ST changes on ECG include ST segment elevation and depression, which usually represent myocardial ischemic injury. Normal ST segment performance on ECG is as follows: V1~V3 leads can be elevated by 0.1~0.3mv, the rest of leads ST segment is basically the same as the baseline level, and the downward shift is usually not more than 0.05mv. If ST segment elevation or depression occurs in addition to these conditions, it is clinically significant. When myocardial ischemia leads to myocardial injury, ST segment elevation or depression may occur. In subendocardial myocardial injury, the ST vector is directed away from the epicardium and toward the endocardium, which depresses the ST segments of the leads located in the epicardium; in subepicardial myocardial injury, the ST vector is directed toward the epicardium and away from the endocardium, which elevates the ST segments of the leads located in the epicardium.