Circulation, Vol 82, 244-260, Copyright © 1990 by American Heart Association
FX Witkowski, PA Penkoske and R Plonsey
The automatic implantable cardioverter-defibrillator has been shown to
dramatically improve survival. The future refinement of these devices
requires a clear understanding of their mechanism of action. We performed
the following study to test two hypotheses: 1) When defibrillation is
successful, fibrillating activity must be annihilated in a critical mass of
both ventricles; and 2) when defibrillation is unsuccessful, at least one
area of the ventricular mass has been left fibrillating. Unipolar Ag/AgCl
sintered electrodes were directly coupled from triangular arrays at 40
epicardial locations (total, 120 recording sites) that covered both right
and left ventricular surfaces and were designed to measure the voltage
gradient generated by the shock at each triangular array as well as the
underlying myocardial electrical activity before and immediately after the
shock. An algorithm was developed and tested that reliably scored whether a
postshock activation was a continuation of the immediately previous
fibrillating activity. This technique was applied to 203 defibrillation
attempts in six open-chest dogs during electrically induced ventricular
fibrillation. There were 139 successful defibrillation attempts and 64
unsuccessful attempts. Monophasic truncated exponential 10-msec
defibrillation shocks (0.5-35 J) were delivered through an anodal patch on
the right atrium and a cathodal patch on the left ventricular apex. In all
cases of unsuccessful defibrillation, at least one ventricular site could
be clearly identified that failed to be defibrillated. In cases of
successful defibrillation two distinct patterns were observed: 1) complete
annihilation of fibrillating activity at all sites or 2) nearly complete
cessation of fibrillating activity with a single area of persistent
fibrillation that subsequently self-extinguished within one to three
activations. This single site in the second form of successful
defibrillation was located in the region of minimum voltage gradient
produced by the defibrillating waveform and was occasionally accompanied by
dynamic encapsulation with refractory tissue as a result of a wavefront
emanating from a region that had undergone successful defibrillation. These
results support the hypothesis that a critical mass of myocardium must be
affected for successful defibrillation and that unsuccessful defibrillation
is always accompanied by residual fibrillating activity in at least one
site. The results also demonstrate that the size of the critical mass
required for successful defibrillation can be less than 100%.
ARTICLES
Mechanism of cardiac defibrillation in open-chest dogs with unipolar DC- coupled simultaneous activation and shock potential recordings
Department of Medicine, University of Alberta School of Medicine, Edmonton, Canada.
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