Time course of changes in ventricular fibrillation threshold in myocardial infarction: characteristics of acute and slow occlusion with respect to the collateral vessels of the heart.

The time course of the changes in ventricular fibrillation threshold (VFT) was determined in 28 dogs by means of electrical stimulation, triggered from the R wave, with DC square wave pulse series (of 140 ms duration) inserted into the vulnerable period of the cardiac cycle. After acute coronary occlusion there was a sudden decrease in the VFT followed by a slow increase such that 30 min after the ligation pre-occlusion values of VFT were reached. The magnitude of the decrease in VFT depended upon the collateral blood supply. There were no other changes in VFT during the remaining course of the experiment. Following slow coronary occlusion the first arrhythmic phase was not detectable. Any decreases in VFT that occurred were smaller and of shorter duration than those occurring after acute occlusion and were independent of the extent of the collateral blood supply.