Ventricular defibrillation with myocardial electrodes in the dog, calf, pony, and pig.

The defibrillation effectiveness of 4-msec rectangular wave shocks was evaluated using myocardial electrodes with 25-kg dogs and (nominally) 100-kg calves, ponies, and pigs as part of an evaluation of the suitability of these animals as models for automatic implanted defibrillator research. The percent success of defibrillation attempts in dogs ranged from 44% with 4-A, 6-J shocks to 93% with 8-A, 16-J shocks. The success achieved in calves with scaled electrodes was uniformly lower and reached a maximum of 28% with 23-A, 107-J shocks. Pigs defibrillated with scaled electrodes at a success level (43%) more comparable to dogs (54%) at the 12-A, 25-J level. Using larger electrodes in calves, only 63% success was attained with 25-A, 52-J shocks; lower (14-A, 22-J) and higher (35-A, 100-J; 47-A, 176-J) shocks were less than 30% effective. Ponies were refractory to defibrillation at up to the 25-A, 100-J level with the large electrodes; with 21-A, 43-J shocks only 42% success in pigs was achieved. In general, there were marked differences between the responses of dogs and calves that are inconsistent with the closed-chest defibrillation behavior of both animals. Failure to achieve reliable and highly successful defibrillation in the larger animals may indicate their possible unsuitability as models in implanted defibrillator research.