Progressive transmural electrographic,myocardial potassium ion/sodium ion ratio and ultrastructural changes as a function of time after acute coronary occlusion

The progressive transmural electrographic, biochemical and ultrastructural changes as a function of time after acute coronary occlusion were systematically assessed in eight dogs. Transmural plunge electrodes with poles 1 mm apart were placed in the ischemic and nonischemic zones, and coronary occlusion was maintained for 4 hours. Transmural full thickness biopsy specimens were obtained from each zone for electron microscopy before, and 1 and 4 hours after occlusion. Endocardial and epicardial layers were also obtained for assessment of myocardial potassium ion (K⁺) and sodium ion (Na⁺) concentrations. Before coronary occlusion, local Q waves were recorded an average depth of 1.0 ± 0.34 mm from the endocardial surface. After 1 hour of occlusion, Q waves appeared at an average depth of 3.8 ± 0.67 mm and progressed to a depth of 5.2 ± 0.7 mm at 2 hours, 6.2 ± 0.5 mm at 3 hours and 7.0 ± 0.5 mm at 4 hours. After 1 hour, ultrastructural changes of early ischemia, including a decrease in glycogen and mild mitochondrial swelling, were seen in the endocardial layer; the epicardial layer showed normal morphologic features. After 4 hours, the endocardial layer showed well developed ischemic changes marked by the loss of mitochondrial cristae, vacuolization, the appearance of amorphous mitochondrial densities, an increase in interfibrillary space and the appearance of I bands. In contrast, the epicardial layer at this time showed only early ischemic changes. At the end of 4 hours, the endocardial layer showed a marked decrease in myocardial K⁺ concentration and an increase in Na⁺ concentration leading to complete reversal of the $\text{K}{}^{\mathrm{+}}\text{Na}{}^{\mathrm{+}}$ ratio (0.7 ± 1.0; P < 0.001). In the epicardial layer, a smaller decrease in K⁺ concentration and an increase in Na⁺ concentration occurred, resulting in a diminution but not a reversal of the $\text{K}{}^{\mathrm{+}}\text{Na}{}^{\mathrm{+}}$ ratio (1.4 ± 0.2; P < 0.005).Thus, the dynamic evolution of an acute myocardal infarction involves a sequential progression from endocardium to epicardium as a function of time, resulting in an epicardial “border zone” in the early stages after acute coronary occlusion.