IEEE Spectrum December, 2007 - 21

HEART ELECTRONICS: An electric
impulse starts in the SA node and
travels to the two atria, which contract,
pushing blood into the lower chambers.
Current then passes through the AV
node to cause the ventricles to contract.

Sinoatrial
(SA) node
right atrium

bachmann's bundle
left atrium
left bundle branch

left
ventricle

conduction
pathways

Electrical
flow
Atrioventricular
(AV) node
right
ventricle
bundle of his
right bundle
branch

corresponding to sites where Taser darts commonly make contact with human subjects. We used a custom-built circuit that
matched the waveform and typical 5-second shock duration of an
X26 Taser gun, but our device could deliver a much larger shock.
To boost the output current, we increased the capacitor sizes in
the device. After inducing ventricular fibrillation, we immediately rescued the animal using an ordinary defibrillator. We then
stepped down the current to determine the highest amount that
could be delivered without inducing ventricular fibrillation.
We calculated that quantity, cast in terms of multiples of the
capacitances, for each of the body sites we'd chosen to test. Of
the various positions we examined, some were a mere centimeter
or two away from the heart, which sits just under the chest wall,
touching it on the inside. Not surprisingly, we found that darts near
the heart had the lowest thresholds for inducing ventricular fibrillation. At the closest spots-with one dart hitting at the lower end
of the chest wall, and the other at the top of the breastbone-such
a cardiac crisis would ensue with about four times the standard
Taser capacitance.
Our experiments were the first to document that Taser-like
impulses, albeit more energetic ones, applied close to the heart on
the chest wall in pigs could have serious cardiac consequences. Even
at the standard output of a Taser, we found that current applied to
the most vulnerable part of the chest was able to drive the heart to
beat up to 250 beats per minute, which is about twice the normal
rate for pigs. These experiments also showed us that the onset of
ventricular fibrillation is related to how fast the heart is driven by
the impulses-which scales with the amount of current used.
Because the standard Taser output proved on average to be
one-fourth what was needed to cause fibrillation, one is tempted
to conclude that the device is fundamentally safe. But there's
another factor to keep in mind: a large portion of the violent
individuals with whom the police have to deal are under the influence of cocaine, methamphetamine, or other stimulants. So the
Taser has to be safe even for those whose physiology is distorted
by the presence of such powerful drugs. Cocaine in particular is

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a concern with respect to cardiac complications because it raises
heart rate and blood pressure and significantly increases the risk
of a heart attack even without any kind of shock.
My colleagues and I supposed that the presence of such drugs
would increase the potential for cardiac arrhythmias, and we later
tested this hypothesis in a separate study, published in the Journal
of the American College of Cardiology. To our surprise, the amount of
current needed to bring on ventricular fibrillation didn't go down;
indeed, it increased significantly when the pigs were administered
cocaine. After some thought, we realized that our initially puzzling
findings were not entirely out of line, because cocaine has certain
anesthetic properties that can affect the electrical behavior of the
heart in ways that protect it against shocks and decrease its vulnerability to fibrillation. Applying enough voltage to a heart cell will
open its sodium-ion channels and start the contraction machinery,
but cocaine stops up the voltage-activated sodium channels, making it more difficult for electricity to trigger a muscle contraction.
Another study carried out at our clinic more recently showed
that implantable defibrillators and pacemakers function normally
after a typical 5-second electric shock from a Taser. It remains
to be seen, however, how well such medical devices stand up to
repeated or longer shocks.
It is a challenge to relate experiments conducted under controlled
laboratory conditions to the vagaries of real life. For one thing, we
obtained our results from anaesthetized pigs with ostensibly normal hearts. It's possible that an abnormal or diseased heart-or even
a heart under stress or one affected by amphetamines-might be
more vulnerable. No one has yet studied the effects of Taser shocks
on such hearts, information that is sorely needed to understand
what might prove to be the greatest danger from Tasers.
Even so, we were comforted to learn that stun guns do not normally pose any cardiac risk. The full length of the Taser dart tip
would have to embed itself into the skin and chest-wall muscle of
a relatively small, thin person to get within the range of distances
where we found the heart to be most vulnerable. Furthermore, the
most sensitive region for the induction of fibrillation covers just a
small area. And it is unlikely that two darts would land there.
Much remains unknown about the physiological effects of a
Taser shot, but the absence of conclusive medical knowledge doesn't
necessarily mean that the devices shouldn't be used-as long as
evidence continues to support their safety. Rarely is any biological
phenomenon or medical device fully understood and tested, and
the Taser is no exception. As more information becomes available,
law-enforcement agencies and their officers will better understand
the consequences of each pull of the trigger.
n

About thE AuthorS
MArk w. kroll is an IEEE senior member who holds more
than 250 u.S. patents as an inventor of electrical medical
devices. he sits on the board of taser International. pAtrIck
tchou is a cardiologist who specializes in treating cardiac rhythm disturbances at the cleveland clinic, a leading
research hospital in ohio.
to probE furthEr
the police Executive research forum's report on standards
for "conducted energy devices" is on its web site at http://
www.policeforum.org/library.asp?MENu=356.
recent u.S. Department of Justice findings on arrestrelated deaths can be found at http://www.ojp.usdoj.gov/
bjs/abstract/ardus05.htm.
the Institute for the prevention of In-custody Deaths has
related research available at http://www.incustodydeath.com.
December 2007 | IEEE Spectrum | NA 31

http://http:// http://www.policeforum.org/library.asp?MENu=356 http://www.ojp.usdoj.gov/ http://www.incustodydeath.com http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum December, 2007