IEEE Spectrum November, 2008 - 32

brillation in half, from 400 J to 200 J, while significantly reducWith the accompanying reduction in peak voltages from
ing the failure rate. The first, which took hold in the 1980s, was 4000 V to less than 2000 V, the gas arc relay was dropped;
to eliminate the bulky paddles.
switching is now performed by modern compact insulated-gate
Unlike the electrons that carry current through wires, current bipolar transistors. These are configured in a classic H-bridge
in the body is carried by ions, chiefly sodium, potassium, and circuit, the component that allows motor controllers to run forchlorine. When the electric potential across a cardiac cell's mem- ward or backward [see "Tender Loving Shock"]. Depending
brane reaches a certain threshold, ion channels open, allowing on which two of its four switches are closed, the circuit can
ions to enter the cell and trigger muscle contraction. In place of deliver a normal or reversed-polarity voltage. Two switches
the defibrillator's paddles, biomedical engineers developed flex- are turned on for 5 to 8 milliseconds to deliver the main shock.
ible adhesive patches coated with a metal chloride gel (such as Immediately afterward, the two remaining switches are turned
tin chloride) to transfer current from the wires to the body. Each on to deliver the residual capacitor charge and perform the
patch has a different polarity: on the patch with a negative polar- burping function of the second phase.
ity, for example, an electron from the wire replaces a negative ion
Taken together, these changes reduced the weight of the
from the chloride in the gel, which frees the chloride ion to pass unit from 40 kg to 1.5 kg and made it safer to operate. Further
through the skin and carry current into the body.
advances in the capacitor, battery, and high-voltage semiThe patches reduced the typical contact resistance from conductors should eventually reduce the size of the AED to
about 150 ohms to about 75 ohms, which allowed for smaller that of a deck of playing cards.
voltages. The lower voltages meant that defibrillators could be
built with higher-density electrolytic capacitors and smaller
he next challenge was to design the brains of the
semiconductor switches. Thanks to the adhesive patches, the
machine. The defibrillator had to figure out, on its own,
defibrillation operation now required only one person.
when to deliver a shock.
However, these gelled patches sometimes dried out, so paraThe heartbeat is most vulnerable during a period known
medics had to check their electrodes every day. Two of us (Karl as the T wave. The T wave occurs just as the heart is beginKroll and Byron Gilman) had the idea of packaging the two ning to relax after contraction and lasts about 100 ms, or about
pads together with a partially conductive release liner between one-tenth of a heartbeat. A shock administered to a nonfibrilthem. Now the AED was able to pass a small shock between the lating heart during the T wave could potentially induce fibrilpatches to prevent them from getting dried out.
lation. If the AED's electrode patches are positioned far from
The other big advance was to switch to the modern biphasic the target, such as on the belly, the current that finally reaches
waveform, which, in addition to its higher efficacy, also reduces the heart could be sufficient-if it arrives during that vulnerthe power requirements for defibrillation [see "Wave Sculpting"]. able period-to induce fibrillation, but it may not be strong
One of us (Mark W. Kroll) was the
first to publish a quantitative description of the biphasic waveform, in
1994. In essence, the first phase of the
shock charges the cell membranes;
the second phase, where the curArc relay
rent reverses, returns the cell membranes to zero voltage. We still don't
Inductor
know entirely why the biphasic waveC
Paddles
form is so effective, and we have yet
(4000 V)
to learn of any analogues in the rest
of biology. One purpose of the second
phase appears to be to discharge and
heal the blasted-open membranes of
cells closest to the electrodes (which
receive the most extreme current)
and to discharge cells that are only
marginally charged. Mark coined the
MicroAdhesive pads
controller
term "burping" (based on the idea of
C
a mother burping excess gas from an
(2000 V)
infant after feeding it) to describe this
strange phenomenon.
Because biphasic wavefor ms
require less power than their precursors, the size of a defibrillator's comEKG system
ponents could also shrink. The heavy
metal film capacitor was replaced by
a lightweight bank of aluminum electeNder lOviNg sHOcK: in the original defibrillator circuit [top], a manually tripped gas
trolytic capacitors connected in series,
arc relay delivered the shock. in modern aeD circuitry [bottom], the shock and the eKg
and the heavy iron inductor was elimsignals pass through the same electrodes. an h-bridge circuit, shown in red, generates
the biphasic waveform by flipping the voltage at the right time. IllUstRatIon: eMIlY CoopeR
inated altogether, as it was no longer
needed to reduce the peak currents.

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2008