Biomedical Engineering Reference
In-Depth Information
arrangements are made for cardiopulmonary bypass. Cardiac paralysis allows a surgeon
to perform delicate procedures on a quiet, motionless heart. A common way of causing
elective cardiac arrest used to be by applying medium-frequency sine-wave stimulation
through electrodes on the surface of the heart to bring about ventricular
fibrillation, but this
method is seldom used today. Instead, a potassium-salt solution is usually injected to stop
the heart intentionally (cardoplegia). When a patient is placed on the cardiopulmonary
bypass machine, the heart and body are cooled to reduce the heart's metabolic require-
ments and oxygen consumption. The cardiplegia solution is then injected to cause cardiac
standstill and further reduce the heart's oxygen consumption.
When the surgery has been completed, the cardioplegic agents are
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flushed out of the
heart with warm blood. The heart resumes contraction, and use of the heart-lung machine
is discontinued. If normal contractions do not begin at once, a de
fl
brillation shock is used.
Temporary pacing leads (“heart wires”) are sometimes left stitched on the surface of the
heart for connection to a temporary pacemaker, since it is not uncommon that the heart
remains bradycardic until all pharmacologic agents are
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fl
flushed out and normal metabolism
is resumed. Chemical cardioplegia is preferred to
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fibrillation because the heart's quivering
during
fibrillation wastes the heart muscle's energy resources that are needed once beating
is resumed. Despite this,
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fibrillators are still used in animal research connected to the
speci
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c mechanisms of
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fibrillation as well as in the assessment of de
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fibrillation systems.
fibrillator. The circuit uses a 555 timer IC to gen-
erate a 50-Hz square wave. This signal drives two sets of six paired CMOS bu
Figure 8.48 shows a battery-powered
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ff
ers each.
This output stage is actually a full bridge which causes doubling of the e
ective voltage
across the heart. Capacitors C2 and C4 block dc components on the output signal.
Resistors R2 and R3 are used to protect the circuit against de
ff
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brillation currents. Since a
certain current
density
threshold has to be exceeded to cause
fibrillating
current and electrode surface contact area are proportional. Using pediatric-use
de
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fibrillation, the
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fibrillate a pig's heart typically requires 10 to 12 V at
50 Hz. Using the outside surface of alligator clips as electrodes (which have a much lower
contact surface area) may take only 3 to 4 V.
As simple as this circuit is, an even simpler battery-powered
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brillation paddle electrodes to
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fibrillator is the most com-
mon method for inducing
y touching the terminals of a
standard 9-V battery to the ventricle (e.g., to the right-ventricular out
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fibrillation in the animal lab. Brie
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fl
ow tract) almost
always
fibrillates the heart. Moral of the story: Don't lick-test 9-V batteries laying around
an electrophysiology lab.
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CONCLUDING REMARKS
There is much more to the design of active implantable medical devices (implants that rely
for their functioning on a source of electrical energy or any source of power other than that
generated directly by the human body or by gravity) than the short explanations and
demonstrations presented in this chapter. The European Union has published a directive
that regulates the level of performance and testing applicable to active implantable med-
ical devices. This standard is:
• EN-45502-1,
Active Implantable Medical Devices
—
Part 1: General Requirements
for Safety, Marking and Information to Be Provided by the Manufacturer
, 1998
As a specialized subset of the implantable devices
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field, cardiac pacemakers and
de
brillators have been around for quite a few years now. Their clinical success have
turned them almost into commodity items with standardized features. Regulators recog-
nize this fact and are trying to establish speci
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c standards for the performance of these
devices. Although at the time of this writing the relevant European standards are still in
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