Recurrent appearance of protective zones after an unsuccessful
defibrillation shock.
Hwang, Chun, Wei Fan, Peng-Sheng Chen.
Division of Cardiology, Department of Medicine, Cedars-Sinai
Medical Center and UCLA School of Medicine, Los Angeles,
California
APStracts 3:0150H, 1996.
This study was designed to test the hypothesis that protective zones
appear recurrently at the initiation of ventricular fibrillation
(VF), and that when shocks are delivered during protective zones
there can be a decrease in the defibrillation energy requirement. A
total of 12 open-chest dogs were studied. Six dogs were included in
protocol-1. Following 8 baseline pacing stimuli (S1) with cycle
lengths of 300 ms, a strong premature stimulus (S2) (73+/-10 mA) was
given to induce VF. In subsequent episodes, a second strong premature
stimulus (S3) was given at progressively longer S2-S3 intervals in 20
ms increments. In protocol-2, we delivered unsuccessful
defibrillation shocks via a transvenous defibrillation electrode
placed in the right ventricular apex of 6 dogs. A second shock was
then delivered to patch electrodes on the right ventricular outflow
tract and the posterior wall of the left ventricle. The results of
protocol-1 showed that the S3 terminated reentry and prevented VF
only when it occurred at specific time intervals after the S2 (the
protective zones). These protective zones appear recurrently up to
375 ms after the onset of VF. The results of protocol-2 showed that
the total energy required for successful defibrillation was dependent
upon the interval between the first and the second shocks. Intervals
in favor of effective defibrillation (protective zones) appeared
recurrently for up to 280 ms after the first shock. When the second
shock was delivered during a protective zone, the defibrillation
energy requirement was decreased by up to 23% (from 13.1 +/- 2.0 J to
10.1 +/- 1.8 J, p<0.003). However, when the shock was delivered
outside the protective zone, a significant increase in the
defibrillation energy requirement was observed. We conclude that
protective zones appear recurrently at the onset of VF, and after
unsuccessful defibrillation shocks.
Received 20 October 1995; accepted in final form 4 April 1996.
APS Manuscript Number H981-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 23 April 96