Comparison of myocardial damage from defibrillator discharges at various dosages

To study the difference in myocardial damage produced by the same defibrillation energy delivered at frequent, low-energy discharges vs infrequent, high-energy discharges, experiments were performed on 48 dogs. In Part I, a total of 3000 Wsec energy was delivered; in Part II, a total of 600 Wsec was delivered; and in Parts III and IV, a total of 1800 Wsec was delivered. In half the dogs, the energy was delivered in three shocks within 135 sec. In the other half, the shocks were delivered by 10 smaller shocks at 15-sec intervals. The degree of myocardial damage assessed by St-segment elevation on precordial mapping or necrotic index from gross and microscopic examination 4 days after the shocks showed no significant difference. However, the three high-energy shocks (600 to 1000 Wsec) resulted in ventricular fibrillation or complete heart block in 33% (6 of 18) of the dogs. These arrhythmias were not observed in the animals receiving low-energy (60-, 200-, or 300-Wsec) discharges.