implantable Cardiac Defibrillator (cont.)
Who should receive an ICD?
Patients at risk of developing sudden cardiac
arrests due to ventricular tachycardias and fibrillations
are candidates for ICDs. ICDs do not prevent the occurrence
of life-threatening rhythms, but can quickly terminate them
when they occur. Recent clinical trials have identified
several groups of patients who should receive ICDs. They are:
- Patients who have survived cardiac arrest;
- Patients with ventricular tachycardias that
significantly decrease the amount of blood delivered by the
heart, resulting in low blood pressure;
- Patients with significant heart muscle damage from
prior a heart attack, and have ventricular tachycardia
episodes that are not suppressed by medications; and
- Patients deemed at high risk for sudden death from
cardiac arrest based on a history of heart disease and
findings from an electrophysiologic evaluation (special
study of the electrical system of the heart).
How are ICDs designed?
An ICD consists of one or more leads (conducting wires
insulated with silicone or polyurethane) and a
defibrillator unit. The defibrillator unit is a small
titanium case containing a microchip computer, a capacitor,
and a battery.
The leads carry electrical signals between the heart and
the defibrillator unit. One end of a lead is placed on the
inner wall of the heart while the other end is attached to
the defibrillator unit. The leads help the defibrillator
unit monitor the natural heart rhythm. The leads also
deliver electrical shock(s) from the defibrillator unit to
the heart when tachycardias occur.
The microchip computer runs the defibrillator, monitors
the natural heart rhythm, instructs the capacitor to send
electrical shock(s) when tachycardias occur, determines the
strength of the shock(s) sent, and also keeps a record of
the heart rhythms as well as the shock(s) sent by the
defibrillator.
Modern ICDs have programmable features that allow the
doctor to change the cutoff heart rate for activating the
defibrillator. Tachycardias with rates higher than the
cutoff heart rate activate the firing of shocks by the
defibrillator. The doctor can also adjust the strength
(amount of energy delivered) of each shock, and the number
of shocks delivered with each tachycardia episode.
Most defibrillators now have built-in pacemakers as
well. The newer defibrillators can have very sophisticated
pacing devices equipped with the ability to pace both the
atrium and the ventricle (dual chamber pacers). Some
defibrillators have rapid pacing capabilities. Rapid pacing
can sometimes convert a tachycardia to normal rhythm
without administering electric shock(s).
The electric pulses and shocks delivered by the ICDs are
of such low energy that they do not harm the patient or
family members in physical contact with the patient.
Next: How are ICDs implanted? »
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