A 90% stenosis of the opening of the first diagonal branch is defined as the presence of a severe stenosis in one of the branches of the coronary artery. The first diagonal branch is a branch of the anterior descending branch of the coronary artery that divides the proximal and middle portions of the coronary artery. The anterior descending branches of the coronary arteries supply the heart muscle in the anterior wall, the anterior interventricular wall, part of the high lateral wall, and a small portion of the lower wall. When the opening of the first diagonal branch is 90% narrowed, it will lead to myocardial ischemia in the blood-supplying area, and there is a possibility of further progression to 100% occlusion, which in turn triggers myocardial ischemia, degeneration and necrosis in the corresponding blood-supplying area, i.e., myocardial infarction. Coronary artery disease can be diagnosed when the lumen area of the coronary artery is narrowed by more than 50%, and when the degree of stenosis reaches more than 75%, it is a severe stenosis, and the patient is prone to angina pectoris or even myocardial infarction, which can be life-threatening in serious cases, and needs to undergo coronary artery angiography and stenting to treat the disease. If there is any discomfort such as pain in the anterior region of the heart, please consult a doctor in time to avoid life-threatening situations.