THE DEFIBRILLATION THRESHOLD - A COMPARISON OF ANESTHETICS AND MEASUREMENT METHODS

In 18 open-chested mongrel dogs (18.0 +/- 1.7 kg) we compared three anesthetics and three methods for measuring the defibrillation threshold. Six animals were anesthetized with pentobarbital (30 mg/kg) and maintained with a pentobarbital infusion (4 mg/kg per hour). All other animals were anesthetized with sodium brevital (10 mg/kg) and maintained with either halothane gas (1.5%, N = 6) or isoflurane gas (1.8%, N = 6). In each dog, we measured the energy required for 50% successful defibrillation (E50) with: (A) a 3 reversal-up/down method; (B) a 15 shock-up/down method; and (C) a percent success method. Anesthetics and methods were selected in a balanced random order. Ventricular fibrillation was induced with 50 Hz electrical pacing. After 15 seconds, monophasic truncated exponential shocks were delivered by way of a spring-patch electrode configuration. The animal was rescued (if needed) and fibrillation/defibrillation episodes were repeated at 3 minute intervals. After each determination of the E50, the E50 was delivered in ten successive defibrillation trials to determine its actual success rate. We found no significant difference in E50 among anesthetics; a significant difference (P < 0.05) in E50 between method C (9.7 +/- 2.6 joules) and method B (8.2 +/- 1.6 joules); no significant difference among anesthetics or methods for the actual success rate of the E50 (45 +/- 21% successful); and method A required significantly fewer fibrillation episodes and number of shocks and less cumulative energy than the other methods. We concluded that the anesthetics tested had little effect on E50 but that the method used to determine E50 could have an effect. Also, the E50 estimated by all methods consistently produced an actual success rate lower than 50%.