Often MIs begin with infarction of the subendocardial layer of cardiac muscle. This layer has the longest myofibrils in the heart, the greatest oxygen demand, and the poorest oxygen supply, Around the initial area of infarction (zone of necrosis) in the3 subendocardium are two other zones: (1) the zone of injury-tissue that is injured but not necrotic; and(2) the zone of ischemia-tissue that is oxygen deprived. This pattern is illustrated in Fig.40-2.