IEEE Spectrum November, 2014 - 59

In a patient with normal heart size and
anatomy, the defibrillator is implanted in
a standard configuration: A battery sits
beneath the collarbone, and a catheter
snakes through a vein into the heart's
right ventricle. But existing devices
weren't designed for children's small
bodies, and doctors frequently have
to implant the bulky battery down in
the child's abdomen. What's more, in
children with malformed or diminutive hearts, the catheter often can't
get through the tiny veins or reach the
proper target inside the ventricle. Surgeons often have to place the electrodes
outside the heart instead.
Pediatric hearts with congenital
defects are so variable and structurally
complex that defibrillator implantation
is a highly individualized art. Currently
there's no reliable way to predict the
ideal locations for the defibrillator
components in a given child, and an
imperfect setup can have serious repercussions. For example, poor positioning of the battery can cause the leads
that twist through the child's body to
bend and stress, creating fractures in
the insulating material. Such cracks can
make the defibrillator discharge unnecessarily or, worse, fail to deliver a shock
when needed.
The exact location of the electrodes
outside the heart is also a question
of great importance. Doctors set the
defibrillator's voltage based on how
much cardiac tissue the current must
flow through and the orientation of the
muscle fibers in that tissue. High voltages can damage tissues and can also
cause great pain, because the current
stimulates nerve fibers. So doctors are
motivated to find the locations that
deliver an effective shock at the lowest voltage.
In a proof-of-concept study, we took
the MRI scans from a pediatric patient
with a congenital heart defect, in which
the right ventricle was dramatically
undersized. We created a 3-D model
of the child's heart and the surrounding torso, and we experimented with
different locations for the battery and
the electrode tips. Our simulations
eventually identified the configura-

tion that should produce an arrhythmia-stopping shock at the minimum
voltage. If doctors adopt such models, it may take the guesswork out of
device positioning and spare young
patients from repeat procedures to
reposition their devices.
There may come a time when all
patients with heart conditions, from
babies to octogenarians, have their

virtual hearts tucked into their electronic medical records, which doctors
can then use to plan their treatments.
I look forward to that day, for I have
g reat hope that these simulated
hearts will be able to prevent some
real human hearts from breaking. n
post yoUr comments at http://
spectrum.ieee.org/virtualheart1114

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2014