Phase 2 Reentry as a Trigger to Initiate Ventricular Fibrillation During Early Acute Myocardial Ischemia

BACKGROUND: Phase 2 reentry caused by heterogeneous loss of the transient outward potassium current (I(to))-mediated epicardial action potential (AP) dome can produce a closely coupled R-on-T extrasystole leading to ventricular fibrillation (VF) under conditions of ST-segment elevation unrelated to ischemia. The present study examined the role of phase 2 reentry in the initiation of VF during early myocardial ischemia. METHODS AND RESULTS: Regional myocardial ischemia was produced in an isolated, arterially perfused canine right ventricular wedge preparation. Transmembrane APs from 2 epicardial sites at each side of the ischemic border were simultaneously recorded together with measurements of extracellular potassium concentration (K+o) and a transmural ECG. Loss of the I(to)-mediated epicardial AP dome in the ischemic zone but not in the perfused tissue resulted in phase 2 reentry and associated R-on-T extrasystoles capable of initiating VF in 7 of 15 preparations during the first 3 to 9 minutes of myocardial ischemia, with marked ST-segment elevation and K+o accumulation. The I(to) and phase 1 magnitude of epicardium contributed importantly to the onset of VF. Phase 1 magnitude and I(to) density at +30 mV in the group with phase 2 reentry-related R-on-T extrasystoles were 32.2+/-1.3 mV and 30.3+/-0.5 pA/pF (n=7), respectively, significantly greater than those (24.0+/-1.8 mV and 23.2+/-1.0 pA/pF) in the group without the extrasystoles (n=8, P<0.01). CONCLUSIONS: Acute regional myocardial ischemia results in markedly heterogeneous loss of I(to)-mediated epicardial AP domes across the ischemic border, leading to phase 2 reentry. Phase 2 reentry can in turn produce an R-on-T extrasystole capable of initiating VF.