Abstract
Ventricular fibrillation thresholds were determined by delivering gated trains of pulses to the ventricle, spanning the vulnerable period. The coupling interval of premature beats decreased and the number of subsequent repetitive responses increased with an increase in the intensity of gated stimuli. Although fibrillation thresholds were determined before and during coronary occlusion at an identical site unaffected by ischemia, they were consistently lower in the ventricle with an ischemic area. The decrease in fibrillation threshold resulted because the ventricle with an ischemic area could be fibrillated by premature beats with longer coupling intervals and fewer subsequent repetitive responses. It was concluded that reentrant activity and resulting fibrillation are more likely to be induced in the ventricle with an ischemic area because of the increased irregularity in the propagation of premature impulses. This irregularity is a result of a marked depression of excitability and conduction velocity in the ischemic area. The results emphasize the greater chance that premature ventricular beats will induce fibrillation in patients with myocardial infarction.
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