FLAKER, G., ET AL.: The Effect of Multiple Shocks on Canine Cardiac Defibrillation. To determine if multiple shocks adversely affect the success of later shocks compared with early shocks, we analyzed the success rates of initial shocks (defibrillation attempts 1–5), first half shocks (defibrillation attempts 1–20) and second half shocks [defibrillation attempts 21–40) in a canine model. Epicardial patches were placed on the right and left ventricle in 28 dogs. Ventricular fibrillation was induced by a 60-Hz shock. After 30 seconds, defibrillation was attempted using 7, 12, 13, or 18 joules with either a uniphasic or biphasic rectangular waveform. The uniphasic waveform was 5 msec in duration; the biphasic waveform was 10 msec, with the lagging 5-msec pulse one-half the amplitude of the leading 5-msec pulse. For uniphasic shocks, the right ventricular patch was positive; for biphasic shocks, the right ventricular patch was positive during the leading 5 msec of the shock and negative during the lagging milliseconds. A total of 960 fibrillation episodes were evaluated; no dog was involved in more than 40 fibrillation episodes. The success rates of defibrillation attempts 1–5, defibrillation attempts 1–20, and defibrillation attempts 21–40 were similar at 12, 13, and 18 joules. This information supports the continued use of up to 40 fibrillation trials in canine cardiac defibrillation. However, at 7 joules defibrillation attempts 21–40 were more successful than defibrillation attempts 1–5, and 1–20. With our methodology, these data are consistent with the hypothesis that low energy shocks create a “sensitizing” effect on cardiac tissue, allowing more successful defibrillation with repeated shocks.