The "border zone" in myocardial infarction. An ultrastructural study in the dog using an electron-dense blood flow marker.

There is controversy over the existence of a lateral "border zone" of intermediately injured tissue in the developing myocardial infarct (MI). The authors attempted an ultrastructural demonstration of this zone in serial subepicardial biopsies from developing MIs in dogs subjected to 90 minutes, 6 hours, or 24 hours of left anterior descending coronary artery ligation (4 dogs per group). After ligation, 400,000 units of horseradish peroxidase was infused through the left atrial appendage as a blood flow marker, to allow distinction of perfused and ischemic myocytes under electron microscopy. Evans blue stain was infused in the same way for gross guidance in harvesting biopsies across the lateral margins of the ischemic region. Interdigitation of perfused and ischemic tissue was observed over a lateral margin 6 mm wide, which caused admixture of perfused and ischemic myocytes in biopsies from this region. Ultrastructural ischemic injury was graded on a five-level qualitative scale (normal, mild, moderate, severe, lethal). Perfused myocytes from control and border sites were equivalently well preserved (79% normal, 21% mild injury, n = 353). Ischemic myocyte injury increased with duration of ischemia. At 90 minutes, 65% of ischemic myocytes had mild injury, 35% moderate (n = 138); at 6 hours, 17% of ischemic myocytes had moderate injury, 81% severe, 2% lethal (n = 115); at 24 hr, 100% of ischemic myocytes had lethal injury (n = 148). Severity of ischemic myocyte injury did not vary from the lateral border to the center of the ischemic region: there was no lateral "border zone." However, if myocytes were not separated into perfused and ischemic subpopulations before statistical comparison of biopsy site to severity of injury, false evidence of a "border zone" was obtained.