Ventricular fibrillation and shortening, alternans and after-depolarizations of epicardial monophasic action potentials during coronary occlusion andreperfusion - Effect of repetition of ischemia

The relationship between the occurrence of ventricular fibrillation (VF) an d repolarization abnormalities of the ischemic and reperfused myocardium is poorly understood. The present study examined the temporal relationship be tween ischemia- and reperfusion-induced changes in monophasic action potent ial (MAP) configurations and the occurrence of VF, and assessed the effects of repetition of ischemia. The left anterior descending coronary artery of 32 anesthetized dogs was occluded twice for 5 min, 30min apart, during con stant atrial pacing while recording MAPs from the epicardial ischemic zone. During the first occlusion, shortening of the MAP duration at 90% repolari zation (APD(90)) and an increase in MAP alternans, defined as the maximal d ifference in APD90 between 2 consecutive beats, were observed. Afterdepolar izations also occurred transiently in 35% of the animals during occlusion a nd in 29% upon reperfusion. VF occurred in 28% (9/32 of the dogs) during th e first sequence, and the incidence was higher in the subgroups with maxima l alternans greater than or equal to 20 ms (p<0.05), maximal shortening rat e greater than or equal to 30%, and afterdepolarizations. During the second sequence, the incidence of VF was reduced to 9% (3/32, p<0.05), associated with a significant reduction in the MAP changes. Thus, repolarization abno rmalities of the ischemic and reperfused myocardium appear to be related to the occurrence of VF. The amelioration of the repolarization abnormalities by repetition of ischemia may be involved in its antifibrillatory effect.