The purpose of this study was to examine the effects of: (1) time to defibrillation and (2) subthreshold preshocks on defibrillation success. We conducted two separate experiments in 19 anaesthetized, open-chested pigs. Defibrillation was attempted using the sequential pulse technique approximately 10 s after electrically induced ventricular fibrillation. Each sequential pulse shock consisted of two trapezoidal puises (approximately 3 ms duration), separated by 0.2 ms. Current was delivered to three mesh electrodes (TX-7, Medtronic) sutured over the anterior right ventricle, posterior right ventricle, and lateral left ventricle. For each animal, defibrillation threshold (DFT) defined as the lowest delivered energy that defibrillated the heart, was measured twice and the average was designated as mean DFT. The energy at 1.4 times the specific mean DFT for each pig was designated as the test shock and 100 volts less than the mean DFT was designated as the preshock.
In the first experiment, test shocks were delivered at five different fibrillation intervals (10, 20, 40. 60, and 90 s). Time to test shock delivery was randomized for 10 to 60s, but, 90 s was always tested at the end of the study. Percentages of success at 10, 20, 40, 60, and 90 s were 94, 78, 94, 83, and 100%, respectively (p = NS). The 12 pigs in which all initial test shocks were successful were selected for the evaluation of post defibrillation arrhythmias. The cumulative incidence of complete heart block lasting at least 5 s were 8, 33, 83, 83, and 83%, respectively. The incidence of complete heart block increased significantly at 40 s (p < 0.001). Ventricular arrhythmias were observed on six occasions at 40 s or later.
Following experiment 1, the pigs were allocated to three groups (n = 6 for each group). Group 1 received one preshock, Group 2, two preshocks and Group 3, three preshocks. Each pig had 16 test shocks, eight with no preshock and eight with one of single, double or triple preshocks within 15 s after ventricular fibrillation onset. The test shock was delivered at 20 s. There were no intra- or intergroup differences in percentage of success regardless of number of subthreshold preshocks.
We conclude that delay of defibrillation therapy up to 90 s and delivery of preshocks have no significant effect on the ability to defibrillate the heart in this model. However, delay to 40 s and longer is associated with a higher incidence of complete heart block and ventricular arrhythmias.