ULTRASTRUCTURAL CHARACTERIZATION OF THE BORDER ZONE SURROUNDING EARLY EXPERIMENTAL MYOCARDIAL INFARCTS IN DOGS

Abstract

In experiments designed to search for such a border zone by EM, 12 mongrel dogs underwent permanent ligation of the left anterior descending coronary artery (LAD). Two to 6.5 (average = 4.2) h later, the hearts were excised, the area at risk (myocardium perfused by the LAD) was outlined by injection of fluorescent microspheres. The myocardial infarct was demonstrated by the nitro blue tetrazolium (NBT) gross histochemical method. Myocardial samples for EM study were obtained from the periphery of the infarct (tissues unstained by NBT) and serially from the immediately adjacent myocardium, which was stained deep blue by NBT. Grossly, the infarcts always involved the subendocardial myocardium, extended for a variable distance in the epicardial direction and closely approximated the lateral margins of the area at risk. When examined by EM, the infarct periphery showed evidence of irreversible damage, confirming the ability of NBT to detect early myocardial necrosis. Multiple samples of the NBT-stained myocardium immediately adjacent to the infarct showed varying degrees of reversible ischemia demonstrating, at the ultrastructural level, the existence of a border zone of intermediate myocardial injury. This border zone was substantial (3-4 mm in width) along the subepicardial aspect of the infarct and very thin (1-2 mm) laterally. A significant border zone was demonstrable by EM in the subepicardial myocardium of 8 out of 12 canine hearts with recent coronary artery occlusion. In the remaining 4 hearts, the infarcts had already reached the epicardium at the time of study and only a thin lateral border zone was present.