in the re-induction of v fib by the second shock that had
already been terminated by the first one.13
In sum, the degree of precision in button-pushing
required to safely and effectively administer dual biphasic
shocks isn’t only daunting, but in light of reaction time
probably not even close to achievable by human hands.
Are Dual Shocks Still Biphasic?
The simple answer to this question is yes and no. Unlike
monophasic shock, the biphasic waveform consists of two
phases: an initially “positive” wave (creating a current going
in one direction) followed almost immediately by a “
negative” wave (or current going in the opposite direction).
The “two-waves-in-one” make the biphasic waveform
more effective than monophasic shock, but also more
complicated and less suitable for dual defibrillation. This
is because double defibrillator shocks result in the superimposition or cancellation of one or the other (or both)
of these biphasic shock waves, depending on shock timing and pad location. For example, if the timing of shock
delivery is such that the two positive waves from a double
shock happen to be partly or completely superimposed
upon one another, the resulting higher voltage gradient
will result in some regions of the heart receiving a greater
or even excessive amount of current.
Alternatively, if the timing and direction of a double
shock is such that the positive wave from one shock is partially or wholly cancelled by the negative wave of another,
the resulting lower voltage gradient means other fibrillating
regions of the heart may receive little or no current at all.
Double defibrillation can thus create a myriad of augmented and cancelled shock waves whose net effect on
v fib becomes an uncertain and potentially risky gamble.
Some might even dare say, a “crapshoot.”
Are There Better Alternatives?
To date, evidence supporting a benefit from dual defibrillator biphasic shocks rests entirely on isolated case reports
or small case series with mixed outcomes. Understandably, resorting to dual defibrillator shock stems from
an imperative to “do something!” when other resuscitation interventions and therapies appear to be failing.
The question, however, is whether there are other, better
alternatives for treating such patients? Consider these:
1. Is the v fib/v tach truly shock-refractory, or just recurrent? If recurrent, a change in shock energy is unlikely
to be helpful. Such patients are better benefitted
by other rhythm-stabilizing medical interventions.
2. Are defibrillator pads optimally positioned?
3. Can resistance be minimized? Applying manual pressure to pads can help lower resistance at the electrode
pad-skin interface. Using gloved hands, press a thick,
folded dry towel(s) on one or both pads during shock
delivery. This can substantially reduce pad resistance,
resulting in higher defibrillation success.14
shocks in a different direction (e.g., anterior-posterior
vs. anterolateral) might afford greater success.
5. Is this a circumstance where rapid transport to a
cardiac catheterization laboratory is the ultimate
solution? Shock-refractory v fib and recurrent polymorphic ventricular tachycardia are common signs of
acute myocardial ischemia and infarction, which may
only be correctable by emergent revascularization.
Apart from anecdotal reports, there’s little clinical evidence
to support a benefit from double defibrillator biphasic
shock. Conversely, there are substantial reasons to question its safety, efficacy and necessity. More research is
needed before dual defibrillation is deployed clinically. In
the interim, given the other available approaches to treat
refractory v fib, it’s best to consider the sage advice from Hippocrates himself: Primum non nocere, or, first, do no harm. ✚
Peter J. Kudenchuk, MD, FACP, FACC, FAHA, FHRS, is professor of medicine in the Division of Cardiology/Section of Electro-physiology-Arrhythmia Services at the University of Washington.
He’s also medical program director for King County Medic One.
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