Reduction of ST elevation in repeated coronary occlusion model depends on both altered metabolic response and conduction property

The aim of this study was to elucidate the mechanisms of altered electrical response to ischemia in repeated coronary occlusion model. To test its dependence on metabolic response, extracellular K+ concentration (eKC), myocardial pH and Pco2 were simultaneously measured with epicardial ECG during three consecutive 4 min of left anterior descending coronary artery (LAD) occlusion separated by 15 min of reperfusion in canine hearts. ECG changes induced by infusion of high K+-buffer (10 mM) into the coronary arterial bed via carotid artery–LAD bypass (referred to as high K+-challenges: HKC) were also tested prior to (the first HKC), and during each reperfusion period (the second to the fourth HKC). ST elevation was significantly reduced in subsequent occlusions (3.14±0.48 and 2.98±0.47 mV in the second and third occlusion, both P<0.05, compared to 4.91±0.78 mV in the first). This was accompanied by significant attenuation of the changes in eKC, tissue pH and Pco2. ST elevation induced by HKC also significantly reduced after repeated occlusion (4.09±0.79 mV in the fourth HKC vs. 5.64±0.68 mV in the first, P<0.05) in spite of the identical changes in eKC during HKC. This progressive decrease in ST changes by HKC was rather consistent with augmented conduction delay (86.4±7.1% increase in activation time in the fourth vs. 54.3±3.4% in the first, P<0.01). These findings indicate that repeated ischemia induces altered electrical response to subsequent ischemia based on both attenuated metabolic response and altered conduction property.