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 (E50) and 80% success (E80) 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 E50 (5.0 +/- 2.1 J vs 5.2 +/- 2.7 J) or E80 (6.3 +/- 2.5 J vs 6.6 +/- 3.2 J) was observed.(ABSTRACT TRUNCATED AT 250 WORDS)