Circulation, Vol 85, 1865-1878, Copyright © 1992 by American Heart Association
SM Dillon
BACKGROUND. It is currently believed that defibrillation shocks act
primarily by stimulating excitable myocardium to abolish wave fronts.
Recent studies have shown that shocks applied during pacing not only
stimulate excitable myocardium but also prolong the depolarization and
refractoriness of myocardium already in a depolarized state. This study
investigates the effects of shocks on fibrillation action potentials.
METHODS AND RESULTS. Recordings of membrane action potentials free of shock
artifact were obtained using the voltage-sensitive dye WW781 during
defibrillation of isolated rabbit hearts. These records showed that the
shocks caused an additional phase of depolarization beginning with an
initial rapid depolarization of the optical signal followed by a slow phase
of repolarization. This occurred throughout all phases of the fibrillation
action potential from just after completion of the upstroke to a time of
near maximal repolarization. Defibrillation shocks, however, had the
additional effect of causing the myocardium to repolarize at a constant
time after the shock regardless of its prior electrical activity--the
constant repolarization time response. This effect was not dependent on the
presence of D600 (methoxyverapamil) or continuous coronary perfusion. It
was accompanied by a similar constancy in the return of myocardial
excitability. Recordings taken from multiple adjacent recording sites also
showed a constant repolarization time among them. CONCLUSIONS. A simple
model of reentry is used to illustrate how the constant repolarization
response, in addition to wave front termination and refractoriness
extension, could play a role in the successful termination of fibrillation
by electrical shock.
ARTICLES
Synchronized repolarization after defibrillation shocks. A possible component of the defibrillation process demonstrated by optical recordings in rabbit heart
Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York City, NY 10032.
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