Biomedical Engineering Reference
In-Depth Information
Implantable Cardioverter De
fi
brillators
The father of the implantable de
fi
brillator is widely recognized to be Michael Mirowski.
His work on an implantable de
brillator began in the late 1960s after his friend and men-
tor, Harry Heller, died as a result of ventricular arrhythmia. His concept was to develop a
compact de
fi
brillator that could provide continuous rhythm monitoring and deliver appro-
priate electrical shock therapy when necessary.
The
fi
brillator occurred in
1980. Five years later the FDA approved the release of an implantable de
fi
first successful human implant of a totally implantable de
fi
fi
brillator for use
in the United States. These early de
brillators were simple devices that would only deliver
high-energy shocks to interrupt ventricular tachyarrhythmias. Much progress has been
made since, and today, the implantable cardioverter-de
fi
brillator (ICD) is the best therapy
for patients who have experienced an episode of ventricular
fi
fibrillation not accompanied
by an acute myocardial infarction or other transient or reversible cause. It is also superior
therapy in patients with sustained ventricular tachycardia causing syncope or hemody-
namic compromise.
The implantable de
fi
brillator is connected to leads positioned inside the heart or on its
surface. These leads are used to deliver the de
fi
brillation shocks as well as to sense the car-
diac rhythm and sometimes pace the heart, as needed. The various leads are tunneled to the
device, which is implanted in a pouch beneath the skin of the chest or abdomen. They can
be installed through blood vessels, eliminating the need for open chest surgery. Modern
implantable cardioverter de
fi
brillators have a volume of 30 to 40 cm
3
. Microprocessor-
based circuitry within the device continuously analyzes the patient's cardiac rhythm. When
ventricular tachycardia or
fi
fi
fibrillation is detected, the device shocks the heart to restore nor-
mal rhythm. De
brillators are also able to provide “overdrive pacing” to convert a sustained
ventricular tachycardia electrically,
fi
and pacing if bradycardia occurs. Implantable
de
brillators are usually powered by internal lithium-silver vanadium oxide (Li/SVO) bat-
teries capable of delivering ampere-level currents needed when charging the high-voltage
capacitors.
Implantable de
fi
brillators usually generate biphasic truncated exponential decay wave-
forms, where a second phase of opposite polarity follows the
fi
fi
first shock phase. Biphasic
waveforms are as e
ective as monophasic waveforms, but at lower energy and with fewer
postshock complications. The mechanism by which biphasic pulses reduce the threshold
voltage is probably related to the
ff
first phase of a biphasic pulse not having to synchronize
as many cells, since the second phase removes the residual charge caused by the
fi
fi
rst
phase, which could otherwise reinitiate re
brillation. Besides lower energy requirements,
biphasic shocks are not as traumatic to the heart as monophasic shocks.
The optimal durations of the two phases are relatively independent of one another.
Theoretically, the optimal
fi
first phase is identical to the optimal monophasic waveform—
about 2.5 ms for a wide range of
RC
values. The optimal second-phase duration is based
on the membrane time constant (about 3 ms). Despite this, several studies have shown that
the second phase should be shorter than the
fi
first phase. The second phase of some bipha-
sic waveforms is so short that it cannot reverse the transmembrane potential caused by the
fi
fi
first phase, yet it greatly increases de
fi
brillation e
cacy. In these studies, as the second-
phase duration increased, the pulse de
ectively and had higher
voltage requirements, which implies that the second phase should be as short as possible,
yet long enough to return the transmembrane potential to a level close to the one that
existed before the shock.
A major di
fi
brillated less and less e
ff
brillators (which apply currents
across the chest from electrodes placed on the skin) and internal de
ff
erence between transthoracic (external) de
fi
fi
brillators (which apply
the de
fi
brillation currents directly to the myocardium) is the energy needed to cardiovert
or de
fi
brillate. Transthoracic de
fi
brillation at 100 J is successful in half the cases, and
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