Proarrhythmic effects of ischemic preconditioning in anesthetized pigs

Conflicting data exist on how ischemic preconditioning affects the incidence of arrhythmias during ischemia. The present study was, therefore, performed to clarify if ischemic preconditioning alters the incidence of arrhythmias during the early phase of ischemia in pigs. Twenty-four pigs (23-36.5 kg) in pentobarbital anesthesia were preconditioned by a 10 min LAD-occlusion and 30 min reperfusion prior to a second 10 min occlusion. The first occlusion served as the preconditioning ischemic period, and the second occlusion as the test ischemic period. To verify that the preconditioning protocol induced protection, infarct size was assessed in an additional nine pigs. The arrhythmic index (arrhythmic beats/min) increased from 12+/-3 during the first occlusion to 38+/-8 during the second occlusion (P < 0.05). Four pigs (17%) fibrillated during the first ischemic period, while 10 pigs (42%) fibrillated during the second ischemic period (P < 0.05). All pigs that fibrillated during the first occlusion also fibrillated during the second occlusion. Mean heart rate was 108+/-4 beats/min before the first occlusion and increased to 113+/-4 beats/min before the second occlusion (P < 0.001). There was a positive correlation between heart rate before the second occlusion and occurrence of fibrillation during this occlusion (rs = 0.42; P < 0.05). The preconditioning protocol reduced infarct size, after a subsequent 45 min ischemic period followed by two hours of reperfusion, from 58+/-3% of area at risk to 40+/-5% (P < 0.05). In conclusion, our data show that ischemic preconditioning increases both the arrhythmic index and the incidence of ventricular fibrillation during the early phase of a subsequent ischemic period.