The relationship between successful defibrillation and delivered energy in open-chest dogs: Reappraisal of the “defibrillation threshold” concept

The traditional assessment of the energy required for successful ventricular defibrillation involves the measurement of a “defibrillation threshold” (DFT), implying a clear-cut distinction between effective and ineffective energies. We examined the relationship between delivered energy and the likelihood of successful defibrillation in 10 open-chest pentobarbital anesthetized dogs, with the use of an internal spring/patch electrode system. An initial DFT was determined by decreasing the energy discharged until a failure first occurred (10.3 ± 3.4 J). Six energy levels in 1 to 2 J increments were then selected surrounding this value and each was administered eight times in balanced random order (total 48 trials). The relationship between energy and percent success in defibrillation exhibited a shallow slope, with a gradual increase in success from 0% to 100% over several energy increments. The initial DFTs showed actual success rates varying from 25% to 87.5% (mean 71 ± 26%). The results were fitted to a sigmoidal dose-response curve by logistic regression analysis and the energy associated with 50% success (E 50) and 80% success (E 80) was determined, as no single value for DFT could be defined in any animal. In 12 other dogs, a defibrillation curve was similarly constructed at baseline and was repeated after 90 minutes. No significant change in E 50 (5.0 ± 2.1 J vs 5.2 ± 2.7 J) or E 80 (6.3 ± 2.5 J vs 6.6 ± 3.2 J) was observed. We conclude that no unique DFT exists, and that the relation between the likelihood of successful defibrilliation and dellvered energy is best described by a “dose-response” curve requiring multiple trials for evaluation. Such curves are reproducible and establish a sensitive method for evaluating changes in defibrillation energy requirements.