Results
Typical ST T sequence of changes in unipolar electrograms recorded from the ischemic myocardium is shown in Figure 1B. These recordings were obtained from an electrode 2 mm below the epicardial surface on needle D10 of the standardized array in figure 1A. Within 90 seconds of occlusion ischemic changes were well developed and by 3 minutes alternans in the ST-T segment become very marked. Such alternans occurred in all of the 15 hearts we studied.
Of the fifteen hearts we studied only five developed ventricular tachycardias that degenerated into ventricular fibrillation. In those animals that did have arrhythmias, the alternans described further evolved into more complex cycles in which each morphology of the electrogram was repeated every 3-7 sinus beats as illustrated in Figure 2A. In these 5 hearts, the last beat of each sequence was usually followed by a spontaneous premature ventricular beat. In 3 of the 5 experiments with arrhythmias the alternans evolved to produce increasingly delayed intermittent potentials of varying magnitude which extended into diastole (Figure 2B). These delayed potentials were at their earliest at the epicardium and extended transmurally with increasing delay towards the endocardium. When maximally expressed, as shown in Figure 2B, they culminated in a ventricular premature beat.
The earliest site of activation of PVC was always found in the sub-endocardium overlying the ischemic region, well away from the border region Figure 3. The epicardial breakthrough always occurred at the border of the ischemic region with separate broad wave fronts sometimes appearing in diametrically opposite locations as shown in Figure 3.