ST segment downshift is a manifestation of myocardial ischemia. Commonly, there are horizontal or sagging ST-segment downshift and upward-sloping ST-segment downshift. 1. Horizontal or sagging ST-segment downshift: ST-segment downshift may be caused by angina pectoris attack. Angina pectoris is caused by insufficient blood supply to the coronary arteries and acute, transient ischemia and hypoxia of the myocardium. ST-segment depression >0.1mv in ECG during the attack, recovered after the attack is relieved, and sometimes T-wave inversion can also occur. 2. ST-segment upward-sloping downshift: no practical clinical significance. ST segment downshift in different leads has different meanings: ST segment downshift in leads V1~V5 suggests extensive anterior wall myocardial ischemia; ST segment downshift in leads II, III, avF suggests inferior wall myocardial infarction; ST segment downshift in leads I, AVL, V8 suggests high lateral wall myocardial infarction; ST segment downshift in leads V7, V8 suggests posterior wall myocardial infarction. When ECG examination is abnormal, it is recommended to go to the hospital in time to clarify the cause of the disease and carry out targeted treatment.