RECURRENT APPEARANCE OF PROTECTIVE ZONES AFTER AN UNSUCCESSFUL DEFIBRILLATION SHOCK

This study was designed to test the hypothesis that protective zones a ppear recurrently at the initiation of ventricular fibrillation (VF) a nd that when shocks are delivered during protective zones, there can b e a decrease in the defibrillation energy requirement. A total of 12 o pen-chest dogs were studied. Six dogs were included in protocol 1. Aft er eight baseline pacing stimuli (S-1) with cycle lengths of 300 ms, a strong premature stimulus (S-2) (73 +/- 10 mA) was given to induce VF . In subsequent episodes, a second strong premature stimulus (S-3) was given at progressively longer S-2-S-3 intervals in 20-ms increments. In protocol 2, we delivered unsuccessful defibrillation shocks via a t ransvenous defibrillation electrode placed in the right ventricular ap ex of six dogs. A second shock was then delivered to patch electrodes on the right ventricular outflow tract and the posterior wall of the l eft ventricle. The results of protocol 1 showed that the S-3 terminate d reentry and prevented VF only when it occurred at specific time inte rvals after the St (the protective zones). These protective zones appe ar recurrently up to 375 ms after the onset of VF. The results of prot ocol 2 showed that the total energy required for successful defibrilla tion was dependent on the interval between the first and second shocks . Intervals favoring effective defibrillation (protective zones) appea red recurrently for up to 280 ms after the first shock. When the secon d shock was delivered during a protective zone, the defibrillation ene rgy requirement was decreased by up to 23% (from 13.1 +/- 2.0 to 10.1 +/- 1.8 J, P < 0.003). However, when the shock was delivered outside t he protective zone, a significant increase in the defibrillation energ y requirement was observed. We conclude that protective zones appear r ecurrently at the onset of VF and after unsuccessful defibrillation sh ocks.