Ventricular fibrillation and defibrillation thresholds in sheep and dogs

Ventricular defibrillation studies normally use dogs rather than other large species. To investigate the suitability of sheep, which are often cheaper and more readily available, we compared ventricular fibrillation and defibrillation thresholds (VFT, DFT) in sheep and dogs. A total of 12 sheep (31 +/- 5 kg) and six dogs (19 +/- 1 kg) were anesthetised with halothane. Fibrillation was induced via epicardial pacing leads, using a 1 s 50 Hz pulse. Biphasic defibrillation shocks were delivered across epicardial patches. Voltage-response curves for both fibrillation and defibrillation were generated. Logistic regression analysis was used to determine 50 and 90% probability of success for fibrillation induction and defibrillation. VFT was similar in sheep and dogs. DFT at 50% probability of success was significantly higher in sheep (369 +/- 14 V) than in dogs (299 +/- 31 V, P < 0.04) but within each species there was no correlation between heart weight and DFT. After defibrillation sheep took longer to return to sinus rhythm than dogs and electro-mechanical dissociation was observed in sheep, but not in dogs. Thus, sheep may not be an ideal model for ventricular defibrillation research but further studies of the intrinsic differences between sheep and dogs may provide insights into basic mechanisms of defibrillation.