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.