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SUMMARY. The present study evaluates the transitional or border zone of intermediate blood flow reduction between nonischemic and ischemic regions after acute coronary artery occlusion in chronically instrumented dogs, using methods that minimize an admixture of ischemic and nonischemic myocardium in the tissue analyzed. The regions perfused by occluded and nonoccluded vessels were identified by tracing the extra and intramural distribution of the coronary vasculature from postmortem angiograms. Regional blood flow was evaluated in serial 3-mm-wide epicardial and endocardial zones from outside and inside the interface between occluded and nonoccluded vessels. The zone of intermediate reduction in blood flow between nonischemic and ischemic regions occurred in the first 3-mm section immediately inside the region supplied by the occluded vasculature. Mean blood flow in this region was reduced to 58 ± 6% and 61 ± 5% (±SEM) of nonischemic region blood flow at the lateral and medial epicardial margins, respectively, and 47 ± 5% and 45 ± 6% at the lateral and medial endocardial margins, respectively. In the remaining ischemic zone, significant differences in blood flow existed between epicardial and endocardial layers; these differences were highly variable between animals. The data indicate that when the analysis of regional blood flow following acute ischemia is based on the anatomic distribution of the coronary vasculature, the transitional or border zone of intermediate reduction in blood flow is limited to a narrow zone immediately inside the occluded vasculature. Studies performed in acutely anesthetized dogs in which the occluded region was perfused via a two-chamber blood reservoir that allowed maintenance of perfusion and exclusion of microspheres from the circumflex region indicate that intermediate reductions in blood flow at the border of the ischemic zone resulted from an admixture of normal myocardium and, thus, do not represent a border zone of intermediate ischemia.

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