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Objective: Acute ischemia comes with two phases of life-threatening arrhythmias, early (within 10 minutes, 1A) and late (after about 15 minutes, 1B). The mechanism of the latter is unknown and in this paper, we test the hypothesis that a phase of intermediate coupling between surviving epicardium and inexcitable midmyocardium underlies 1B arrhythmias., Methods: Pig hearts (n=26) were retrogradely perfused with a blood Tyrode's mixture. The left anterior descending artery was occluded. We investigated (1) inducibility of ventricular fibrillation (VF) with programmed stimulation, (2) tissue impedance (Rt) heterogeneity within the ischemic zone, (3) multiple subepicardial and midmyocardial electrograms, (4) subepicardial lactate dehydrogenase (LDH) and glycogen content., Results: In nine of ten hearts, one--three premature stimuli caused VF between 14 and 53 min of ischemia. This typically happened when the Rt of the ischemic zone had increased up to 40% of its final value. More uncoupling terminated the period of VF inducibility. The excitability of the surviving subepicardial layer was depressed during the same period with partial uncoupling, but recovered when the uncoupling from the midmyocardium had progressed further., Conclusions: We show that 1B-VF can be induced within a distinct time window and coincides with a distinct range of Rt rise. Subepicardium is electrically depressed, presumably through coupling with midmyocardium, complete uncoupling causes subepicardial recovery and terminates the substrate for 1B-VF. Hence, we suggest that the substrate for 1B-VF consists of intermediate coupling of subepicardium and midmyocardium.