Relationship Between Myocardial Infarct Size

We compared myocardial infarct size produced by permanent occlusion of the middle left anterior descending (LAD) or circumflex (LCx) coronary artery in the anesthetized dog. The dogs were killed 3-10 days later, and the occluded coronary bed was visualized by postmortem arteriography. The outlines of the infarct and occluded bed were marked on tracings of weighed left ventricular (LV) rings and the size of the infarct and occluded bed was calculated by planimetry. For both arteries, infarct size and occluded bed size were linearly related to each other, but LAD infarcts were larger relative to occluded bed size (52.0% vs 32.3%, p < 0.05). A smaller occluded bed was necessary for the appearance of an infarct after LAD occlusion than after LCx occlusion (8.3% vs 18.5% of the left ventricle, p < 0.005). Reconstructed LV ring maps indicated a significantly wider margin of noninfarcted myocardium at the lateral edge of the occluded bed for LCx infarcts than for LAD infarcts. For dogs with similar occluded bed sizes in the range of 20-35% of the left ventricle, infarct size was considerably larger for LAD occlusion (15.9% vs 6.1% of the left ventricle, p < 0.001). In this subgroup, blood pressure and heart rate 10-20 minutes after occlusion were not significantly different for the two arteries, but collateral flow, measured with 9-,u radioactive microspheres, was approximately 50% lower after LAD occlusion. The relationship between the amount of myocardium with reduced blood flow and developed infarct size was similar for the two arteries. We conclude that occlusions of the middle LAD and LCx are not equivalent. For a given occluded bed size, LAD occlusions produce larger areas of infarction, apparently related to lower levels of collateral flow delivered to the occluded region.