being successfully terminated, is considered recurrent.
Distinguishing between these two presentations of
post-shock v fib can be clinically challenging since, under
current American Heart Association Guidelines, CPR is
typically resumed immediately after shock delivery. During this time, the ECG is usually distorted by chest compression artifact and not easily interpretable until the next
pause in CPR about two minutes later.
being located too high, too low, or too medially in relation to the heart) may misdirect needed current to surrounding organs instead of to the heart itself, resulting
in a failed shock regardless of its strength.
If placed too close together, the current will take the path
of least resistance and be shunted between the patch electrodes themselves, bypassing the heart completely.
In studies where rhythms during these two minute periods of post-shock CPR were formally analyzed by experts
using techniques that minimized chest compression artifact, v fib was most often found to have been successfully
terminated by the shock; but its effects were short-lived
because v fib subsequently recurred.1,9
The direction of current flow across the heart can also
be a factor in defibrillation success. Biphasic defibrillation
takes advantage of this by changing the direction of the
shock in midstream, resulting in a current that heads in
one direction, followed by taking the opposite direction.
Such recurrent v fib is usually caused by ongoing myocardial ischemia or other characteristics of the heart fostering
electrical instability, not shock failure, and is best treated
with rhythm-stabilizing drugs or other medical measures
along with standard shocks. Deploying dual shocks in
such circumstances isn’t only unnecessary, but hazardous.
Similarly, changing pad location (e.g., from anterolateral to anterior-posterior) creates a differently-oriented
shock. Because this shock traverses the heart from an
alternate direction, it might more effectively reach different regions of fibrillating myocardium.
In addition to risking greater electrical injury to the heart,
dual shocks can expose the circuitry of one defibrillator to
a large shock from the other, potentially damaging one or
both devices and rendering them useless.10 Accordingly, this
practice is discouraged by all defibrillator manufacturers.
Indeed, the success frequently attributed to dual defibrillator shocks may be due to differences in pad location and
the altered direction of the second shock compared to the
first, rather than to the effects from the dual shock itself.
Why Do Shocks Sometimes Fail?
Finally, defibrillation also has an element of chance. During v fib, cells in the heart are in varying states of depolarization, repolarization or resting. Each state responds
differently to shock, such that the effect of a shock can
vary from one moment to the next depending on the
phase of those cells at that instant.12
The answer here requires a brief lesson in electricity. Defibrillator settings are displayed as joules, but “joules” don’t
actually defibrillate the heart; it’s current (amps) applied
over time that accomplishes this task.6 Here’s how it happens. The defibrillator shock creates a voltage gradient
through the tissues between the patient’s two electrode
pads (i.e., high voltage on one end, and low voltage on the
other). Sustaining this voltage gradient for a sufficient
time interval is what ultimately “pushes” enough current
across the heart, resulting in defibrillation.
This explains why, all other things being equal, an identical energy shock may defibrillate on one occasion but
not on another. In fact, the occasional apparent efficacy
of a double shock doesn’t necessarily mean that another
standard shock wouldn’t have worked as well!
If Used, How Quickly Should Dual
Defibrillator Shocks Be Given?
The magnitude of the current, in turn, depends on the
resistance between the two pads. This resistance is related
to electrode size and skin contact, along with the resistance of the organs lying between the electrodes.
Timing is critical when successive shocks are administered
from two separate defibrillators, and is perhaps the least
appreciated pitfall of dual defibrillation. Human reaction
time is about 200 milliseconds (0.2 seconds).
Anything that increases resistance along the path of this
voltage gradient results in a smaller current. For example,
if the interface between the patches and skin creates an
unusually high resistance, much of the energy from the
defibrillator will be dissipated in the patches or at the
skin level, with little current making its way to the heart.
Thus, even one person pushing two separate buttons
on two separate defibrillators, whether simultaneously or
sequentially, doesn’t guarantee that the resulting shocks
will be administered with the desired timing; these can be
off-target by 200 milliseconds in any direction. By comparison, the therapeutic window for double shocks is
much shorter and its margin of safety extremely narrow.
This often occurs because of poor electrical contact
between the pads and skin. Resistance—not energy—is
often the major roadblock to effective defibrillation.
Although applying higher energy can partly compensate for the problem, if resistance itself isn’t addressed,
the effort will usually be self-defeating.
To illustrate, experimental work has shown that the
interval between the delivery of dual biphasic shocks must
either be < 10 milliseconds (0.01 seconds) or 75–125 milliseconds (0.075–0.125 seconds) apart to improve defibrillation efficacy; whereas if the second shock follows the first
by longer periods, its effects are more likely to be one-in-the-same as two separately administered single shocks.
Suboptimal defibrillator pad location is yet another correctable obstacle to successful defibrillation.11 Defibrillator pads that fail to encompass the heart completely (by
More ominous, if the delay between the each of the dual
shocks is 10–75 milliseconds (0.01–0.075 seconds), v fib
becomes more difficult to defibrillate or can even result
A Supplement to JEMS