The branches of coronary arteries arising from the aortic root are distributed on the epicardial surface of the heart. These in turn provide intramural branches that supply the cardiac muscle. Myocardial ischemia generally appears first and is more extensive in the sub-endocardial region since these deeper myocardial layers are farthest from the blood supply, with greater intramural tension and need for oxygen. Ischemia in this area prolongs local recovery time. This generally results in a prolonged QT interval or increased amplitude of the T wave or both as recorded by the electrodes overlying the subendocardial ischemic region.
Transmural ischemia is said to exist when ischemia extends subepicardially.
This process has a more visible effect on recovery of subepicardial cells compared with subendocardial cells. Recovery is more delayed in the subepicardial layers, and the subendocardial muscle fibers seem to recover first. Return to Top Injury Injury to the myocardial cells results when the ischemic process is more severe. Subendocardial injury on a surface ECG is manifested by ST segment depression, whereas subepicardial or transmural injury is manifested as ST segment elevation. In patients with coronary artery disease, ischemia, injury and myocardial infarction of different areas frequently coexist, producing mixed and complex ECG patterns.